Air to Water Heat Pumps for Radiant Heating And Cooling

John Ruhnke

Hi Everyone,

I am starting a new thread about Air to Water Heat Pumps for Radiant Heating And Cooling. The last thread was under the category of refrigeration and AC systems. In order to install a Monobloc ATWHP you don't need to hold any refrigeration certificates or know anything about refrigeration. My knowledge of refrigeration systems is not at a high level anyway. I am still learning. My knowledge of hydronic heating systems and ATWHP is very high. Though I am always still learning.

Ask away and I will answer any questions.

John

Hot_water_fan

Caleffi covers this so well, I’m not sure how much this will be needed. One question I do have is are there any air to water heat pumps with low head loss? Seems like most require high flow and have high head loss.

DCContrarian

I have an air-to-water heat pump in my own home, that I use for heating and cooling. I believe that air-to-water has a number of fundamental advantages, which I outlined in this thread:

https://www.greenbuildingadvisor.com/question/are-air-to-water-heat-pumps-about-to-boom

But there are advantages and disadvantages compared to other technologies. The key is to maximize the advantages and minimize the disadvantages.

First, I've never been able to get "radiant" cooling to work, I need lots of dehumidification in my climate and that comes from air being blown over a cool surface, so I use fan coil units for cooling.

The principal advantage is that you can deliver small, targeted amounts of cooling, which means superior comfort. You can put a small fan coil in a small room, like a bedroom, and run it at very low capacity. My heat pump is a Chiltrix, but they sell the same PHNIX fan coil units as Arctic, rebadged. The smallest one available is rated 3,000 BTU/hr — about a third of the smallest minisplit head typically available. You can run it on the ultra-low fan speed which might produce 10% of rated output, you can have it cycle on and off for even less. There's just no way to do that with a minisplit, let alone a traditional ducted system.

Another advantage is higher efficiency. I encourage anyone interested to read this article:

https://arstechnica.com/science/2024/07/the-hunt-for-the-most-efficient-heat-pump-in-the-world/

Basically when you're really searching for high COP nothing compares to air-to-water.

I would say the biggest drawbacks center around the fan coil units, and the availability of better ones. The PHNIX ones seem to be the only residentially-sized units broadly available in the US. They are great for quiet comfortable operation, both in heating and cooling. But — they're ugly. No too ways about it. They're hard to install. Real hard. They're not really made for the US market, and everything about installing them is a challenge. They are dimensioned for US framing, they don't use US plumbing fittings. They're quite inflexible in the positioning of the plumbing. And I wonder about their durability, like most minisplits they don't have a disposable filter but rather a "nylon stocking" filter that seems to let a lot of dust through.

They're also quite expensive. The Chiltrix website has pricing, they start at about $800 for the smallest unit.

Kaos

While setting up my monoblock we had a late fall heat wave so I've played with radiant floor cooling. I can definitely say it is a bad idea in anything tiled/concrete. The bathroom zone is slaved to the rest of the house so it runs if any zone is calling for heat. This mean the tile quickly got to near loop temps (16C in my case) and management firmly told me that is unacceptable. It is indeed hard on bare feet.

I have small kids which means I spend a lot of time on the ground and even on hardwood floor it was not the best. Where it did work is in the bedroom as I did not have to run the wallmount there for cooling.

The house has a wallmount that does the bulk of the cooling and dehumidification so this was mostly looking if this was worth adding in. Currently, I would say skip it.

Hot_water_fan

Q: what specifically can make an air to water heat pump more efficient?

I can name low temps for radiant floor/wall heating. I also see distribution efficiency via a circulator vs a blower, although I think that’s pretty minor since my ducted blower uses probably 50w. Is it negated by the high head loss through the heat pump? Both also apply to water to water of course. Outside air temps can be higher than ground temps, so I certainly see that. But that applies to air to air too. What else is there?

