hydronic heat temp comparison

[thirdmouse]

In all I've read here on these, I haven't seen anything which answers this question. Assuming a well insulated shop, how closely does the air temperature in the room follow the temperature of the floor? Has anyone taken any readings of this? Is the floor much warmer than the air in the room, or not at all?

On a separate note, what is the typical water temperature as it comes out of the boiler or water heater or whatever and before it enters the floor loops?

 

[RPH]

According the texts I have on this the air tempeture is about 5 degrees F cooler than the concrete.
Siegenthaler's - Modern Hydronic Heating.

 

[Burl]

My 30 X 32 garage has R13 in the walls, R13 in the ceiling (with 1" styrofoam) and R17 doors. The air temp is only about 3 deg. less than the floor, at the 12' ceiling its about 2 deg. warmer.

 

[larry_g]

In my old poorly insulated pole building the ambient would run about 10 degrees cooler. Water at about 100 degrees. If working physically I find the cooler ambient welcome.
lg
no neat sig line

 

[ScaldedDog]

I run the boiler at 124*. I'll have to measure the floor temp vs. air temp.

Mark

 

[jklingel]

Third: I think your answer is somewhat dependent on your heat loss, but the approx value of 5 degrees is what I have read, too. I was told a 3.5* difference for my house. What I have not yet correlated is the water temp and the slab surface temp, assuming PEX is on the XPS and a 4" SOG. Maybe in Sieg's book?

 

[tdkkart]

Check the charts on page 204 of this document:
http://zurn.com/images/pdf/ZPM02101.pdf

And then read the rest of it and you will learn way more than you'd thought possible about radiant floor heating

 

[trythis]

My house is an old garage with a new concrete floor, the air is conditioned (heat and AC) separately from the floor. (2500 sq feet 14 foot ceilings, R38 ceilings, nothing on 2 walls but 3 courses of brick, R19 in 2 walls)

All day, every day, the air is 10 lower than the floor without the air heat kicking in, UNLESS, it gets below 15 outside for more than 4 hours, OR if it stays above 40 for more than 24 hours, in that case it gets hotter (air) than the usual 10 degree difference.
It took a while to find this balance. If I set the air too high, the air system would stay on too much, taking the heat out of the water that also heats the floor.

If I set the floor too low, the air heat was just blowing all day,

They are different zones of the same water to water ground source heat pump.
I leave my floor at 83, and the air stays 72, but often it is 73 anyway.

 

[jklingel]

Check the charts on page 204 of this document:
http://zurn.com/images/pdf/ZPM02101.pdf

And then read the rest of it and you will learn way more than you'd thought possible about radiant floor heating

•• Nice charts. Looks like they confirm my suspicion. I surely read that article soon. I hope they have under slab temps, too; that is something I am still looking for. thnx. j

 

[tdkkart]

Here's another insteresting and informative document:
http://www.ttherm.com/PDFs/SEISCOSpaceHeating.pdf
This document is geared to electric powered heating, but still contains alot of good general info.

The majority of manufacturers seem to recommend a supply temperature of 100-120*, with a designed temperature drop of 10-20* through the floor loops.

I think there are a couple considerations in supply water temps.

Technically if you designed the system for a 70* air temp with a floor surface temp of 75* you could simply supply the floor with a continuous flow of 75* water and it would maintain forever as long as your heat loss remained constant.
Since I have my shop set at 50*, and it seems to hold there with a 53-55* floor temp depending on the outside temps, I could theoretically pump ground water through the system and maintain my desired floor temp. Of course this would require a well, or a large heat-pump style ground loop system.

Supplying input water that is too hot I believe would cause problems with over running your desired floor temp.

 

[thirdmouse]

Technically if you designed the system for a 70* air temp with a floor surface temp of 75* you could simply supply the floor with a continuous flow of 75* water and it would maintain forever as long as your heat loss remained constant.

