Radiant floor cooling with Air To Water heat pump

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MikeT2

Member
Jan 23, 2019
21
Vermont
I have a mostly radiant floors with a few Smith PSU profile fan convectors for the second floor. Currently I use a Biomass wood boiler in winter and a propane boiler the rest of the year. I also have an indirect hot water tank in my loop.
The newest Air to Water heat pumps have caught my eye, especially with the rebates now. Some ATW pumps produce temps of 140 which would activate my PSU units too. My home is too large for one unit in the winter but this ATW may be best for all other times of the year with the reduced load instead of buying two units, so I am thinking a single large ATW would work for these low load times of the year.

I have been reading how ATW pump systems actually can help cool by putting cool water through the flooring. Does anyone have experience with this? Does it actually work?

I could stay where I am with wood heating and cooling provided by window units to some rooms, go to a ductless heat pump for certain areas if that makes financial sense, or go with a ATW heat pump.

Thanks for any information!
 

EbS-P

Minister of Fire
Jan 19, 2019
1,747
SE North Carolina
Quick thought. Will it always run above the indoor due point to avoid condensation? Seems like there are better solutions.
Evan
 

peakbagger

Minister of Fire
Jul 11, 2008
6,837
Northern NH
Funny I just sat through a free webinar by the heating guru John Siegenthaler two days ago and he touched on the cooling option and has touched on it in other courses I have taken. (He is big fan of those air to water units) The course was sponsored by Myson US which is based in Williston VT. John doesnt come right out and say it but he always seems to use fan coil units with drip pans for the air source heat pump cooling. He is based out of upper state NY . These units can be mounted in ducts or can look like a minisplit head. My strong guess is that east coast heat is different than desert heat. If you look at long term temp records our east coast hot days usually align with high dewpoint (or high relative humidity which are related ). Desert areas out west are quite dry with very low dew points. So unlike a western desert type heat we have to remove a lot of water vapor out of the air when we cool it and if we try to cool surfaces they can drop below dewpoint and water can bead on them once the surface temp drops below the dewpoint. That is one of the reasons why few folks in the east even know what a swamp cooler is (AKA evaporative cooler) but many folks out west depend on them as their daytime dewpoint is low. My guess is it way too easy to go below the dewpoint for a household system and cause potential mold with radiant slab cooling unless the area has dehumidification to drive the dewpoint down in advance.

The bummer is radiant cooling is a big energy saver as the losses are much lower for moving the cooling with liquid compared to using big ducts to move air but that dewpoint issue is still there. Commercial installs that need cooling with 24/7 conditioned spaces with low inlet air volumes and lots of internal heat do use setups called chill beams and I think they are a radiant cooling approach.

Note there are two type of air to water units. There are monoblock units that keep all the refrigerant outdoors and heat or cool glycol for heat transfer and there are also split types that use refrigerant to indoor evaporator coils (its an evaporator when its cooling and a condenser when its heating). Usually they are variable refrigerant flow units (VRF) but down in the size households would use the definitions get blurry. This requires more skill to install but no need to mess with glycol and the pipes are smaller. The larger VRF units get interesting as the large units can move heat or cooling from one space to another. So on sunny day, it may be cooling the front of the house and using the heated refrigerant to heat a space at the back of the house. All it takes is big checkbook.
 

MikeT2

Member
Jan 23, 2019
21
Vermont
The condensation/dewpoint issue is something I tried to get info on. Really not much out there other than the generic info that say you may need dehumidifiers, but I think Peakbagger is right about mold being an issue in this region. If I would have to run dehumidifiers in every room that takes a big bite out of energy savings too. Some of the ATW makers sell the chill beams but its not an approach for a residence.
Good info.

A mini split may be the way to go over the window air units.
 

gthomas785

Minister of Fire
Feb 8, 2020
581
Central MA
Dewpoints in new England can get up around 70F in the summer. If you cool the air you will most likely get condensation, and if you don't get condensation the humidity will be close to 100%. Not comfortable. Basically with any a/c system in this area you need a way to drain off the condensate, and that's non-negotiable.

When I hear people talk about radiant cooling with floors or ceiling panels my mind immediately jumps to an image of a cold dark cave with water dripping down the walls. Just.... No.
 
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EbS-P

Minister of Fire
Jan 19, 2019
1,747
SE North Carolina
I was doing some reading on passive houses. And the latent heating load is significant. (I am going make this up and say 20-30% of the total cooling load). This is just the energy required to dehumidify. Lowering the dew point by 5 degrees or about 10% rel humidity, increases the cooling capacity of my heatpump by 10% increasing the efficiency. Small room dehumidifiers take out the water but release the latent heat back into the room. So I have determined heatpump AC is the most energy efficient way to dehumidify but it may not have sufficient capacity. My air handler is variable speed and has different settings to help with this running the fan on low for 5 minutes in addition you can change the CFM/ton (350 is the standard but for the hot humid south 300 is probably better). Mini splits are really good at this and better than whole house units as the blower and compressor are both variable speed. ATW for cooling really needs a unit similar to a mini split.

Evan
 

peakbagger

Minister of Fire
Jul 11, 2008
6,837
Northern NH
My guess at this point is just like minisplits is that the manufacturers are selling to established HVAC installation businesses that will only sell systems. The Myson seminar and a prior seminar from a US manufacturer was oriented to a systems approach where the HVAC contractor sells a complete system. Might be great for new installation but not so easy for an existing home. Air source units really need a buffer tank and low temp radiant distribution system to take advantage of.
 

begreen

Mooderator
Staff member
Nov 18, 2005
91,437
South Puget Sound, WA
What ATW heat pumps are now available and how long have they been on the market?
 
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Brian26

Minister of Fire
Sep 20, 2013
628
Branford, CT
What ATW heat pumps are now available and how long have they been on the market?
Take a look at the qualified product list in the link below. Looks like the AWT are Nordic units.

Vermont has a really great incentive on them at $1k a ton. I wonder roughly what one costs installed after the rebate there ? Even the smallest Nordic unit gets a $3k incentive plus a $500 income bonus.

 

Highbeam

Minister of Fire
Dec 28, 2006
19,549
Mt. Rainier Foothills, WA
I see 7 brands of ATW (air to water) heat pumps on the approved list. COP at 110 degree water for these are around 2. Better than straight electric I guess but way lower than ATA heat pumps.
 

NorMi

Member
Nov 18, 2021
110
Northern LP, MI
My Bosch Max A2A drops to a 2:1 COP somewhere around 0F outdoor temps and heating performance bottoms out at -22F. The Max line delivers nameplate heating BTU down to -5F as well before output drops which is pretty amazing. It seems like they use the next size up ODU to achieve this, for instance my 27k ODU matches the weight/size specs of the normal 36k BTU model. Perhaps the main benefits for A2W might be the inclusion of domestic hot water and integration into existing boiler systems? Performance wise, especially price/performance it is very hard if not impossible to beat a A2A mini split! I was skeptical with using it for all-winter after I installed mine, but even up here in zone 6 it did good year round. One thing I think A2W would solve/improve, especially with storage, would be the unfortunate defrost chill in the air you get at low temperatures. Every two hours +/- on A2A in the winter you get a sharp little reminder of the outdoor temps!
 
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