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About this blog: Climate change, despite its outsized impact on the planet, is still an abstract concept to many of us. That needs to change. My hope is that readers of this blog will develop a better understanding of how our climate is evolving a...  (More)

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How to electrify a boiler

Uploaded: Aug 27, 2023

If a boiler powers your home’s heating system, then warm water flowing through radiant floors, hydronic baseboard heaters, radiators, or even valance heating on the ceiling warms the interior space. Hydronic heat is common in decades-old Eichler homes as well as in newer houses. It uses less space than forced air -- the 3/4-inch-diameter copper or PEX tubing employed carries the same heat energy as an 8x14-inch sheet-metal duct in a forced-air system. Radiant floors themselves are essentially invisible heating appliances, and they are especially effective at warming people in high-ceilinged rooms.

Boilers in California are almost always powered by natural gas, but hydronic heat can be electrified with an air-to-water heat pump.

This gas-powered boiler was replaced with an efficient electric air-to-water heat pump.

If you have a hydronically heated home and your boiler is aging, or you are interested in adding some cooling to your house, consider replacing your boiler with an air-to-water heat pump. These efficient electric appliances extract heat (or cold) from the air using a refrigerant, then warm (or cool) the circulating water with that heat. Air-to-water heat pumps are used in Europe and Asia and increasingly in the US, even in the cold northeast. California’s building code recently recognized just how efficient they are, which is starting to drive more adoption. The temperate Bay Area is a particularly easy place to configure an air-to-water heat pump. I have had one for over two years and it’s been great at both heating and cooling the house.

A Chiltrix air-to-water heat pump sits outside a home.

These heat pumps have two components, a large external box with a fan that goes outside of the house and a buffer tank that goes inside. The outside box contains the elements needed to warm (or cool) the water that will circulate through the house as well as those to warm (or cool) the refrigerant. (1) Indoors, a passive buffer tank connected to the outside unit helps the system run more efficiently (at a slow, steady pace) by acting as a thermal battery. Tanks vary in size, but 20-40 gallons is common. The tank will typically be placed where your boiler is since it will have many of the same connections.

A larger buffer tank makes for more efficient operation, which saves electricity. The one shown is 37 gallons. These tanks have hookups for electric resistance elements for backup heat if you live in a colder environment.

Inside the home there are a number of ways to use the water from the buffer tank to condition your space. Most common is a radiant floor. Air-to-water heat pump systems work seamlessly with radiant floors as long as the temperature used to heat the floor is no more than about 110F. Many of these air-to-water heat pumps cannot heat water above 120F.

This 104F temperature is in the sweet spot for many air-to-water heat pumps.

If your boiler is set to 110 or less, then you can use a reversible heat pump like those made by Aermec, Chiltrix, and SpacePak. These provide cooling as well as heating. Cooling can work with few changes if you have a radiant floor. In our arid area, we don’t have to worry much about condensation on a cool floor, though controls are available that will raise the temperature of the circulating water when needed. (2) There are also products on the market -- air-emitting fan coils and radiant convectors -- that provide efficient and quiet cooling (and heating) when paired with a heat pump. They can be mounted on walls high or low, freestanding on floors, or hidden in soffits, under floors, or behind walls.

Jaga offers a variety of low-temperature convectors. Source: Jaga

If your boiler is set to a higher temperature, it may be because you have heaters in the home that are designed for high temperature water, such as cast iron radiators. Or it could be because your radiant floor has aged and become less effective so the temperature has been increased to compensate. If you switch to a low-temperature heat pump, you can replace older heaters or supplement an aging floor with the emitters mentioned above, which are designed for lower temperature water and provide both heating and cooling. Alternatively, there is a heat pump made by Harvest Thermal that does put out higher temperature water, though it can be tricky to get it working well with radiant floors. (3) This heat pump is not reversible, so it provides heating but not cooling. On the plus side, it can handle your domestic hot water at the same time. Chiltrix says it is coming out with a reversible high-temperature air-to-water heat pump soon.