John Ruhnke

Another company out of Belgium, Jaga makes some fan convectors that go inside the wall or ceilings. They also go into floor trenches. ww.Jaga-USA.com. They are more expensive than the Phnix ones we sell. They don't offer wall mounted units like Phnix does and Phnix doesn't offer anything like Jaga offers. They compliment each other well. It is good to have options for clients.

As far as radiant cooling goes, the best way to control that is with a dewpoint sensor operating a three way valve. The dewpoint sensor reads humidity levels and adjusts the water temps to stay above the dewpoint. That way you don't get any condensation issues. Out west we have reps working in dry climates that we have installed systems that can do 100% of the cooling load with out supplemental cooling. It works great in those conditions. Out here on the east coast our humidity level is high and you need to do dehumidification to get the humidity levels down. Of course the AHU or FC are doing cooling as well. So yes you need a back up source of cooling on the east coast. The key to success for radiant cooling on the east coast is to increase the energy transfer of the radiant surface so you can get a higher transfer of energy at a lower delta tee. That way you can use the radiant cooling more often than the back up. A company called Messana has a radiant heating and cooling ceiling system that does this very well. They are also expensive. Messana has projects in well insulated and tight houses on the East Coast that can do radiant cooling with out back up. Though most Messana systems on the East Coast do require a back up.

Most people like radiant floors. The radiant floor output can be increased for radiant cooling. You can decrease the delta tee by installing bigger pipes in concrete at tighter tube spacing. Warm Board and Ecowarm are two premanufactured panel board systems that do a good job to increase the energy transfer and lowering the delta tee to get more radiant cooling out of your floor on the East Coast. Rahau has a system with thick aluminum transfer plates installed on top of the subfloor that have excellent energy transfer too.

Another great advantage of lowering the delta tee of your radiant system is that this will also increase the average seasonal COP of your ATWHP!!

There are lots of people on the East Coast that have radiant cooling systems designed right and are very happy.

Hot_water_fan

Most people like radiant floors.

Where is the evidence for this? If >50% of Americans like them, why do ~1-5% have them? That’s a huge gap!

John Ruhnke

Lots of PHNIx fan convectors have been installed without a problem and are operating trouble free. DCContrariann must have an older unit not developed yet for the USA market. The newer PHNIX fan coils have American NPT threaded connections. They are easier to install. They look a lot nicer too. Phnix has improved the look and they are elegant looking now.

Jamie Hall

https://forum.heatinghelp.com/discussion/comment/1812171#Comment_1812171

I can think of lots of reasons, @Hot_water_fan , and I dare say — from previous conversations — that we would agree on most of them! For instance — installing a radiant floor in existing construction is… difficult. Not always impossible, but difficult. And most folks own or rent in existing construction. Or… some people like rugs and carpets. They don't play well with radiant floors.

It's not that they aren't quite good — like all other ways of staying warm or cool, there are applications where they are wonderful and applications where… not so much. Nor is it that they are too novel — after all, Roman villas in Britain had radiant floors 2,000 years ago which worked just fine, and my "home" cathedral — Durham — has radiant floors which were installed about 1,000 years ago and are still in use.

As always — when you have a project, identify what you want to do, scan the technology available, pick the best fit for the project and the budget, and go for it.

Hot_water_fan

@Jamie Hall that’s my point! Plenty of things are difficult but are accomplished. This one is difficult and isn’t being done. I lean towards actions, not words. If you like something and aren’t willing to pay for it, then maybe you don’t actually like it that much.

John Ruhnke

@ Hot Water Fan,

I was basically comparing radiant ceilings to radiant floors and saying most people would rather have radiant floors than radiant ceilings. In Germany 67% of new construction is radiant heating. Nearly 100% of systems are hydronic. In America 95% of systems are forced air but here on the East Coast we have a lot of older hydronic systems it is nearly 50%. In new construction I think radiant heating in America is around 7%. Radiant Heating is like buying a Mercedes. Many people want it but they can't afford it. You can easily spend 400% or more for a good quality car vrs a cheaper car. The same thing goes with heating and cooling systems.

