Radiant Floor Circulating Pump

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Old 01-21-13, 05:40 PM
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Radiant Floor Circulating Pump

I recently installed a radiant floor underslab PEX tubing system, which I heat using a Navien boiler. My circulating pump provides less flow than I thought it would based on my head loss calculation. I am only getting about 1 gpm through four 300' 1/2" tubing zones.

I have 4 - 300' zones under my slab, which is about 1000 SF, so the spacing is about 10" or so. The supply water temperature is 120*F, and the return temperature is 90*F, so by my calculation, at 1 gpm, 30* dT, I'm only putting about 15,000 BTU/hr into the house. Does anybody know if increasing the flow rate through my tubes will allow me to get more heat out of my boiler? Right now I'm at 10-15 BTU/SF, which seems low, but I'm not sure if the pump is the problem or if my slab is just maxed out on the heat it can provide. Yes, I have tried bleeding the system of air time and time again, so I'm confident that isn't the problem. Anybody that has experience with the effect of pump size on a radiant system?
 
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Old 01-21-13, 05:55 PM
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Tell us about how you are feeding the manifolds for the PEX to/from the boiler, and about the 'near boiler piping'. Primary/secondary? Mixing valve? etc etc

I don't think the calcs are correct though... because if you flow FASTER, the delta T will DECREASE.

Can you share the calculations that you used?

How deep in the slab is the tubing? Did you insulate UNDER and AROUND the slab?

What pump are you currently running?
 
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Old 01-21-13, 06:35 PM
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I put down R-10 rigid foam insulation, four zones of 1/2" tubing, 300' long each. The concrete is 3" thick over the tubing. The tube spacing is about 11", and I ran the tubing around the exterior perimeter of each zone before looping back through the interior of the zones. The edges are insulated with foam as well.

The tube zones are all manifolded together into a single supply manifold and single return manifold. I have a Navien CH-240 combi boiler, which has its own boiler circulating pump inside it, so you can set the supply and return temperature of the boiler to whatever you want and the pump will modulate itself. The manifold is supplied with water from the main boiler loop through a set of "close spaced tees" and I have a single Taco 007 pump feeding the manifold.

The supply temperature AT THE MANIFOLD is 120*F and the return temperature AT THE MANIFOLD is 90*F. The four flow meters on the manifold, if they're believable, say that the flow through each zone is about 0.25 gpm for a total of 1 gpm. So here's what I get: Q = m_dot * CP * dT, or Q = 1 gpm * 8.3 lb/gal * 1 BTU/lb-R * 30 degrees = approx 15,000 BTU/hr.

Or, you can use the universal hydronic formula, Q = 500 x gpm x dT , which gives the same answer
Q = 15,000 BTU/hr.

So, let's say I get a bigger pump and double the flow rate. Is my dT going to cut in half? Because if I double the flow rate and my dT cuts in half, my heat load is identical. But if I double the flow, and dT cuts back to 20 degrees, then I've actually gained something.

Put another way, I'm getting about 15 BTU/SF out of my slab, which seems low. Why, and what can I do to increase the output of my floor?

Thanks!
 
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Old 01-21-13, 07:15 PM
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The concrete is 3" thick over the tubing
That's pretty deep... closer to surface is better... but too late to do anything about that now!

The manifold is supplied with water from the main boiler loop through a set of "close spaced tees
What size, and how long of tubing run to/from the manifolds?

The four flow meters on the manifold
OK, I see what you did... Your first posting almost made it sound as though you had somehow calculated the FLOW based on the DT... No, the flow was 'known'... good.

So, let's say I get a bigger pump and double the flow rate. Is my dT going to cut in half? Because if I double the flow rate and my dT cuts in half, my heat load is identical. But if I double the flow, and dT cuts back to 20 degrees, then I've actually gained something.
I think you will gain SOMETHING by increasing the flow rate... here's my logic:

As the water runs the tubing, it's cooling off. By pumping more, you will achieve a smaller DT, meaning more of the tubing will be flowing hotter water which will transfer more heat. So I THINK the answer to this is probably:

Double the flow will not necessarily cut the DT in half.