In my house, a reversible heat pump warms radiant floors in winter and cools them in summer. I installed two behind-the-wall fan coils for additional cooling in the common rooms. (4) A Nest thermostat in that area is configured for multistage heating and cooling to control both the floor and fan coils. The heat pump itself is on the small side for a 2100 sf home, providing 2.8 tons of heating and 2 tons of cooling, but it works fine for the well-insulated home. On the coldest days when the unit runs less efficiently because it has to work harder to extract heat, I find that it helps to close off the large master bedroom, which is a heat sink with its high ceiling, large panes of glass, and poorly insulated roof.

This Nest thermostat is set to two-stage heating and cooling to manage both the radiant floor and the fan coils in the common areas. The two-stage setup is shown on the left, while the heating and cooling options are shown on the right. In other areas where only a radiant floor is available, the thermostat is configured for standard (single-stage) heating and cooling.

This past winter, which was a cold winter for the Bay Area, the system used between 630 and 870 kWh per month, as measured by a circuit monitor I have installed. The cost for the power, from $155 to $215 per month at the new Tier 2 prices ($.25/kWh in Palo Alto), is similar to what I would pay for gas here, the equivalent being around 65-85 therms. The installation cost was around $25,000 for the radiant conversion, plus an extra $5,000 for two fan coils, which were optional. The conversion was not cheap, and may not get cheaper for a while due to lack of qualified installers.

This chart shows the power used and cost of running my air-to-water heat pump this past year. Billing is shown as green bars (see left axis) and power consumption as a blue curve (see right axis). The chart assumes electricity costs of $.25/kWh, the new Tier 2 Palo Alto rate. Source: Eyedro circuit monitor

In fact, the most difficult part of these conversions is probably finding a skilled contractor. Replacing a gas-based hydronic system with an electric one requires plumbing skills, HVAC skills, and some electrical know-how. More experienced installers will do a cleaner and more efficient installation by insulating pipes carefully to conserve heat, reducing bends in piping to conserve pressure, sizing appropriately for ample but efficient heating and cooling, and so forth. These conversions take time and there are not enough tradespeople to go around. One hydronics expert I spoke with is looking to design drop-in replacements for some common configurations, which could make retrofits faster and more scalable. But because his schedule is so busy, he is having trouble finding the time to do the necessary engineering.

I would also add that this technology can be finicky. In researching this blog, I found that for the last few months my heat pump has been drawing 100 watts even when it is idle. Diagnostics indicate that the water pump is not turning off, so I’m waiting on tech support to help with it. I do recommend installing an inexpensive circuit monitor with these power-intensive, variable speed appliances in order to better understand how much power they are using and why.

A peek inside

For those who are interested in seeing the innards of one of these appliances and learning a bit more about how they work, here are a few pictures.

This Chiltrix heat pump has most of its equipment in a compartment on the right. The left side consists of a fan and a large coil heat exchanger. Both are described below.

If you open the side of the box that faces to the right, you see this. The important components in the photo above are the large heat exchanger (HX), the compressor (Comp), and the water pump (WP). The plate heat exchanger takes as input (at the top) cold water (CW) and warm gas refrigerant (WR), and outputs at the bottom warm water (WW) and cold liquid refrigerant (CR). A water pump (WP) moves the warm water back to the house, while also pulling cold water from the house. A compressor (Comp) powers the refrigerant cycle. The cold refrigerant leaving the plate heat exchanger travels to a large thin coil heat exchanger wrapping the inside of the box (see photo below), where a fan pulls air over the exchanger to warm the refrigerant. The refrigerant then goes back through the compressor and into the heat exchanger as a warm gas. The compressor uses most of the power in this box, though the water pump also uses some. The power to each is automatically adjusted as needed, saving energy. The photo above also shows a flow meter (FM) on the water circuit. In the middle of the photo, unlabeled, is the reversing valve that can switch this appliance between heating and cooling.

