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New Radiant Zone, Heats for awhile then stops

Hello, I am hoping someone can suggest or help me here.

I had a contractor replace what used to be a hot water baseboard zone of heat with Radiant in slab and under floor heat.

It is a Taco 007 pump on 3/4" copper leading to a watts radiant manifold with 2 feeds and 2 returns on it. 

I have 2 other zones in the house 1"  copper baseboard.

Boiler is 135000 btu weil mclain and I also run a 1943 EFM coal stoker once the heating season gets colder.

In any case this new zone has approx 250 ' of pex on the side that heats the slab and 200' of pex on the under floor run.

I realize that it will take awhile to get that slab up to temp but here is where my problems are starting.

when I start the thermostat to call for heat (lets say 65 deg)

The zone starts pumping.

Input temps around 120 to 140 and output temps 90 to 100

the under floor zone obviously heats up a little faster than the slab and I know I can control that with the flow controls on the return side of the watts manifold but the issue is generally speaking the zone will start heating and run for say... 30 minutes to an hour and then it seems to stop circulating even though the pump is still running and the zone if still calling for heat.

when I look at the GPM needles on both zones here is what I find.

Initially I have about 1/2 gpm into both zone.  a touch less into the under floor zone but still close to 1/2 gpm.

After this run of an hour and the zone seems to stop circulating the GPM needles are both at 0 even though the pump is running.

The pump I assume is OK since that at first the zone starts to heat as you would expect it to.  but then it seems to stop flowing.

I have vented the air and actually there was quite a bit of it initially when I started but now things seem to not be getting any air out of it.

also vented my other 2 zones are initially the contractor failed to do this.

I think the Pex is 1/2" rated at 180 deg

The contractor is aware of this issue but has yet to come back to remedy it.  So for now I'll just keep the remaining 75% of their money which I assume will inspire them but who knows.

In the mean time I would like to figure this out myself.

All my pumps are pumping away, I do not have a micro bubble air eliminator.  I do have an air scoop and manual bleeders as well as 2 auto bleeders in the system.


  • rox5488
    rox5488 Member Posts: 4
    Pump Sizing?

    I'm not an expert at this but I was trying to do some pump sizing.  I don't know if my 007 in this radiant zone would act like it is acting if it is undersized?

    Run for awhile and then lose flow?  Hopefully someone will be able to help with that.

    In any case I was doing the calculations and coming up with about 9.2 GPM at 9 Feet of head and if I allow 2 feet of head that goes up to 11 for air removal?

    In any case if that is right I should be using a taco 0010 rather than the 007  I just wish I knew if an undersized pump would act like this.  Loop length of pex is approx 270.  room is 7800 BTU which might be high as it is a new room but I used average insulation value.  250 sq feet of surface area on floor.

    Any help is appreciated.  Thanks
  • Ironman
    Ironman Member Posts: 6,868
    Several Issues and Questions

    Are there separate zones, manifolds, pumps, mixing valves and t'stats for the slab and the wood floor?

    You seem to indicate that the pump is loosing flow while running?

    Pump sizing has nothing to do with air removal. You don't run the pump while purging air. What was your system pressure? What was the pressure while you were purging? For a small radiant zone, the 007 is probably sufficient.

    125 deg supply water is much too high for a radiant floor. Having an 85 deg. return means that you have a 40 deg. Delta T. It should be 10 deg for residential radiant. This indicates air in the system or incorrect piping or pump sizing. But the 007 is normally sufficient, as I said.

    Can you post some pics or a piping and control diagram?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • rox5488
    rox5488 Member Posts: 4
    more info

    The under floor and slab is all one zone which are adjoining rooms,  there are flow controls on the return of the manifold where I can increase or decrease flow to the zones so I can put more flow into the slab and less under the wood etc. 

    Yes the pump is losing flow while pumping.  after about an hour or so the flow or GPM needles on the manifold drop to 0.   They normally were about 1/2 GPM when running.

    Pressure is at 15 PSI to 20 PSI when heating.

    I seem to have doubts it is air only because the zone heats fine at first and only after a period of time with the pump running does the flow go away.  I would think I wouldn't have any flow at all if it were an air issue but again I am not an expert so that is why I am asking!

    Would a 40 deg delta T stop a zone from flowing?  if so maybe I need to adjust the mixing valve to accomplish this.   Right now though I am just trying to get the zone to flow correctly.   I can always adjust for heating properties after it works right if this is the case...  is that correct?  Thank you for any help or suggestions with this
  • Gordan
    Gordan Member Posts: 891

    Mixing valve, eh? I'll bet that you have the mixing valve piped in wrong, and once the slab begins to get saturated and the return water temperatures go up, it shuts off flow from the boiler to the zones. Post pictures or a piping diagram. Or both.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    edited November 2010
    Let's Start

    With simple radiant fundementals. A slab should not be on the same zone with the joist heating. There is no way a slab should be in the same water temp as joist heating. Typical slab water temp at design should only be around 100 degrees. Joist somewhere between 115 - 120 at design using extruded plates.

    Without seeing the job and playing I am only making assumptions but I tend to believe the huge delta-t is being caused by all the cold water in the pex in the slab. The therm is proably getting satisfied from the joist and you are really never getting the slab up to temp so it stays off for long periods of time meaning no mass to keep the slab or the water in the tubing in the slab warm.

    The pump is plenty big enough. It's the design that is bitting you. If you want to keep them on the same manifold then you have to zone the slab separate from the joist. Add actuators and put in a slab sensor to keep a min slab temp. Your delta-t in the slab should be 10 degrees on the joist 15 degrees.

    Once again we see how the most important part of a radiant application is screwed up. The design phase. Where's the heat loss and radiant design? Was one even done? Is the pump on the radiant supply side out to the manifold of before the mixing valve?
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • rox5488
    rox5488 Member Posts: 4

    The mixing valve appears to be piped correctly.  H to the boiler side of the supply  C to the return side from the zone and then the output goes to the zone pump and downstream to the supply side manifold for the pex.  So to me that looks ok I guess. 

    I can't answer if any of the design was done by the contractor.  if it was he didn't share it with me but I can't speak for them.

    The pump for this zone has never run long enough to get the slab up to temp because it only will run for an hour or so and then flow is lost.  The pump and zone is still running and calling for heat but there is no flow on the needles.  I agree the delta T is a large swing due to the colder water from the slab but I am just trying to get the zone to flow.  After a few hours we can get that slab up to temp.  now as far as the 100 vs 120 for the 2 different types of materials.  I can't really say how well that will work since I haven't had it running yet.  I think the contractor's plan was simply to reduce the GPM Flow in the under floor pex to help equal it out.  I don't know if that will work really or not though.

    I turned the pump on again today to see what it did and today I have no flow in that zone at all.  So I am starting to question if the pump has failed...

    The contract is supposed to be out monday so we'll go from there I guess.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Hold On A Sec

    Ask the contractor for the heat loss and design. Your paying for the system are you not? The contractor doesn't have to live in your house, you do. There is no way I would ever put a slab and joist heating on the same manifold never mind the same zone. You are going to have a very uncomfortable system.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
This discussion has been closed.