Hot_water_fan

agreed! In the US, new build hydronics is a luxury product no doubt.

Jamie Hall

I run Chevys. Used Chevys. I would love to have a fancy new car, or even a less fancy one. But at 3 times my annual income? Perhaps not. I'd love to have another chunk of Cedric's home have a heat pump for summer cooling and shoulder season heat. Can I afford it? I cannot.

Funny thing. In this country, if you make something people want and can afford, they'll buy it. If they can't afford it they won't. If the only product legally available is too expensive, they'll go without.

John Ruhnke

1/2 the wealth in this country is owned by a small number of families. The top 10% has more money than they know what to do with. Where I live they get huge wall street bonuses. If they think you are a commodity or just like everyone else they will shop you around to get a better price. If they think your knowledge and workmanship is more like a fine piece of art then they will want your services. They want the best. If you can deliver that they will pay for that. We deserve there money. We make and build things. We reduce fuel consumption with our designs. We are reducing energy use and fighting climate change. We are building out the electrical grid needed for the next generation.

They have all the money and they will pay you what you are worth. You just have to show them you can design systems that your competition can't. Radiant Cooling is the new buzz word. Give them a design that is well thought out and is better than what your competition can do. When your competition says they can't!! You tell them that is because they don't know how to design it properly. Tell them you can!! A very good proper design including things your competition can't do is most definitely very valuable to the top 10%!!! You deserve to get paid!!!

John Ruhnke

Also I want to state that yes I did some high end projects for wealthy clients but I fixed a lot of leaky toilets for middle class folks like me. I charged based on how much time and effort it took. Some of theses complex projects I did took extra time and cost me extra money to make it work. I had to charge more.

I also changed out a lot of boilers to MOD CONS for average people that saw there fuel bills cut in half!! I saved them money!! KISS (Keep It Simple Stupid) always applied and I could give good prices on boiler change outs.

We also have ATWHP change outs for boilers that are reasonably priced. We have a High Temp Heat Pump system that comes in cheaper than changing to a mini split system. So don't think ATWHPs are just for the wealthy. Lots of middle class people buy them too.

Hot_water_fan

@John Ruhnke could you please speak on my efficiency question above?

John Ruhnke

I will answer the question. Just not now. I know this is going to be a few paragraphs. I will get to it early next week.

I could go on and on about efficiency. Saturday Night approaches!!

DCContrarian

I live in Washington, DC — the DC in "DC Contrarian." It's fairly hot in the summer — design temp of 92F — and quite humid. In my house I have a sensor that tracks the temperature and humidity every 10 minutes, so I know that over the course of this year's cooling season, June 15 to September 15, the lowest humidity ever got was 59% RH, the highest it got was 69%, and the average was 63%. I had the thermostat set at 77F all summer.

That's not a terrible range, but I'd prefer it a little dryer, maybe an average of 55%. But it's not terrible.

I'm going to go off on a little aside that is probably elementary for most folks here, but I want to make sure we're all starting on the same page. The reason they call it "air conditioning" instead of just "cooling" is that air conditioning does two things: it cools, but it also removed humidity. Without dehumidification cool air is damp and uncomfortable. If you cool air below its dew point — by removing heat — some of the moisture in it will condense out. Some of the heat that is removed goes toward lowering the temperature of the air, that's called "sensible heat" because you can sense that the air has changed temperature. The rest of the heat goes toward removing humidity, that's called "latent cooling."

At any given moment, the cooling that is being performed is a mixture of sensible cooling and latent cooling. The common measure of this is the "sensible heat ratio," or SHR, which is sensible heat divided by total heat. If an air conditioning process does no dehumidification then the SHR is 100%.

If your air conditioner is meeting the set point but the humidity is higher than you would like, the SHR is too high. This is an incredibly common problem here in DC, if you get a humid day that's not that hot the SHR needs to be really low, lower than air conditioners can typically provide. A lot of people here run dehumidifiers in their basements in the summer.