The higher angle in this photo shows an expansion tank (EXP) in back, the reversing valve (RV) in front of it, and a pressure release valve (PRV) for the water circuit.

If you open the right side of the front of the box, you see the control board on the top, the inverter board lying flat above it, and the compressor wrapped in a cover below. The control board makes adjustments to pump and compressor speeds based on multiple inputs. The inverter board manages power to the unit.

The entire left part of the box consists of a large finned-tube coil heat exchanger. The refrigerant tube wraps through it and the fan draws air over the exchanger so the refrigerant can warm up (or cool down) as needed. Tiny fins effectively extend the surface area of the long and winding refrigerant tube, enabling it to better capture heat from the air blowing across it.

The garage-mounted fan coils have a similar finned tube coil heat exchanger. Both have a fluid (refrigerant or water) exchanging heat with the air. In the case of the garage-mounted exchanger in the fan coil, the goal is to heat or cool the air passing over it. In the case of the outdoor coil in the heat pump, the goal is to heat or cool the refrigerant inside the coil. But both air and fluid change temperature during these heat exchanges.

Congrats on making it this far! Please share any comments or questions you have about these air-to-water heat pumps in the comments section below.

Notes and References
00. Email notifications for this blog have not been working for several months, but I think they are working now. Please lmk if you are having any problems being notified when a new one is available. Update; Argh, I think they're still not working.

0. I’d like to thank my HVAC contractor for reviewing this blog post.

1. These all-in-one systems are meant to be easier to install, partly because they keep the refrigerant enclosed in one outdoor box. However, there are also split systems with an outdoor unit that captures heat from the air (so it has a fan, an outdoor coil, and a compressor), and an indoor unit that transfers the heat to the water (so it has a heat exchanger and water pump, plus the electronics). These require refrigerant to be piped between the two systems. You can read more about different configurations in this helpful writeup.

2. I have used a radiant floor for occasional cooling for two years with no sign of condensation. However, it’s possible that with the changing climate we will see more frequent high dew points in coming years. When air has a higher moisture content, water condenses at warmer temperatures. More tropical storms could mean that a floor surface cooled down to 65F would be more likely to experience condensation. On August 23 this year, humidity hit 95% in Palo Alto, and we had a dew point of nearly 70 a few hours later.

Humidity was 95% at 7:37am on August 23 in Palo Alto. Source: KCAPALOA199 Personal Weather Station

The dew point was 69.7F at 11:27am on August 23 in Palo Alto. Source: KCAPALOA199 Personal Weather Station

3. Harvest Thermal uses the Sanden heat pump, which is designed for domestic hot water. It is a terrific hot water heater, with an outdoor component and an indoor component. The Sanden uses carbon dioxide as a refrigerant, which has much less warming impact than standard refrigerants. But CO2-refrigerated systems work best when the fluid coming (back) into the heat pump is much cooler than the fluid leaving. That is true for domestic hot water (the incoming fluid is just cold water). But radiant floors typically don’t drain that much heat out of the circulating fluid -- if they did, the rooms at the end of the radiant loop wouldn’t get heated properly. So some contractors report difficulty getting these systems to provide enough radiant heat. The systems work better if hot water is running. In Europe with similar systems they sometimes reverse the direction of the heat pump circuit every 30 minutes, so that every room is on the warm side of the circuit each hour.

4. The fan coils were more of an experiment than anything else. The two common rooms are both adjacent to the garage where the HVAC closet is, so the location was convenient. Connections could be short and simple, plus there was a nice way to mount the fan coils behind the walls (i.e., in the garage) so only a grill would be visible from inside the house. In addition, the kitchen area has no radiant floor, so the fan coils provide extra value. It turns out that the air cooling is really nice in these rooms on hot days, but I’m not sure it was worth the cost and effort to put the fan coils in.