I'm getting about 15 BTU/SF out of my slab, which seems low. Why, and what can I do to increase the output of my floor?
What can you do NOW that it's cast in stone? Increase the flow rate and run the hottest water you can without damaging the slab.

I don't understand why the 007 is only flowing 1 GPM though... that don't sound right. Does the manufacturer give any stated accuracy for those flow meters?
 
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Old 01-21-13, 07:20 PM
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Was this system just started up cold and this is what you are seeing after only a few hours? OR, has it been installed and running for some time?

I believe that as the slab warms, your DT will decrease... and by your calcs this would mean that there would be LESS heat output, no?

By the way, what did your calculations predict that the flow rate would be?
 
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Old 01-21-13, 07:23 PM
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Holy cow... you went with a 200K BTUH boiler to heat 1000 sq feet? How come why?
 
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Old 01-21-13, 09:33 PM
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I live in northern WI, and it's pretty standard to put down 2" of foam, staple the tubing to the foam, and pour the slab over the top, so that's nothing new.

Each run of tubing is 300' long, so I am thinking along the same lines as you...the temperature of the water in the tube probably drops almost all of the 30 degrees in the first 100' or so maybe. So by speeding up the flow, I could get hotter water deeper into the zone where it could lose more heat to the room.

I bought the 007 pump based on the pump curves for that pump, so I was stunned that I'm only getting 0.25 gpm through the flow meters. But the flow meters may be inaccurate. They're the meters that came attached to the manifold that I bought, so I have no idea how calibrated they really are.

The Navien CH-240 is a 199kbtu boiler, but it's a combi...it modulates way up to heat domestic hot water, and it's capable of modulating down to 20k btu at low fire for space heating. So, I don't need to get a lot of extra heat out of my slab, just 20,000 btu/hr would at least get me into the minimum fire range.

I hate to buy another pump only to find out that it doesn't do the trick. How confident are you that it will fix my problem?
 
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Old 01-21-13, 09:38 PM
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Also, the house is 2500 SF. The rest of the space is upstairs. I just haven't had a chance to set up my upstairs hydronic system yet. Also, it has been running for a couple weeks now. When the slab was ice cold, it'd run with about a 60 degree dT at the manifolds.
 
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Old 01-22-13, 03:27 PM
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What size, and how long of tubing run to/from [the boiler to] the manifolds?
standard to put down 2" of foam, staple the tubing to the foam, and pour the slab over the top, so that's nothing new.
No, nothing new, but in my opinion, not the best way to do it. When the tubing is buried that deeply in the slab, the response time and ultimate output of the slab go down... I feel that the closer you can get the tubing to the surface, the better, within limits of course!

I hate to buy another pump only to find out that it doesn't do the trick. How confident are you that it will fix my problem?
I would hate for you to do that too... and I'm not really that confident that it will 'fix' the problem. It will certainly improve it though.
 
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Old 01-26-13, 04:19 PM
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i surely am no expert but i believe your water is moving too slow you should be aiming for about 2 or 3 gpm of flow i have a small room with only 1 loop of 1/2 pexala pex 250 feet in length and my delivery temp from my otdoor wood stove is about 170 then cuts back to 110 with a mixing valve at the point of entry to the loop(using cool water from the floors return line) and the return line where it exits the floor is 190. i would believe that with a loss of 30 degrees in the floor there is a good part of your 4 loops that is not giving up near the btu's it could if the water was hotter through to the ends of the 4 loops? i do think i read somewhere on teh net that a 1/2 loop shold not exceed 300 feet so you are at teh max for 1/2 pex (i used pexalpex which i think has a bigger ID than regualr also)
 
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Old 01-26-13, 07:28 PM
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cuts back to 110 with a mixing valve at the point of entry to the loop(using cool water from the floors return line) and the return line where it exits the floor is 190
I just KNOW there's a typo there!

110 IN and 190 OUT ? House built on a magma dome?
 
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Old 01-26-13, 07:48 PM
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Heya Meche.. Im just north of you across the border !