Radiant cooling has to have a SHR of 100%. You want to prevent condensation. You need to prevent condensation, it can be incredibly damaging.

If you mix a cooling system with a SHR of 100% with another cooling system, the combined SHR will be higher than the second system alone. If the second system already has a SHR that is too high, the combined system will only make the problem worse.

In a climate where conventional AC struggles to get the SHR low enough, adding radiant cooling is not going to improve comfort.

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1812168#Comment_1812168

"As far as radiant cooling goes, the best way to control that is with a dewpoint sensor operating a three way valve. The dewpoint sensor reads humidity levels and adjusts the water temps to stay above the dewpoint. That way you don't get any condensation issues."

I see two problems with that approach.

First, humidity can change quickly. As I noted above, I monitor temperature and humidity in my house, and humidity typically peaks around 9AM when everyone has just showered. It peaks again in the evening when everyone is home and dinner is cooked. The biggest source of indoor humidity is occupant activity. I've seen the indoor humidity move by 20 percentage points in a few hours.

Changing the temperature of the water isn't going to change the temperature of the surface that quickly. The response time of the surface is going to be somewhere between hours and days depending on how it's constructed.

The second problem is that with a heat pump efficiency is all about temperature difference, if you're cooling water and then mixing it to warm it back up again you're giving away a lot of efficiency. As I said earlier, the key to getting the most out of any technology is playing to its advantages and minimizing the disadvantages. With a heat pump, if you're tempering, you're doing it wrong. Also true with heating.

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1812168#Comment_1812168

"Another company out of Belgium, Jaga makes some fan convectors that go inside the wall or ceilings. They also go into floor trenches. ww.Jaga-USA.com. They are more expensive than the Phnix ones we sell. They don't offer wall mounted units like Phnix does and Phnix doesn't offer anything like Jaga offers. They compliment each other well. It is good to have options for clients."

I've looked at the Jaga units. Their website is almost unusable it's so disorganized, but from what I've been able to gather few of their units support cooling, which is really the hardest part.

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1812166#Comment_1812166

Fundamentally heat pump efficiency is driven by temperature delta, so yeah, low temperature delta is what makes heat pumps more efficient. Specifically, that it's practical to run at lower temperature deltas with an air-to-water than with other technologies.

For example, imagine a house with a 40K BTU/hr heating load. You could heat it with water leaving at 90F and returning at 80F, you'd need 8 GPM or a 1" pipe. That's doable. It would be a challenge to get 40K BTU/hr from water with an average temp of 85F, you'd need a lot of surface of radiation, but that's basically what the guys who do the extreme efficiency challenge do.

Now imagine trying to heat the same house with an air-to-air heat pump producing 90F air. It would be 20F above room temperature, you'd need about 1900 CFM of air flow. You'd need about a 20" round duct to provide that or maybe a 10x34. You start running into practical issues finding room for ducts that size and it's easier just to bump the heat pump temperature up.

I will concede that with cooling it gets more complicated because your choice of temperature gets driven by the need for dehumidification.

Hot_water_fan

@DCContrarian thanks. Agreed you start to hit that constraint. I wonder how big the efficiency penalty of the heat exchanger here is - because you’re not comparing 90 degree water to 90 degree air - you’re comparing whatever temp the heat exchanger must be to overcome this extra heat exchange. And how does the lower air return temp affect this?

Overall, I’m looking for efficiency advantages unique to air to water, not something that is also true of water to water.

John Ruhnke

DCContrarian,

I understand what you are saying, but it works. Radiant cooling is operated off of a dewpoint sensor that mixes with a three way valve. Everyone designing radiant cooling is doing it this way. Messana, Chiltrix and Arctic Heat Pumps all have systems like this. It works and there are no condensation problems. We have lots and lots of radiant cooling systems out there without problems. Also our competition is also having equal success. HRV and ERV systems also run and pull air out of bathrooms and kitchens. This also helps. If you follow the proper design guidelines condensation is not an issue. Again I say proper design guidelines must be followed. Opening a sliding glass door is also not an issue. It just works. The radiant slab is never as cold as the water in the pipe. I think this is part of the reason it works.