From inside the house the vents look like this, with the intake on the bottom and the outflow on top.

The garage side looked like this midway through construction. The contractor had to build a fireproof box to go around the fan coil because it penetrates the drywall.

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Posted by Asher Waldfogel, a resident of Old Palo Alto,
on Aug 27, 2023 at 8:57 am

Asher Waldfogel is a registered user.

Great post!

This is a passion project with no financial payback at electric tier 2. Have you run any numbers on how big a PV panel would zero out the electric costs, and if there's a net payback on the combination?

Curious if you have any sound pressure level measurements or specs for the outdoor unit?

Posted by Sherry Listgarten, a Mountain View Online blogger,
on Aug 27, 2023 at 12:58 pm

Sherry Listgarten is a registered user.

@Asher: FWIW, for the outdoor unit, in real life I don't notice it. It's in a side yard. If I think to check, I usually have to look at the plants (are they moving?) to see if it's on unless I stand pretty near it. But, for you, I just measured :)

Right next to it it's about 68. About 20 feet away it's about 55. About 15 feet away with a thin wood gate in between it's about 50 -- I have to strain to hear it. Around the corner of the building with no gate, I don't hear it at all and the sound meter didn't pick it up beyond ambient (around 44).

Re PV, it depends on pricing, which is evolving. I wouldn't recommend it in Palo Alto unless you like the extra reliability the battery provides. But I imagine pricing still favors this in some areas. More deets: If you want to size for winter, then it's around 25 kWh per day. If you get 4 hours of full sun, then that is a 6 kW roof. You'd also want a battery since you are going to be doing a lot of heating when the sun is low. One 10 kWh battery might not be enough, but it'd be a good start. This will be way too much in other seasons, but you could use it for something else.

If I were installing a heat pump, I'd look into energy efficiency before PV. But YMMV.

Posted by Mondoman, a resident of Green Acres,
on Aug 28, 2023 at 3:22 am

Mondoman is a registered user.

Could you describe roughly what was involved in the $25k conversion price? Is that just the boiler or does it include new convectors or even new radiant flooring?

Posted by Asher Waldfogel, a resident of Old Palo Alto,
on Aug 28, 2023 at 8:16 am

Asher Waldfogel is a registered user.

68 dBA in a side yard setback would be annoying. 50dBA at the lot line is not an issue.

re PV: I was looking for a way to make an economic case in favor of the heat pump conversion. I've analyzed PV several times over the last 20 years and payback has always been close to forever. I'll probably do it when I replace my roof, mostly as a hedge against future electric rate increases.

But not clear I'll be able to avoid electric rate increases even with PV. I'm still waiting to hear how PAU expects to bill out several hundred million $ of distribution system upgrades and I suspect there will need to be a large fixed fee which I hope will be equitably divided.

I've discussed heat pump radiant boilers with my plumbers and it's not a good fit for me, but it's great to see it can work in an Eichler.

Posted by Sherry Listgarten, a Mountain View Online blogger,
on Aug 28, 2023 at 3:45 pm

Sherry Listgarten is a registered user.

@Mondoman: The $25K for the boiler conversion broke down as about $10K for materials and $15K for labor. I had another estimate that was higher, more than $35K, though that would have done hot water as well. (I did hot water separately for $5K.) It was pulling teeth to get even two estimates.

@Asher: FWIW, I just went to measure from my neighbor's side yard and the meter couldn't hear it at all. But it wasn't running at full power (it's variable speed). This time it was 62 dB right next to the box. There's a fence between us and some vegetation. In my experience, the sound is not an issue.

The number you didn't ask for is what it takes to replace a boiler with a gas boiler in Palo Alto. I don't know. But imo if your boiler is old or you are looking for cooling, the economics would be fine with a few more tradespeople, especially because gas is very likely to get more expensive relative to electricity over the life of that new boiler. Do you want to lock your house into gas at this point?

