Hydronic radiant floor heat

[crater]

Posted: Sun Feb 24, 2008 4:36 pm Post subject: Radiant hydronic floor heat

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I am building a new 1900sf home with a attached 400sf garage, I installed my barrier pex tubing on my own without much of a design, I placed pex 16oc with a max length of 250', with double bubble insulation blankets instead of 2" ridged styrofoam. I have set up 3 zones using 3 circulating pumps, Taco 3 zone module, a Thermolec 15kw(50K btu) electric boiler,expansion tank,RPZ valve,Spirovent, and other run of the mill fittings, The boiler came with a genral illustration of how it was to be plumbed, as I did. However I can't seem to get the supply water temp over 80 degrees. I have tested the boilers current draw and other thing on it, it checks out ok. I have been told that the insulation does'nt really supply ampel insulation and that I probaly have my pex too far apart, they should have been 12oc. Knowing all of this what would the least expensive solution to make the floor system work? Any ideas would be great, my next step is call in a professional, which I think will only tell me what I already know, I screwed up.

[seaneys]

The size and distance of the coils ultimately limits the amount of heat you can receive from your slab. Where do you live? Have you done a heat load calculation to see if you are in trouble? If you are in trouble, you might want to consider augmenting with baseboards, PEX in the walls, or PEX in the Ceiling.

I'm not sure from your post what is going on with your water temp. What is your boiler temp?

Steve

[Bill Arden]

However I can't seem to get the supply water temp over 80 degrees.

This would indicate the opposite in that the water is not getting hot enough.

The "15kw(50K btu) electric boiler" seems about right, but you don't specify R-Value or "Design temp"

The real question is "Is the room staying warm?"

I already know, I screwed up.

Actually it looks good.

The difference between 12oc and 16oc is minimal.

[alternety]

First of all - the bubble stuff is quite close to zero insulation value.

How is the tubing attached to the floor. Is it in poured concrete or what. The thermal mass involved can keep the water temp low for quite a while (days) at start up. With slab-on-grade and bubble insulation the earth in the immediate vicinity is part of the mass you have to heat. That can have a pretty significant impact. How long was the system running while you did your test?

Get the Slantfin free heat calc software and see what the house needs. That size house, if it is tight and well insulated, probably does not even need 50 KBtu.

Is it a closed system (i.e., not tied into your DHW).

Is your building closed in and insulated? How do the rooms feel while you were testing. If it is a tight house you may not need 120 degree water. Figure that out after you are sure it is working.

What is the temp of the water going into the water tank. A Btu raises 1 lb of water 1 degree F. Your boiler can produce about 50 KBtu/hr.

How did you decide what the circulation rates/pump sizing requirements were? You have three zones. How are your 250' loops interconnected? Is there just one loop per zone? If not, what manifolds and loop balancing controls are you using?

[crater]

All loops are in a 4" concrete slab (garage-slab on grade, and basement)
I had the system running for 9 days @ full capacity and water temp was only 80degrees, room temp was 45degrees.
After 2 days I shut down garage loops, and 4 of the basement loops, this leaves 2 basement loops running. The water temp went up to 140degrees quite quickly. I ran it this way for 1 day then opened up the other basement loops (4) and the water temp dropped rapidly. I ran the sytem in this config for 6 days more days and couldn't get the water temp up past 80dgrees.
I have a delta-T of 10 degrees ie: 80d out 70d back in.
When I had just 2 loops running, the floor temp(where the active loops were) was somewhere around 84 degrees, when I turned on the other 4 I couldn't get the floor temp past 54degrees anywhere.
Maybe I just need to finish off the main floor so I can retain the heat that it is producing (along with some help from the forced air system) and maybe it will work.
My biggest ? was is there a major loss from 12"oc spacing to 16"oc spacing, as far as the insul goes, I didn't think it would replace 2" extruded styrofoam either until I tested it.