Since I have my shop set at 50*, and it seems to hold there with a 53-55* floor temp depending on the outside temps, I could theoretically pump ground water through the system and maintain my desired floor temp. Of course this would require a well, or a large heat-pump style ground loop system.

This is just exactly what I was leading up to in a through-the-back-door kind of a way. Anytime I'd ask this straight out of a heating/AC guy, I'd get told it wouldn't work, but I remained unconvinced by the complete lack of thought put into the answer. Given a well insulated shop, if the air temperature will hold within 5º or even 10º of the floor temperature, it seems to me like one could run ground temperature water through the floor non-stop and hold the shop in the neighborhood of 45º - 50º or so. Past installation costs, this sounds like a dang cheap way to heat a shop to a comfortable enough working temperature. No heat source needed, just the cost of running a pump all the time. The only concern would be to make sure there is enough feet of tubing in the ground vs. the feet of tubing in the floor in the various loops that the ground temperature remained dominant. I don't know what ratio that would have to be. But I can't figure out why it wouldn't work. Even 45º would be much better than the 20º I am working in now in my too small garage. And if one wanted it warmer than that for a short time, fire up another heater of one sort or another for a little bit. It wouldn't take much. Tell me why this won't work.

Even if it wouldn't work the way it sounds like it would, with a little care ahead of time to make sure there was enough tubing to operate correctly, throw a geothermal heat pump in the system and then have all the heat you want. (I was told by one heating/AC guy that these put out about 80º water.) At the very least you could still use the floor tubing with a boiler of some sort and the attempt wouldn't be a total waste. It won't be anytime soon, but I am filing ideas in the back of my mind for when I can build the pole barn I want. But when the time comes, tell me why I shouldn't try this experiment.

 

[thirdmouse]

No thoughts at all? Did I kill it off?

 

[mebedave]

thirdmouse
I have been kicking around the same idea. I’m very close to pouring a slab in my 40x36 shop (pole barn) and I have to think about heat very soon. I have a geo ground system with heat pump in my house and LOVE IT. I think the idea is worth at least installing the piping onto the slab, so the idea could be tested later

The other idea I have is an outdoor wood burning stove that could somehow heat water then pump though the slab. I'm cheap so this is something I would need design tips on so I could build a home made one.Any ideas out there?

I have 45 acres of hardwood timber so the firewood is free. The only pain here is I have to keep the fire burning

 

[sneezer41]

I think the flaw in your logic is that in order to get a x degree surface temp you need much warmer water. As the 'load' on the floor drops, it will drop the surface temp of the floor if you do not have a warm enough water source. Put another way, 50 degree water will not produce a 50 degree floor temp, and as the room temp drops when outside temp drops, your floor temp will drop. No delta T no heating.

I think in actual cold weather you will find 50 degree ground water in short supply.

 

[jvitez]

Our home's has geothermal well to well/water to water heating, with 4 zones, each using a fan coil with high velocity ducting. Each zone has a 40 gal water tank with an aquastat. Each tank is set to 115F high limit and 105F low. A geothermal system can definitely go higher than 80F.

Our well water is 53F in the summer and 43F in winter. No benefit to running straight water using these temps. Maybe in Florida.....

 

[trythis]

I have a similar system to jvitez, the system is a closed loop. Geothermal water to water 115 to 105 system as well. I was running it at 125 but the heat pump would shut down. However, I keep my floor at 83. It will maintain 83 if the water in the tank is 83 but the pump runs constantly, this is only when 10 or below outside.

 

[thirdmouse]

I think the flaw in your logic is that in order to get a x degree surface temp you need much warmer water.

Therein lies the next question. Given the water circulating 100% of the time, how much colder would the floor be than the water temperature? I had thought that if there were enough tubing in the ground vs. how much tubing is in the floor, the ground water temperature would remain the dominant temperature. That may require more tubing in the ground than a standard heat pump setup normally does. I don't know.

