Hi! First time poster. Have really enjoyed the wealth of info here on the forums. Been reading tons, here and all over the web, as I've been researching heating sources for over a year for my new home build.

So I'm sold on radiant floor heating... and have been exploring all the different ways to heat the water for radiant.

The fuel comparison charts I've seen show heat pumps as 200+% efficient, and geothermal heat pumps as 300+% efficient. This took me a while to wrap my mind around how something could be more than 100% efficient like electric heat. Then I finally realized the potential for moving (pumping) heat from one source to another verses creating heat. <lightbulb=on>

So since heat pumps (air source-"regular" and water/dx source-"geo") are the most efficient way to "generate" BTUs. So why are they not used to heat radiant floor heating water?

From my searching, it looks like some Geothermal heat pumps are capable, so why no air-source heat pumps?

Generally, "Air Source" heat pumps do not work well below, say, 45* or so. They can be fitted with defrost controls, but the energy used to do that defeats the energy saved running the heat pump. They are great for the southern climates, but northern climates not so much. They can have their place in a northern climate, but with limitations.

They are used for heating.
Here is a company that has been building them for many years. Their claim to fame is air source heat pumps working up near the US-Canadian border (heating mode)

They make a "reverse cycle chiller" that can produce 115 degree water @ 0*f outdoor temperature.

http://www.aquaproducts.us/home/index.php?option=com_co...cle&id=27&Itemid=189

To make the system have a good payback utilization, air cooling for summer could be implemented with a simple fan-coil unit.

Bill, have you actually used any?

this is technology that keeps "bobbing and weaving" into my peripheral vision but I have yet to speak to anyone actually using such a system with radiant.

I'm getting set to install my first system with an air-to-water heat pump. Aqua-Source is the vendor however there are at least three or four other manufacturers making RCC for residential use.

Air sourced RCC are more common in europe and I've investigated a couple of manufacturers in Germany, Sweden, Denmark and the UK. These product appear generally very well built, but they're expensive. However they are very well documented with regards to their COP, particularly performance as a function of temperature.

I have to take issue with the comment by the Guest regarding performance below 45F. Yes, performance does fall off, but generally the crossover point at which the HP becomes less efficient that a high efficiency water heater (assuming $0.10/kw electricity and $1.50/therm gas) is more like in the mid to low 30's). He is correct that defrost is an issue but all of the newer HP's come with computerized defrost to minimize the cycles. Still to be conservative I'd say that if you assume the heat pump is good to the high 30's you are pretty close.

Not that that is a problem. In my system the HP feeds a storage tank that just happens to be a gas hot water heater. There is an outside thermostat that drives "kills" the HP when the temperature falls below the crossover point and enables the pizo starter on the gas heater. This not only allow me to heat with the most cost effective fuel, it gives me backup in case of a power outage and allow me to up the water temperature for those days when its cold out (the HP is set to supply very low water temp around 105F because it operates more efficiently and the gas water heater being less sensitive to this is set higher).

Sort of a nice set up. Other advantages are the same HP supply cold water for air conditioning (via air handlers), preheat the domestic hot water and supply hot water to heat the pool.

I'm expecting a whole system COP of around 3. The heat pump itself is rated at 3.4 at 30F, 3.8 at 40F and 4.1 at 47F so I think that is pretty conservative. During the summer when I'm heating the pool I could easily see a COP of more than 5, which is pretty sweet since the only additional thing I need to buy to run the pool is a heat exchanger.

Stay tuned.

Interesting feedback. I'd love to talk to people using these "cold weather" heat pumps. they really sound too good to be true.

you've got a slick "low tech" setup there sbe.. elegant, I like it... As long as the water heater is an acceptable choice for your heat load/degree day count below 30 Low load situation?

quote:

I have to take issue with the comment by the Guest regarding performance below 45F. Yes, performance does fall off, but generally the crossover point at which the HP becomes less efficient that a high efficiency water heater (assuming $0.10/kw electricity and $1.50/therm gas) is more like in the mid to low 30's). He is correct that defrost is an issue but all of the newer HP's come with computerized defrost to minimize the cycles. Still to be conservative I'd say that if you assume the heat pump is good to the high 30's you are pretty close.

Fair enough, but... January of 08 never once got above 32* where I live here in the good ol midwest. January of 09 only got above 32* a couple of times. Thats not helping me out much.

Like I said, depending on where you live, it's a beautiful thing.

82 cents per therm here in Mpls BTW. Electric lovers exaggerating for affect I think.

If only I were exaggerating. I'd love to pay $0.82/therm as the decision would be easy.

A couple of updates. I spoke the president of Aqua Products yesterday. He claims that the heat pump efficiency curve is better than I'm supposing and I won't hit crossover until 24 F. We'll see. Not that it matters since we spend so little of the year below 35-40F anyway.

There are more advanced HP that do better at low temperatures but they are spendy and the whole point of this is to use off the shelf components to keep the cost of the equipment down.

To which end here are some more details this installation. I've found a source for the 5 ton Aqua Source Chillers for $898 on closeout (these are last years model). York (and others) is selling their top of the line HP for around $1000. So for under two grand I can put together 5 tons of heating for the floor, cooling, pool heating and domestic hot water preheat. That's got may attention.

Assuming it all comes together I'm going to give it a try. Worst case I lose $1800 on the two RCC because I'm configuring the Firstco air handlers with both gas and hydronic coils (very small upcharge). If the whole thing is a total bust, which I don't expect, I can pull out the RCC and trash them, hook the freon lines up the air handlers with no big loss. If it work as well as I hope then I'm going to make this part of my program.

I'll keep the board posted. In the meantime if anyone else is interested they have about 18 of the 5 ton units and a bunch of the smaller stuff. I'd be happy to put you in touch with the distributor if you want to give it a whirl.

Its certainly cheap enough.

Isn't the problem with heat pumps that they just can't put ou high enough temperatures? I've been doing heat-load, flux requirement, floor temp calculations and it looks like I'm going to need 137 degree water temp under the living room to provide 82 degree floor temp. I don't think that kind of temperature is available with a heat pump is it? Not even with 50 degree ground-source heat.

getting water temps down is paramount with any heat pump of any kind I am aware of.

supplemental heat may be necessary in your case.

Yes. My floor temps are 105. The efficiency falls off slightly between that and 115. Above that it falls of more steeply.

Why such high water temps? Very high heat load? Staple up? Both?

The structure I'm heating is extremely energy efficient (well insulated and very low infiltration) plus I'm in a mild climate (Portland, OR) and the installation is plates between the subfloor and flooring. It doesn't get better than that. Most of the time we're under 10 btu/sf and often well under that.

It sounds like you're running closer to 30.

That was supposed to be water temp of 105, not floor. Sorry

I have used the Aquachiller product on jobs, but not the reverse cycle chiller. I want to put one in my house but I don't have the OK from the wife yet to spend $4k on more equipment for the house.

I can make it work for my house because I have air handlers for chilled water cooling. Radiant floors for heating.

Also, since I have a wood gasification boiler its hard to justify the expense for heating, and I have a 3 ton (undersized) chiller alerady for the house cooling. I usually only run the chiller between 10 & 20 days per summer.

Bill,

Can you tell me which of the chillers you used and how it worked out?

As I mentioned they have the SCHP RCC's for about $900 which seems pretty reasonable given it will supply both heat for the floor and cold water for the air handlers.

Did you use the remote chiller or the all in one where it mounted under the York and sits outside?