I placed a heat lamp in front of 3"s of extruded styrofoam and 1 layer of the bubble/bubble insul. I found that there was a 3 degree difference with the styrofoam winning, but I contributed that to the 3"s of styrofoam was made up of (2) pieces of 1.5" styrofoam so there probally was a air gap between the pieces thus giving a higher R-value than there really was with a single piece of 3" styrofoam. Plus this was 1" more styrofoam than the bubble stuff was supposed to replace so I was convinced that the bubble/bubble stuff really did replace at least 2"s of extruded styrofoam.
The pump size was a guess. I went into the local Plumb Supply and told him I needed to plumb the boiler in the config. the boiler illustration showed and he gathered up all the parts for me. ( I don't think I am having a problem with flow rate, as the most pex on one pump is 1000' the other 2 pumps only have 500' on them) plus I can shut off half of the 1000' zone and still not get up to temp.
IE: 2 - 250' loops = 140degree supply water 130d return
4 - 250' loops = 80d supply water 70d return
6 - 250' loops = 70d-80d supply water wnad 60d-70d return
8- 250' loops = forget it as the last 2 are garage loops (slab on grade)maybe 60degree supply
Boiler is set to supply 180degree water(high limit)

[jadnashua]

If the room temp is only up to 54-degrees, are you sure you're getting good flow? If you were getting good flow, I'd expect the return to drop closer the the room temperature. where are you measuring the input temp? If it is only 80-degrees going in, that doesn't seem like enough.

Is it possible that the pumps are fighting each other? How are the manifolds connected. With one on could one of the other pumps be trying to push the other way, essentially cancelling the flow?

The pumps can't push through a large air pocket very well...are you sure all of the air is purged out of the loops?

On a cold day, I'll get a 50-degree delta on the water going in verses out of my radiant floors. With 80 going in, you'll not transfer a whole lot of heat.

[crater]

The supply temp comes from the boiler temp/pressure gauge. I installed other temp. gauges but I'm quite sure they are lieing. like the supply says water is 60d and the return is 70d meaning the return is warmer than the supply which can't be happening, I think there is air at the gauges not giving me a correct water temp. although I did try to bleed out air at the gauge to prevent that from happening.
My point exactly, the supply is not warm enough to warm maintain temp. let along warm the mass of concrete. I can see that a 10degree delta-T would warm the concrete, but talk about inefficient, at this rate I would have to run the system all year round just to keep the concrete warm. Plus I think the disc in the electric meter is getting a high-speed wobble from turning so fast Seeing my $$$ go bye bye is not what I really had in mind.
I can't really see how I have to flow wrong either, each pump is hooked up to its own supply manifold and all returns are hooked up to one central manifold.

[crater]

I read on net I could calculate the size in BTU's needed like this is it correct or even close?
tight house = .375 in upper states*amount of sf to heat*target temp

.375*1940sf*70degrees(target temp) = 58200btus
thus making my 51000 btu boiler undersized especially because of the 16"oc tubing and lack of good insul. and adding on another 400sf for the garage.

[Bill Arden]

Is the garage slab exposed at the walls or at the door?

The heat has to go someplace.

It does not matter what the pumps do in that if the heat is not pulled from the boiler it(the boiler) will turn off in order to prevent the boiler temp from getting too high.

--
I also have to say your R-testing method was not correct. You need to use two heat conductive plates and even pour materials to conform to the bubble wrap and compress it as it would be in the ground.

http://www.espenergy.com/foam_board_insulation.htm
2 inch foam = ~R6 to R8

I am guessing two layers of bubble wrap would be ~R1.5 to R2

Next ground water/vapor can pull more heat away.
--
For comparison.

My garage slab has 2 inches of foam under it and 2 inches all around the outside.

I do have a exposed section where the door is, but that zone is not heated.

--
My house on the other hand has a 4 inch band of exposed concrete and the concrete inside the house got so cold that a water pipe that was laying on the floor froze!

[Bill Arden]

.375*1940sf*70degrees(target temp) = 58200btus
thus making my 51000 btu boiler undersized especially because of the 16"oc tubing and lack of good insul. and adding on another 400sf for the garage.

For comparison. (since I am in a cold area too)

Let me check it another way. *pulls out February electric bill*

I used 5,409kWh for electric heat (1,005kWh for general service)

So that averages 7.27kW

Knowing that you need more for when it's really cold, a 50KBtu boiler would barely cover my needs until I get some more insulation.