One heating/AC guy I asked about this said it wouldn't work because the water needs to be warmer than that. But he's also thinking of household systems where you want the interior temperature to be 70º - 75º. Then he told me that the water out of the heat pump needs to be only 80º to get that and it will still cycle on and off, not running 100% of the time. He said that's what his systems do. That doesn't seem like very much temperature difference to me. So how closely would the floor temperature follow the water temperature if the water did run 100% of the time?

Our well water is 53F in the summer and 43F in winter. No benefit to running straight water using these temps. Maybe in Florida.....

Not a bad idea. Sometime here I'll measure our well water temperature straight out of the ground and see what it is. Heck, even if it kept the floor only 40º, that's better than the 15º that it is right now.

 

[nate379]

Don't think it'd work around here. My well water is around 40-42* year round.

Funny thing though, I was gone for 3 weeks and I turned the heat down to 45* For the most part the house stayed closed to 50* without the boiler running.

 

[Flatland Dave]

Thirdmouse, I too am contemplating the same idea, usnig well water. We have artesian up about a foot out of the ground about 60*. I need to investigate this as well. I'd love to have a shot at a 50* shop when it is -20 out, with minimal cost. The only worry I have is power outages. In 15 years we have had 2 major ones at 7 days each with cold weather. What happens to the slap if the pex freezes?

 

[lodemia]

Bumping this thread. Thirdmouse/Flatland Dave, have either of you implemented your experiment and can you comment on how well it works/didn't work? I have the same idea, my groundwater is 53 degrees, and if I can just recirculate 53 degree water/antifreeze, 100% of the time, it seems as though that should keep the shop a constant 45-50 degrees.

 

[tdkkart]

Bumping this thread. Thirdmouse/Flatland Dave, have either of you implemented your experiment and can you comment on how well it works/didn't work? I have the same idea, my groundwater is 53 degrees, and if I can just recirculate 53 degree water/antifreeze, 100% of the time, it seems as though that should keep the shop a constant 45-50 degrees.

Re-reading this from last year, I suddenly came to a realization of why it might not work so well.

The problem is, a floor running at a temp of say 50* has more than one direction of heat loss. First, and the one we think about the most is upward heat loss, this is the one that heats our building, and in my case a floor temp of 50* will result in an air temp of about 45-47*.

However, unless obsolutely isolated from the surrounding world, the floor is also losing heat downwards and outwards into the ground and/or the outside air depending on how your slab is situated. How many BTUs is this downward and outward heat loss?? Could well/ground water supply enough BTU's to overcome the losses?? I'm not sure??

 

[lodemia]

Keeping with that thought, it's common to put 2-3" of high density foam insulation under and around the slab, which is R-6.5, resulting in an R13 or R19.5 barrier from the downward and outward heat loss.

 

[keweenawbee]

I think I am missing something. Unless you have some sort of buried loops what are you doing with the expended well water that has lost it's heat in the slab? I suppose if you re on a city sewer you can just dump it down the drain. I like the concept but I would overload my septic system.

 

[lodemia]

I have 5 acres and a backhoe, so I thought I could just bury a bunch of loops. Again, I'm not looking for it to be 70 degrees, I'm just looking to keep it a little above freezing (45 degrees or so).

 

[lodemia]

The other thing that people do is "pump and dump" systems where there are two wells, placed a ways apart. One well to pump from, the other to pump back into. Or if you're rural, you could always run a field tile or tie into one that already exists.

 

[keweenawbee]

This area rarely needs air-conditioning but there are a few days each year there where I just have to suffer through the heat and high humidity. I wonder how long it would take and how much water I would have to dump off to cool down my garage. I have about 1250 ft of 1/2" pex in my 1000 sq. ft slab. I think the well water is around 40 degrees in the summer. I know water is getting precious, but I am a few feet from Lake Superior so I am hardly wasting water by letting it sink back to the water table. I actually had to raise the whole area for my structure and septic system so I have a perfect run-off to just dump water. And what about the humidity, would this all be a waste of time? They say this application works best for cooling when you run the radiant pipes in the ceiling. Makes sense as heat rises.