120 at the manifold output might be a little hot if you are putting down any laminate or wood floors (warpage, separation etc), and prolly too hot for tile (hot toes), but pretty good for carpet. (I use a thermal gun to check my tile floor near the start of the slab lines to check actual max floor temperature).

With all this talk of delta vs speed of your pump, I missed if you said how the house 'feels' ?

We went with a air handler for the 2nd story here, with inlets only in the main floor ceiling. Before I built, I had read about ppl complaining about 'stale' air in hydronic houses, so I thought Id keep some kind of airhandling going on..
 
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Old 01-26-13, 08:05 PM
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yep typo 90 out sorry! my floor is ceramic tile on the cement floor and have not loosened any yet...been a few years
 
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Old 01-26-13, 08:34 PM
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Wow, can not believe you did all these calculations. My system was just slapped together for the most part. Not to bore you but I have 3/8" tubing in the kitchen, hallway and foyer. There is only one tube through the transfer plates underneath the tile, cement, plywood and then the transfer plate. So I am probably about 2 to 2.5 inches below the tiles and then I have the insulation. The family room has 2 transfer plates through each joist. Total square feet is about 1,300 with the radiant heat. Other rooms have radiators. I have never really measured the distance but I am definitely at over 1,200 feet if I remember correctly. I although run my water temperature at 140 degrees going in and about 110 coming back. I have never had issues with cracked tiles either. Just recently my boiler went nuts and the temperature was about 190 running through the pex tubing and nothing cracked. Probably about 30 minutes but I caught the problem. My B&G Pump is 1/12 hp. It does take some time to heat the areas if you allow the system to cool down too much. For example is the temperature outside is 30 degrees or less and you allow the system to drop to 62 at night it will literally take about 1 hour before the temperature reaches 66-68. So I keep the system @ 66 degrees all the time and manually adjust the temp. to 68-70 just for a few hours. So the deeper you are from the surface the longer it will take. I did read once the temp. should be about 20 degrees from supply to return. Maybe increasing the temperature by 10 degrees going in and 10 degrees coming back may make a difference. I also believe there is a way to check the heat of the surface so you can make sure you are not running the system too hot. Radiant heat no matter how efficient will not do its job is you have cold air coming in through cracks, etc. Also, I would recommend you close any duct work for the AC as heat escapes through the supply and returns, etc.

Good luck and keep us posted.
 
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Old 01-26-13, 08:43 PM
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if teh return water is 30 degrees cooler than the feed water then that would make for a very uneven floor temp as there would be cool spots likley in the floor. plus (not being an expert i am guessing here) cooler water gives up less btu's so by raising the speed of the water in the loops you would increase the overall temps of the loops and that i think would let the floor absorb more btus but i think getting 20 btu per square foot out of a radiant floor system would be about the limit unless you were to use larger pipe in teh loop?
 
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Old 01-26-13, 08:53 PM
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if information from the net is any good? then i have found a chart from a pex supplier that states a 1/2 pex loop should be maxed at 300 feet in length...spaced 8 inches apart in the slab and a flow at .3 gpm would yield about 20 btu per square foot per hour...with just an increase of .1 gpm the output of the btu will increase 5 btu per square foot per hour. i have no idea if this is true, i am just passing on what i have seen. the experts can approve or disprove
 
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Old 01-26-13, 08:56 PM
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You can get more than 20 BTU/SF ... not necessarily bigger tubing, but closer spacing. I usually use 25 as a 'rule of thumb'.

Always go to closer spacing around the perimeter of a slab, and under windows and in front of doors.

Some installers will put two (or more) loops in each larger room, and run them so that the supply comes in off the manifold and goes one to the far end of the room and snakes back, then the other goes to the near end and around and back. This gives better balance.

What seems to be MOST important though is to have all loops off of a single manifold the exact same length so that they will share the flow as equally as possible... if any loop is shorter, it should have a flow balancing valve so it doesn't 'hog' the flow.

We are 'drifting' meche's topic a bit though... not sure if he's coming back or not though... let's not get too far 'out there'. If someone wants to talk about radiant design, I suggest starting new topic.
 
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