BTW, rooftop PV is a very expensive way to generate electricity. If it's not priced that way it's because it is (very inequitably) subsidized.

Source: Lazard (2023)

Posted by Eric Muller, a resident of Los Altos,
on Aug 28, 2023 at 6:50 pm

Eric Muller is a registered user.

I have been looking at this stuff recently, here is what I found.

1. there are a number of Air2Water heat pumps available in the US/North America.,,,,,,,, Pretty much all the big Japanese and European manufacturers have some, but none import in NA. In California, there is in principle the additional complication that the equipment needs to be listed in the MAEDbS database of the California Energy Commission.

2. the monobloc designs (the refrigerant stays in the outside unit) limit the concerns with refrigerant leaks. However, some of the brands above use refrigerants which are being phased out starting this year.

3. some/most systems can do space heating, space cooling and domestic hot water from a single heat pump (typically, domestic hot water has priority over space cooling). Some have a box, placed inside, with all all the necessary valves, pumps, etc, making it almost a plug and play system.

4. most systems can be used with ducted air. There are air handlers that take hot/cold water in, and transfer the heat/cold to air, which is blown in the ducts. However, a2w heat pumps generally warm the water less than a gas furnace heats the air, so you may need larger ducts than those you currently have.

5. it seems not too difficult to install underfloor hydronic. That involves PEX tubing and aluminium plates under the floor to tranfer the heat. Then you can seal/isolate just below the underfloor and keep your crawling space outside the enveloppe.

6. the sanco heat pump is really designed for domestic hot water, and space heating is a little bit of an add-on. It can deliver 4.5 kW, but they recommend to use only 3 kW for space heating (or ~10,000 btu/h)


Posted by Eric Muller, a resident of Los Altos,
on Aug 28, 2023 at 6:50 pm

Eric Muller is a registered user.

7. noise can easily be an issue. Here in Los Altos, a typical geometry is to place the heat pump in the side yard, at most ~6 feet from the property line. And we have a noise ordinace of 45 dB at night at the property line (so the equipment must be rated no more than 51 dB @ 1m).

8. a nice property of most of those heat pumps : they are "inverter" hp, meaning that the fan speed can be adjusted continuously, and is typically not full speed; that can help with noise. However, you can't really tell much from the specs. I am not sure about the air handler (if you use one): the harvest thermal system is claimed to be much quieter than a typical forced air/gas furnace (which alternates between full on and full off).

9. one issue that is not discussed much: integration with a "smart home". My want is very simple: I have PVs, so the best kWh to use (in my home) are those that I would otherwise send to the grid. Yet few of those hp mention anything about interfacing with a system what would see my production, my consumption, and can arbitrate the various controllable loads (heating, cooling, car charging, etc).

10. the TECH rebates are only available if you use a qualified installer. And there are relatively few of those. If you add that there are few installers familiar with a2w heat pumps, etc, then that's the real bottleneck.


Posted by Sherry Listgarten, a Mountain View Online blogger,
on Aug 28, 2023 at 9:29 pm

Sherry Listgarten is a registered user.

@Eric, thanks for the detailed comments. A couple of things:
1. Re brands, I only mentioned those that I've heard directly or indirectly about good installs. That is limited and happy to hear about other experiences.
2. Re refrigerant, yes, the companies are updating theirs as needed.
3. I don't get the ducted air thing, so didn't mention it, though I know of it. If you have room for ducts, why not go with air-to-air which is a lot cheaper? I didn't have time to dig into this.
4. Yeah, as you say, they could have grid integration. But (a) the thermostats can have that; and (b) these things work best with a pretty steady temp. So I can also see why they haven't prioritized this.
5. Yes, installers are the clear bottleneck here. Maybe now that T24 finally recognizes these that will start to improve.

Thanks again for sharing your findings.

