Air-to-water vs. Air-to-air Heat pump
Hello,
I’ve recently read a few older posts discussing air-to-water heat pumps and wanted to get some insight into my particular situation.
Me and the family recently moved into a 22 year old home with a 4 zone hydro air heating system powered by a fuel oil boiler in Central Massachusetts. In the process of looking into updating the system with air conditioning, I stumbled upon air-to-water heat pumps and became quite intrigued. I asked a local HVAC contractor about them and he said, “Air to water heat pumps are inefficient and we don’t install them. Since your system is > 20 years old, why don’t you just get some air-to-air heat pumps, new air handlers, and keep your oil-fired boiler for deep winter heat? I’m happy to give you quotes.”
Given the age of the system, this seems like reasonable advice, however, I’m still kind of stuck on the idea of an air-to-water heat pump. Since we already have the plumbing in place, it doesn’t make sense to me to add in new refrigerant lines. While two of the air handlers are in the basement, one is in the attic of the main house and the other is in the attic above two rooms over the garage so I would anticipate we would require quite a lot of refrigerant line.
Am I missing something? Is there any validity to his claim that air-to-water heat pumps are inefficient (they seem to be quite popular outside of the US)?
In theory, wouldn’t a very basic solution to air conditioning be to send chilled water to the existing air handlers during cooling season? If we were to go with air-to-water heat pumps, any major differences between monobloc and split systems? I don’t mind a short run of refrigerant if it saves me from filling the system with glycol.
My biggest concern with an air-to-water heat pump is finding a reliable installer and making sure I have a good maintenance technician.
Overall, a gas boiler isn’t an option without burying a large propane tank and I’m hoping to add solar panels in the next few years so I quite like the idea of heat pumps.
Thanks for your input!
Taylor
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Replies
Not much truth in the contractor’s statement. There’s some variation between the thousands of air-to-air models and the handful of air-to-water, but it’s slight. The efficiency difference between an oil/gas boiler and a heat pump is much larger.
However, there are some issues depending on the water temperature you’ll need for the air handler. The efficiency decreases as the required temperature increases, so they’re not a perfect match for air-to-water and efficiency will depend very much on the heat loss. Don’t sweat the glycol, you won’t have much water volume.
Unfortunately, it is not as easy as reusing the existing distribution pipes, so you’ll be replacing those either way (chilled water must be in insulated and vapor sealed tubing otherwise condensation will form). Having 4 separate air handlers is definitely a point in an air to water heat pumps favor.
However, the concerns about contractor experience and manufacturer availability are real and a major hurdle. You’d be an early adopter for sure.
Ah, so it looks like I would need to either run refrigerant lines or new chilled water lines.
I'm not opposed to replacing the air handlers, particularly if it helps reduce the noise and on/off cycling.
Thanks!
Usually air to water seems over engineered for ducted systems like yours, but with 4 zones it starts to make more sense!
In an interesting turn of events, it appears the two air handlers in the attic are already plumbed with refrigerant lines accessible in the basement and drainage (drainage from the air handler and both are in plastic drain pans).
The ability to add radiant floor to a bathroom or two in the future is still a major selling point for air to water.
Good! Don’t worry about bathroom radiant - that shouldn’t be the deciding factor between air to water and air to air since electric resistance can be used cheaply for bathrooms.
Taylor you should look for a contractor specializing in geothermal and who knows what they are talking about. Water is a much more efficient medium for thermal transfer than air.
Good luck!
Good idea, thanks!
The good ones can be very efficient. The problem is that it's hard to find installers that understand them.
One question: how do you do air conditioning now? In theory you could do it with chilled water in your existing fan coils, if you had a heat pump so supply the chilled water. But, that would require insulating the plumbing meticulously to prevent condensation on the pipes.
We currently have window AC in the bedrooms only and that helps cool the downstairs a bit.
Most of the existing pipe is accessible but there is some in the wall that may require opening a wall in a closet to access, which would probably need to be done even if we ran new pex or refrigerant lines.
An option is to valve off parts of the distribution system in summer and only use a few of the fan coils for cooling. Unfortunately, the ones you want to run for cooling might not correspond to the ones that are easiest to access for insulation. But new pre insulated PEX can be fished through some spaces that you can't access directly.
Enertech Advantage is another one to look at, in addition to Chiltrix. Enertech is a better established company with a better reputation, and they might also have a bigger network on installers to refer you to.
https://www.theradiantstoreinc.com/enertech-advantage-air-to-water-heat-pump
Thanks! I found an instate geothermal installer who advertised a "long awaited" air to water heat pump in late 2021. Still waiting to here what it is.
Seems to me it is very unlikely an air to water heatpump will produce water cool enough to remove much if any moisture from the air. Where I live removing moisture is about 60% of what we need my AC to do.
Note air to water HPs are rare birds and tend to be very expensive. The low production numbers mean the units tend have bugs that need to get worked out. In fact your installer is unlikely to have installed one before and the local distributor is unlikely to stock any parts for such a rare unit.
Note your current hydro air units seem unlikely to be equipped to collect and drain any water that would condense.
Note your current piping seems unlikely to be insulated for cold water and may condense water on the pipes that would likely drip in unexpected locations staining walls and ceiling.
Walta
Thanks for your input Walta. In my head, I was thinking that drainage and insulation would potentially be issues but didn't write it.
Your points about parts and QC are well-taken as well.
There are several systems like SpacePak and Chilltrix that focus on dehumidification in the available literature but as above, this is still currently niche.
Chiltrix has a neat feature where you can set both a temperature set point and a humidity set point. It adjusts the water temperature to hit both. I think controls like this will become more common.
If I can find a local installer, I’ll give Chiltrix some serious consideration.
An air to water heat pump can easily produce water at 35F. That's about as cold as you want a cooling coil to get, otherwise you risk icing. At 35F with indoor air at 72F and 50% RH you get a sensible heat ratio of 67%, which is more dehumidification that most buildings need.
Air-to-water heat pumps are the norm for commercial buildings. What's rare is seeing them small enough for residences.