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Open Loop radiant heat system high pressure problem

Hello,
I just finished my open loop heat system. It's underfloor and has a 2 port manifold that pushes to 700' of 5/8" tubing. Whenever the water pump from my well kicks on if someone is using the hot water tap / shower and the water heater then turns on, the pressure relief valve set to 30psi begins to open every few minutes. Then the system is fine until the house calls for more water and the problem happens again. The pressure on the well pump / expansion tank feed is static at about 30 psi, when the well pump kicks on I see it climb to about 45-50 psi. I am assuming that since all the piping is connected to the water heater, the pressure is rising and opening up the pressure relief valve? What are your suggestions to fix this? I was going to get a relief valve set to 45psi online, any other suggestions?


I have a Bradford White propane EF heater that has two ports on the side for space heating which I used. Model of the heater is: RC2PV50H6N
I have a Taco 007 pump
1" and 3/4" piping that feeds 5/8" PEX
Check valve after the first 5 gal expansion tank.



Comments

  • SuperTech
    SuperTech Member Posts: 1,691
    To fix this you must correct the dangerous flaws in the design. Domestic hot water needs to to be separated from space heating for legionella bacteria protection among other concerns. A plate heat exchanger could be used for this purpose.  
  • k2krafty
    k2krafty Member Posts: 8
    edited November 2020
    Hello, thanks for your concerns. After a lot of research this system is safe as long as you run the water through it at least once a day. I could always put a UV filter into the house feed too. It is still the most cost efficient way to do radiant heat also.
  • Jamie Hall
    Jamie Hall Member Posts: 17,122
    edited November 2020
    k2krafty said:

    Hello, thanks for your concerns. After a lot of research this system is safe as long as you run the water through it at least once a day. I could always put a UV filter into the house feed too. It is still the most cost efficient way to do radiant heat also.

    So they say. I wouldn't use it, nor -- when I was inspecting -- would I have approved it.

    That said, if you are pushing through all 700 feet in series, your head loss is going to be on the order of 40 psi or so at a normal flow rate of, say, 3 gpm. Your well pump is certainly capable of producing more than that -- perhaps quite a bit more (there is a reason that water heaters are normally equipped with 125 psi ratings).

    I do NOT recommend a higher pressure relief valve. Assuming you want to stick with an open system -- which, I reemphasize, I would neither use nor approve of -- you need to find a way to reduce those pressure peaks, such as with a much larger expansion (pressure) tank. There is, however, a fundamental incompatibility here: in most of North America, plumbing fixtures and fittings are intended for feed pressures in the 20 to 30 psi range (some, such as some water closet fills, won't work at all at less than that). A larger expansion tank would allow you to work with a smaller pressure swing on the well pump without destroying the pump. You might also consider installing a pressure reducing valve on the pump feed line, to hold the maximum pressure down to, say, 25 psi. That could also -- and perhaps better -- be placed in the output line after the pressure tank, allowing you to run the pressure tank and well at a more normal set of pressures -- say 30 to 50 psi -- while not overpressuring the system.

    I might add that the "run water through it everyday" theory does have some validity -- provided that by saying that you intend "exchange the water in it every day". That's a lot of hot water to use in a day...

    All that said -- it's not an acceptable system.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    HVACNUTkcoppSuperTech
  • k2krafty
    k2krafty Member Posts: 8
    Thank you for the thorough answer. You both have made me think some more about the open loop system. I suppose the hard part of installation is done. Based on my photo in my post I wouldn't have to add much except a dedicated heater, correct? Also would you recommend an electric instant heater? It will take some time to get the new parts so I think in the spring that will be a project to tackle. In the mean time for this winter I will get that pressure reducing valve after the well expansion tank, great idea and thanks again.
  • Jamie Hall
    Jamie Hall Member Posts: 17,122
    As you say, @k2krafty , you've done almost all the hard work already. The only thing really needed at this point is some way to positively separate the heating loop(s) from the domestic water supply. You could either add a dedicated heater -- I would avoid electric for the heating, unless your electric rates are very low, but there are some very very good LP gas fired heating boilers with high efficiency (if properly sized for you application, as high as 96% much of the time) which take very little space. Another option would be, as has been suggested, a heat exchanger -- keeping the heating water and domestic water completely separate, but using a single heat source.