Posted by Eric Muller, a resident of Los Altos,
on Aug 28, 2023 at 11:24 pm

Eric Muller is a registered user.

> If you have room for ducts, why not go with air-to-air which is a lot cheaper?

One reason would be to include domestic hot water. Another is to use the hot water storage as a battery. Also, efficiency, since a2w has typically a lower temperature rise/drop and therefore is more efficient. Finally, I think that with a2a, you need to run refrigerant lines between the outside and inside; even with precharged lines, you still need an HVAC guy and equipment, while any plumber (or diyer) can deal with a monobloc a2w (it's just ordinary plumbing).

I am not saying that one approach is better or cheaper. My point was mostly that if you have a gas furnace with air ducts, and you switch to a a2whp, there is a good chance you will have to change your ducts for largers ones.

> But (a) the thermostats can have that

No thermostat knows how much my panels produce, and how much the rest of my house consumes (i.e. what's available for thermal storage, for example).


Posted by Bruce Hodge, a resident of Palo Verde,
on Aug 29, 2023 at 8:08 pm

Bruce Hodge is a registered user.

Thanks for this great write up! I think the principal barrier is the cost. The lowest quote I've received so far is $18.5K which is still high compared to an air to air heat pump system. Other quotes were for $27K and $34K (includes DHW). We have a lot of Eichers in Palo Alto, so hopefully we can figure out a way to drive these costs down more. All of my quotes incorporated Chiltrix HPs, with some variation in the sizes quoted. Looks like a Chiltrix CX-34 is sufficient for my Eichler and most other Eichlers in Palo Alto.

Posted by Bruce Hodge, a resident of Palo Verde,
on Aug 29, 2023 at 8:22 pm

Bruce Hodge is a registered user.

Perhaps it's not clear from my previous comment, but air-to-air heat pump systems require ducts to distribute the heated air, and hence are problematic for Eichlers (which don't have ducts unless a previous owner decided to add ducts). The Eichlers with copper distribution tubing seem to still be good for the most part. The non-copper systems have mostly failed and were replaced with various solutions, with ducting being one solution.

Posted by sustainability, a resident of another community,
on Aug 30, 2023 at 6:06 pm

sustainability is a registered user.

Question for Eric Muller: Loads of interesting info. Curious which A2W HP companies have the plug and play box set up?

Posted by Eric Muller, a resident of Los Altos,
on Aug 30, 2023 at 10:58 pm

Eric Muller is a registered user.

@sustainability these are the ones I remember:

Web Link

Web Link (indoor hydro box)

harvest thermal

Posted by Sherry Listgarten, a Mountain View Online blogger,
on Sep 1, 2023 at 6:50 am

Sherry Listgarten is a registered user.

FWIW, for those who are interested in learning more about this technology, one reader pointed me to these videos.

I'll have more to add tomorrow.

Posted by Eddie, a resident of Fairmeadow,
on Sep 1, 2023 at 7:21 pm

Eddie is a registered user.

Is the fan/compressor that you show the same as one that could be used for a heat pump water heater system?
Could the water tank you show be used as the tank for an heat pump hot water system?

Or... if you wanted to have a heat-pump based water heating and house heating system, do you need 2 fans/compressors and 2 water tanks?


Posted by Sherry Listgarten, a Mountain View Online blogger,
on Sep 2, 2023 at 11:26 am

Sherry Listgarten is a registered user.

Hey, sorry for the delayed replies, I've been out of town without a computer...

@Eric: My duct beta comes more from installations of ducted air handlers in air-to-air systems. Of the people I know who have installed one, all in older homes (1970 or older), none installed larger ducts, even though technically they might be called for. But it's something to be aware of. I think leaky ducts can be a worse problem, but also easier to fix?