I will agree for efficiency purposes that mixing the water temperature to something hotter is not smart. There are many applications for radiant that require multiple water temperatures. Mixing is something we just have to do. There are staple up radiant heated floors with higher temperature requirements being mixed down to supply cooler temps for concrete slabs There is a balance between efficiency, comfort, convenience and price of the system. We also boost DHW to kill legionaries disease and mix it down to the faucets so as to not scald people.

John Ruhnke

Enhanced Vapor technology is unique to Air sourced Heat Pumps. It cannot be used by geothermal systems. This is the technology that helped Air source HPs catch up to Geothermal.

@Hot Water Fan

I will write that efficiency piece you asked for. I spent a lot of time studying energy efficiency in heating systems. It is one of my areas of expertise. I just want to think about it a little while and write something good.

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1812290#Comment_1812290

"Overall, I’m looking for efficiency advantages unique to air to water, not something that is also true of water to water."

The big advantage of water-to-water is that water supplies tend to be more stable in temperature than the outside air.

One of the things you see across the heat pump business is that technical innovation tends to show up in the most popular product types first and in the less popular types later. Right now new technologies typically are introduced in minisplits first, then air-to-water and water-to-water even later.

So the most advanced air-to-air give air-to-water a run for their money and in some ways surpass them. Water-to-water seems to be mostly running 20 year old technology and a lot of the theoretical advantage dissipates.

It's also clear that the technology is still evolving rather rapidly.

John Ruhnke

Our Air to Water Heat Pump has all the latest advantages and have had them for 9 years now.

Hot_water_fan

EVI is useless if you’re ground source yeah? You don’t need low temp performance because the lowest temp it would see is ~32F.

All else equal, doesn’t EVI lead to lower efficiency vs an equivalent ASHP? It looks that way for the Mitsubishi’s I’m familiar with. You trade low temp performance for COP.

John Ruhnke

@ Hot Water Fan,

EVI increases COP and efficiency by a huge margin. Mitsubishi's hyper heat does not outperform EVI it just does the same thing a little differently. COP does go down with temperature but that is the same with all heat pumps. But at equal temps EVI outperforms and has a higher COP.

Hot_water_fan

That doesn’t seem correct based on what Mitsubishi has published. Anyway, I’ll wait for your long answer.

John Ruhnke

I have not looked at the latest from Mitsubishi. maybe they got something new. Next Thursday I am going to a Trade Show and will learn more.

The long answer is going to turn into a paper. Now that I think about things I should have written something years ago.

Jamie Hall

I wonder if it's time for my challenge again. It's still out there. National Register Landmark house, so no change to appearance inside or out permitted, except in the basement. One outside compressor or equipment unit permitted. Noise level less than 40 dB outside or inside. Known peak heat loss 275,000 BTUh. Known minimum outdoor temperature -15 F. Design indoor temperature 75 F.

The challenge: You install a heat pump system capable of meeting the heating requirement. If, at the end of 20 years, the all up cost, including operation, maintenance, capital investment, replacement if end of life, and lost capital opportunity, is less than the all up cost of the existing oil fired steam system, and if the maximum down time for repair or maintenance has been 2 hours or less, we'll buy it from you at that time. If it isn't, you will bear all costs involved in restoring the building including existing heating system and grounds to the present condition.

There's the table stakes, gentlemen. Meet 'em or fold.

Hot_water_fan

ha Jamie give it up. We aren’t interested in your challenge

Jamie Hall

https://forum.heatinghelp.com/discussion/comment/1812318#Comment_1812318

yeah. i know. I just like to poke the bear now and them!

hot_rod

https://forum.heatinghelp.com/discussion/comment/1812318#Comment_1812318

yeah, I doubt the industry will develop a system specific to Jamies needs. That is not a mainstream application.