    You will be able to continue to use the well pump control expansion tank, but you will need another one and an associated pump for the heating loop. All the associated replumbing can be done in your existing utility space -- unless it is insanely small.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 15,048
    While I’m no fan of combined open systems, they do happen, still code approved in some areas.
    Really no reason to have a 30 psi relief, everything in that system can operate at 6o psi or more, the pump, tube, manifolds, etc.
    It is critical that the factory T&P valve remain in the tank in the appropriate location, top1/3 with the probe into the tank.
    If you want a second pressure relief, get a 60 or 75 psi. 
    Come up with a plan to separate the systems with a HX soon.
    hopefully you have  two 350’ loops?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • k2krafty
    k2krafty Member Posts: 8
    Thanks for the additional replies. I bought this LP high efficiency water heater shown in my first post to heat the house water and the radiant floor, it was a big expense. Bringing in another one just for the floor would be costly, I'm leaning to the heat exchanger idea. Does anyone have a good diagram or articles I can read up on this? Physically I cant imagine how water is heated that high just through a heat sink! Crazy stuff.
    Thanks Hot Rod- I have one loop with 325 and then another with 375 - I know i'm supposed to keep them as close as possible.
  • DYI
    DYI Member Posts: 12
    One source for these type of heat exchangers is Brazetek. Here's a table they provide for sizing a heat exchanger for hydronic applications.


    .
    SuperTech
  • Jamie Hall
    Jamie Hall Member Posts: 17,122
    Radiant floors don't require -- in fact, don't really like -- high temperatures. The hot water from a good water heater is plenty high enough to get the water in the radiant floor up to where it needs to be.

    Do note in the table above, though, that the assumption is that the water heater output is 140 F -- which is as is it should be. However, you really should have a thermostatic mixing valve on the domestic hot water supply, to mix that hot water from the heater down to 110, to avoid scalding the unsuspecting.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • k2krafty
    k2krafty Member Posts: 8
    Thanks for the diagram and the advice. Brazetek looks like a good company from their site, I'll look to them for the exchanger. Jamie, funny enough I already installed a Honeywell mixing valve because of this very reason, so we are ready on that front. Anything else I need to know regarding the heat exchanger? I looked online and it looks like I will need another pump for the water heater circuit side. Thanks again for the help everyone.
  • hot_rod
    hot_rod Member Posts: 15,048
    A fairly simple upgrade, although a bit $$
    A stainless pump, air vent, 2- ball valves heat exchanger, some connection fittings. Connect the radiant into the other two ports of the HX.
    Both circulators wire together, run any time a heat call.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • k2krafty
    k2krafty Member Posts: 8
    Thanks HotRod, this is extremely helpful. I was reading that adding anti-freeze is needed but is not a great conductor for heat. Is this a requirement and if so, what would be the ratio for best conductivity?
  • Zman
    Zman Member Posts: 6,994
    Why do you need antifreeze? If you don't absolutely need it, don't use it. If you do add antifreeze, use propylene glycol mix intended for heating systems with a concentration of 30%
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • k2krafty
    k2krafty Member Posts: 8
    Ok thanks Zman I was reading online that closed systems use some sort of antifreeze or I guess propylene glycol.
  • hot_rod
    hot_rod Member Posts: 15,048
    k2krafty said:

    Ok thanks Zman I was reading online that closed systems use some sort of antifreeze or I guess propylene glycol.

    Only if you have piping is an area where they could freeze, unheated crawlspace for example or in an outside wall. Obviously snowmelt tune in a concrete slab:)

    I would consider a hydronic conditioner in the closed side of the system once you isolate it with a HX.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • k2krafty
    k2krafty Member Posts: 8
    Thanks Bob, I will go ahead and get a water conditioner once I make the swap.