--
800Sq house on slab (R10 in attic, 4 inch walls, leaky windows, Exposed slab)
900sq garage (R48 attic, 6 inch walls, Insulated slab, no windows )
1700sq total

FYI: Dual Fuel/off Peak = $0.057 $/Kw

[crater]

the garage loops are totally shut off ,I am not trying to heat the garage until I can get the basement to warm up.
I had a thought today though, I thought I may have the loops hooked up backward to the design. In other words I would be sending the hotest water to the center first then out to the edges, thus as the water cools it would be going to the colder concrete then back to the boiler. And this was the case, so after a real pain I have reversed all the loops supply and return lines,now at least the fluid should be traveling in the correct direction according to the design anyhow. I'll have to wait and see if it starts to warm up, I really can't see a hugh diff though.
Now I am curious if the pumps flow enough? as there are 2 pumps with 2 loops on each pump, If I run either pump alone the temp will come up, and one of them is the garage loops(the coldest floor) only when I turn on the pump with 4 loops does the temp fall dramatically. when I turn on the 2 pumps with 2 loops each the temp does fall but not as much and does stablelize and maybe even start to rise(very slowly)

[alternety]

Crater, Bill's estimate for R value of the bubbles is about right. I don't think you will see much difference in water temp overall by changing the circulation direction. Heat transfer will be a little better at the input to the loop but less than it was by the end. I am guessing that they will pretty much balance out.

It sounds as if you are running this thing without the main floor closed up and insulated. But apparently upstairs will be heated with a separate forced air system. Or is it a boiler driven fan coil? If there are two systems, and your 1900 sq ft is the sum of basement and main floor, your boiler is twice as big as needed. There are quite a few things we really don't know.

What sort of temperatures have you had for the last month. Your concrete was at this temp when you started if the house is not fully closed up.

Your results in varying the number of loops running seem consistent with bringing a large volume of cold concrete and the earth beneath up to a usable temperature. You simply get higher boiler temps because the water is transferring fewer BTU/unit of time. You are currently taking heat out of the boiler at a much higher rate than it can recover. That is why the temps are as they are.

Your calculated heat load really does not have a lot to do with what is currently going on in the slab. A note on floor temps. When a system is well designed, if you walk on the floor in bare feet it should feel "not cold" to a little warm.

You may actually be circulating the water too fast. I don't remember off hand what a good temp drop in the water after passing through the tubing should be.

What happened to the temp of the 84 degree section when you went back to more zones running. If you bring one section up to that kind of temp and then turn that loop off, the temp drop can give you some feel for how much heat is being lost to the ground and the interior space. If you do just one zone at a time and they are the same size, they should get to about the same temp in about the same time. If they don't, you may have a kinked tube in there or the earth underneath is causing a much higher loss. If there is any ground water moving down there, it could make a huge difference. Also just static water in one place and not another.

If you had some idea how much water you were moving in the loops that would help you understand how many BTUs are moving.

[enosez]

Are you using a tempering valve setup?

If so, I found out that the pumps need to go after the valve as they are very restrictive and create a lot of head pressure before the valve.

Also, are you using a by-pass? With the tempering valve, and having the pumps before the valve, I've seen a lot of the flow "bypass" the manifold completely. (Assumming you have a bypass)

I've even seen the temp gauge on the manifold read 120 supply and 90 return, yet the actual flow and temp of each zone was no where near that. That was due to the temp valve and bypass being pipe right into the manifold.

Maybe a picture of your setup would help the proffessional see your set up and gtuide you.

Good luck and keep us posted.

[crater]

ok I'll take some pics, no tempering valve, yes to bypass, but only I installed bypass after I seen the system was not working properly.
A square foot will always be a square foot, weather its in the basement or on main floor. For my calculation the basement is 1940sf the main floor is also 1940sf. I'm not trying to heat the main floor though just the 1940sf of the basement. And yes there is no insul in ceiling of main floor, so I know I'm not goinig to be able to retain the heat the floor produces, but I would think that it would still heat up.
Location: Iowa; temps have been in the high teens and lower, this week we are breaking 20 into the 30s even then back down to the 20s later this week.
I did started the floor system 2 weeks ago when we were in the single digits.

[crater]

here are some pics of the setup