Radiant floor heat temperature and pressure

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Blazedog, Nov 7, 2017.

  1. Blazedog

    Blazedog New Member

    Joined:
    Nov 7, 2017
    Location:
    Pennsylvania
    hello,

    I have been lurking here for a long time as a nonmember and I'd like to say upfront how much useful information I have found and have seen you share with others. Thanks for helping the plumbing industry.

    So, we bought this house a few years ago and it had a radiant floor heat system installed in the kitchen. We turned it on and heard the pump humming so we knew it powered up but never used it. This year we have the need to use it and I have some questions about it.

    I'd also like to say that the contractors who built this addition to the house for the previous owner did a lot of questionable things so im not even sure if everything was done that should have been on this system. From what I've read there should be floor sensors. As far as I know there are none.

    So heres the setup: 30 gallon electric hot water heater. Outlet has a pressure and temp gauge right above the tank. A few inches later is the water pump. An aquastat is on the pipe that cuts the signal from the thermostat to turn off the relay which turns off the pump. Then it goes to a 2 zone manifold and plastic lines go into the cement/tile slab. Returns cone back to another manifold with an auto air bleeder and then back into the tank.

    So if I set the tank temp higher than A then the pressure in the system climbs to about 50 psi. It seems high so I lowered the temp back to A. The aquastat settings are a mystery because someone messed with it before me and I don't know what they started on. It's a Honeywell L6006C aquastat. The differential wheel doesn't seem to do anything. I've set the limit at about 125 with a differential of 10 and assume that means hot water pumps to 125 and then shuts off until it drops to 115. I'm also assuming the off time is to allow the water heater to make more hot water. Is that what the aquastst is supposed to do?

    If I set it lower than 125 the pressure stays lower but im seeing the floor return water temp rising until it levels off at about 80 and then the water in the tank can't seem to keep up with the temp loss so the pump keeps running until the return water temp starts to drop and never satisfies itself. Am I just not waiting long enough for the temp to stabilize? Ive waited about 1-2 hours.

    So I've thought about movingbor adding an aqua stat to the return and having it shut the pump off when the temp gets too hot. Is that a reasonable thought of a way to control the water temp?

    There also is an expansion tank before the supply manifold. There is air in it but I haven't set or checked the pressure. Just to mention the water in the system was cloudy and probably never flushed so I completely flushed it until it all ran clean. I even drained the tank and with the two bleeders on top and the auto sir purge upen I refilled it with wTer until all the Air came out. Pump runs pretty quiet and I don't see bubbles pass through the lines as I did before I flushed it. I also filled it up about 12 psi. What is recommended fill pressure cold and for the expansion tank?

    I know it's ally but I'm trying to eliminate as mAny return questions as possible to help solve.

    Thanks,
    Nick
     
    Last edited: Nov 7, 2017
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    A floor temperature sensor is not required, but can help, if done well.

    For the pressure to rise that much during the heating of the water, says to me that you either do not have an expansion tank, or the one you do have has failed. In a closed loop heating system, you want an expansion tank to limit the pressure rise. When setup properly, it won't change pressure more than a few pounds from cold to hot.

    Trying to initially heat a concrete slab can take a very long time, and longer, when your inlet temperature is low. There's also a maximum inlet temp you should attempt. It might not be uncommon for it to take several days when first turned on to bring the slab up to desired temperature, but once there, things would run much less to maintain it. It would be normal for the return water temperature to be quite low, which would trigger the tank to keep trying to heat the water. Again, this could literally take a couple of days to bring the slab up to temp with the tank running constantly. An electric WH typically only has about a 4500W maximum heating element, so picture that trying to heat up tons of concrete.

    Once things are up to temperature, 125-degrees may be too hot. It would depend somewhat on how deep the tubing is in the slab and whether and how much insulation there is underneath it. Most of the time, you don't want a slab temperature much over 80-degrees or so.
     
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  4. Blazedog

    Blazedog New Member

    Joined:
    Nov 7, 2017
    Location:
    Pennsylvania
    I know the post was long so maybe you missed it but I did mention at the end that there is an expansion tank. I was hoping for some advice on pressure settings for the tank and system.

    Can you reread what I wrote about the aquastsat and let me know if I understanding how it works ?

    Thsnks
     
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    IF you have a working expansion tank, the pressure would not fluctuate as much as you're seeing. That implies either it isn't charged up properly, is way undersized, or has failed. To specify an ET's size, you need to know the total volume of water in the tank and all of the piping.

    In a closed system for radiant heat, you only need somewhere in the order of 15psi as long as you're not trying to pump water up 3 or more stories. Since you're using a water heater, verses a boiler, the water heater is designed to function with typical home pressure up to 80psi, so you could run it with more if you wanted. Once you decide on what pressure you wish to run it at, you must precharge the expansion tank to that same pressure, or you will potentially damage the bladder in the thing, or basically, fill it up so there's no room for expansion, stretching it way beyond its design capabilities. Not all expansion tanks can take a precharge of 80psi. Make sure your ET is rated for use with hydronic heating systems, not all are. If you use one designed only for potable water, it will fail quicker because of the heat since they are designed to go on the cold supply of a water heater. Won't work when you're circulating hot water.
     
  6. Blazedog

    Blazedog New Member

    Joined:
    Nov 7, 2017
    Location:
    Pennsylvania
    Ty for the info. I'll ltry ok into whstvthe predsure in the er is and try to determine the size. The wh is 30 gallon but I have no idea on the lines. They go right into the floor and I have no idea how they were laid out.

    Can you help me with understanding how the aquastst will work. It is on the supply side right after the pump. It breaks the thermostat wire and is connected to the r and b terminals. I'm not sure how it would work because if it is set at let's say 105 and it has a differential of 10 does that mean that it will come on when the water temp is 105 and go off when it's 95? How does it come back on because the temp reading is on a pipe that is not in the floor so does it just cool off with the ambient air and then turn the pump back on? Seems a bit odd that way because it's not the water temp in the wh or the floor.

    I'm also not sure if I need to set the thermostats stat in the wh at higher then A because it seems like the elements won't heat the water fast enough and it just keeps running. I hooked up the two temperature probes on a testo 550 to the supply and return lines and the supply temp starts to drop and the return goes up to a point then it just can't get warmer and the pump keeps running.

    How would you put an aquastat on a system like this if you were installing it? Would you put one on the return to shut the pump off when the temperature got too high? Would that then give the wh time to reheat?
     
    Last edited: Nov 8, 2017
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