@Bruce: That is super encouraging about the price coming down. FWIW I have definitely seen ductless installs at $18K or even more. They will have ~4 heads, a sizable outdoor unit (e.g., 4-ton) and nice finishing on the outside. Not everyone wants that, but it's not crazy. (I've seen bigger.) Re failed Eichler floors, a lot of people seem to move to ductless minisplits (see here). Others keep their low-temp boiler and move to valance heating or low-temp baseboards. Those would work fine with one of these air-to-water heat pumps.

@Eddie: There are two types of air-to-water heat pumps: low temperature ones (these can heat water to about 110 or maybe 120) and high-temperature ones (these can heat water to 160 or more). For domestic hot water, you need temps higher than the low-temp systems can provide. If you want to do both your radiant floor (say) and your hot water heater at the same time, you have a few choices:

1. Try Harvest Thermal (see above)

2. Try Chiltrix -- they have a special hot water tank that can sort of work with their AWHP, though my hvac contractor steered me away from it as too complex

3. Use a standalone HPWH (e.g., AO Smith, Rheem)

4. Wait for the US to catch up with Europe and Asia wrt high-temperature air-to-water heat pumps. Chiltrix says they have one coming out "soon" (possibly end of this year).

5. Use an electric boiler (resistance heat) between your buffer tank and your domestic hot water to get the additional degrees you need beyond what the low-temperature heat pump will provide.

There are probably other ideas too :) I went with (3). It's simple and it's fine.

Posted by Sherry Listgarten, a Mountain View Online blogger,
on Sep 2, 2023 at 11:34 am

Sherry Listgarten is a registered user.

I got a really interesting email from a reader in the northeast, a retired mechanical engineer, whose been thinking about how to electrify the many homes there with hydronic heat, typically using high-temperature radiators or baseboards. He's frustrated that the US is so behind other countries when it comes to the higher-temperature air to water heat pumps. I haven't gotten permission from him to post his whole letter, but here are a few things he mentioned:

1. He's very worried about refrigerant leaks from the air-to-air systems. "We have a 2-head air-to-air mini-split in our 2-story house that I am not too happy with. Refrigerant leaks are difficult to find and expensive to repair and with 4 flare-nut joints for each indoor head, I suspect leaks are more common than we know of. My unit has lost 3 full charges (15lbs total of R-410A) in the span of 5 years. All told that cost about $3000 and has caused the global warming equal to 3 years of burning gas in our condensing boiler. How is that going to be good for the environment when done at a mass scale?"

2. He lists these producers of high-temp air-to-water heat pumps available overseas: Viessmann, Bosch, Mitsubishi, LG, Toshiba, Feenstra/Vattenfall, Valliant

3. He really likes the modularity of these systems, which is an important point given how fast tech is changing. "With ATW heat pumps all of the indoor distribution and water storage tanks are generic and completely reusable. You would just replace the outdoor heat pump unit as you would if your boiler failed."

Posted by Eddie, a resident of Fairmeadow,
on Sep 2, 2023 at 6:55 pm

Eddie is a registered user.


> I went with (3). It's simple and it's fine.

So just to be clear, does that mean you have 2 fans/compressors and 2 water tanks (one of each for hot water and one of each for radiant heating)?

I just posted a comment on you previous article about my experience with the Palo Alto Heat Pump Water Heater program. I was disappointed with the lack of help in dealing with the issue of how to fit everything, given that we currently have a tankless water heater.


Posted by Sherry Listgarten, a Mountain View Online blogger,
on Sep 2, 2023 at 7:37 pm

Sherry Listgarten is a registered user.

@Eddie, I have a Rheem heat pump water heater (which, yes, looks like a tank, and it has a compressor built into the top part of it) and a buffer tank for my radiant floor. The former tank is bigger than the latter. I put the Rheem where my old gas (tank) water heater was, and I put the buffer tank where my boiler was. So it all fits fine. I have just one of those box-like outdoor compressors, which is for the space heating/cooling. Does that answer your question?

I did see your comment on the other blog post, and hope that you will share your feedback with the folks running the program. I would try

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