If the oil fired steam works well, stick with it

Kaos

https://forum.heatinghelp.com/discussion/comment/1812300#Comment_1812300

At least on min splits within Mitsu lineup, EVI does not increase COP. It does increase maintenance capacity by great deal. You can look at two similar units one EVI:

EVI:

https://ashp.neep.org/#!/product/31992/7/25000/95/7500/0///0

non-EVI:

https://ashp.neep.org/#!/product/107511/7/25000/95/7500/0///0

Even at low modulation, the non-evi COP is higher. The unit does fall flat at 5F but still well above on COP of the cold climate unit. The one big issue with looking COP only, is the non-EVI unit will blow warm air which is definitely uncomfortable.

John Ruhnke

https://forum.heatinghelp.com/discussion/comment/1812315#Comment_1812315

Hi Jamie,

I am interested in exploring this option. Should be an easy bet for me to beat Steam. I will have to look at electrical costs for the area you live in. I can design something super efficient though. I was going to ask to include environmental costs for our kids and next generation but I decided not to go there. To many people that don't know how to analyze engineering and science information lurking. I don't want to launch a huge debate right now. I won't go there. Don't even try.

I will accept this challenge and easily win as long as I get to convert those Steam radiators to low temp hydronics and hook an Arctic Heat Pump to it. Also there may be more than one heat pump outside. They will be piped and wired to run together though as one unit. Staging is required. (But first I need to explore things a bit further)

John Ruhnke

@Kaos,

Does that non evi mitsu unit have hyper heat? because that is a similar version of EVI. Also our govt standards are biased and horrible. Most likely captured by industry insiders with political motives. It will send everyone off in the wrong direction. I am exploring options using proper M&V. I like openenergymonitor.org. It will take a while for me to learn from what that community is doing. They are heading in the right direction. OpenEnergyMonitor.org is a very good starting point.

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1812347#Comment_1812347

NEEP is not a government agency.

As far as I can tell they are as close as exists to a non-biased source of information about heat pump performance.

John Ruhnke

To DCContrarian and Hot Water Fan,

I have a lot to learn about NEEP. I don't want to comment on something I am not yet an expert at. I need to do more research. I may like it. You might even change my mind on things. If you do that, it means I am learning something new. I love learning new things. I love to grow as a person. This is't anything that will happen fast it will take time.

DCContrarian

Just to lay out the parameters of the challenge, @Jamie Hall says the heat loss is 275,000 BTU/hr at -15F, indoor of 75F.

That's 90F difference, so 90 degree-hours per hour. At 275K BTU/hr that's 3,055 BTU/degree-hour, or 73,333 BTU/degree-day. We don't know where you are, but it's somewhere cold, let's say 6000 degree-days per year, or 440 million BTU per year.

A gallon of fuel oil is about 140,000 BTU, if the net efficiency is 70% that works out to 100,000 BTU/gallon net and that 440 million BTU per year takes 4400 gallons of fuel oil.

At $4 per gallon that's $17,600 per year for the annual fuel bill. Does that sound about right?

That same 440 million BTU is equivalent to 130,000 kWh. Let's say electricity is $.20 per kWh, you'd have to have an overall COP of 1.48 just to break even. Anything in excess of that could go to pay for the capitalized cost of improvements.

My gut feel is that even in a climate that hits -15F you could average out to a COP of 1.48. At the same time, it very rarely makes sense to replace working equipment just for efficiency gains, that's true of all types of equipment.

lkstdl

@DCContrarian ,

In a system with radiant cooling, rather than using mixing valves to raise the water temperature, why couldn't we either:

• run the whole system at a higher water temperature, and use dedicated dehumidifiers (either several small units throughout the building, or a central one integrated to the HRV)
• run the system at water temperature low enough to get dehumidification, but plumb the fan coils in series with the radiant panels, and control the flow so the water leaving the fan coils is above the dewpoint. Optionally their could be a bypass around the panels to allow control of the SHR.

Luke