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Hot water and underfloor heat

mikeglmikegl Member Posts: 5
Ok. Hopefully this isn't lengthy. My apologies. So. We have water underfloor heat in a place we recently bought. It has a wall thermostat downstairs. The main furnace thermostat is upstairs. The underfloor heating system runs from the same hot water tank we use for the whole house. I'm trying to figure out how the underfloor heating affects the hot water tank as every once in awhile, we will have no hot water. Does the underfloor system suck it out every so often? And installing a separate system to control the underfloor heat is not an option. I'm just trying to understand what happens. Thanks. The hot water tank is gas.


  • Jamie HallJamie Hall Member Posts: 12,859
    Well let's see here. You refer to a gas hot water tank. Do I presume that you are referring to a gas hot water heater? Or are you referring to a gas fired boiler? If so, what are you using for domestic hot water?

    What heats the rest of the house (you refer to the "main furnace thermostat")?

    How is the downstairs heat piped? From the hot water tank around (circulator pump?) and back to the tank? Is that pump what controls the circulator? What turns the gas on in the "gas hot water tank"?

    And last -- how does your domestic hot water fit in to this?
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • mikeglmikegl Member Posts: 5
    Ummm. Wow. Ok. Terminology.
    1) yes, I guess I am referring to a natural gas hot water heater.
    2) a natural gas connected furnace heats the house.
    3) the piping runs from the tank and back to the tank. I don't know what controls circulation, I'm going to confidently say a pump yes.
    4) what turns the gas on? I'm assuming an ignitor in the tank itself, which in turn is controlled by a wall thermostat.
    5) how does the domestic hot water fit into this?? Not sure what you mean.

    It's I'm thinking it's to technical a question and while this is not really a huge issue, perhaps I need to have a plumber come have a look.
  • Jamie HallJamie Hall Member Posts: 12,859
    There are several possibilities... the thing which concerns me about it is that it sounds as though the underfloor heating may not be separated from the domestic hot water. Honestly, that is not good practice, as there are a variety of dismaying bacteria which can grow in the nice warm underfloor heating pipes -- and get back into the domestic hot water, where you really don't want them.

    You probably do need to have a plumber -- or better, a person well qualified in heating -- come and look at the arrangement and, most likely, suggest changes which will give you better heat, better hot water, and better safety. Have you checked the "find a contractor" tab above for someone near you? Or if you tell us where you are, we may know someone who serves your area.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • mikeglmikegl Member Posts: 5
    Thanks. I'm in Canada. Red Deer to be exact. I'll post a picture of the set up later on when I get home.
  • GroundUpGroundUp Member Posts: 970
    Just spitballing at the info provided, but I am going to guess that the radiant underfloor and domestic are both fed from the same water heater as an open system. Even if it is separated with an exchanger (which it should be), if the radiant draws more BTU than the water heater can recover from, there will be little or no heat left to use for domestic water after the radiant has been running for awhile and it needs time to recover. Pictures will hopefully tell the story
  • SuperTechSuperTech Member Posts: 1,336
    edited March 2019
    It looks like an open system, I certainly don't see a heat exchanger. It looks like this water heater was originally designed for use with an air handlers. I don't like it. I'd make sure that circulator is bronze or stainless steel. I'd consider getting a plate heat exchanger and changing the pipe on the T&P to copper. PEX isn't supposed to be used for that. You may be running out of water due to poor sizing of the system, or excessive build up of scale.
    Get a boiler and maybe use the tank as a buffer tank.
  • mikeglmikegl Member Posts: 5
    Thanks SuperTech. I think our vision of boiler is different. When I think boiler, I think of those big old things in the basement of buildings. Lol. But i am going to get some quotes to separate the two systems and go from there. PEX pipe is used up here for everything.
  • hot_rodhot_rod Member Posts: 13,315
    I believe that is a legal system in Canada, a combined radiant/ DHW, you do have a stainless circulator. The Apollo is a listed tank for combined systems. I think the Canadian code requires the circulator run every 24 hours for a period of time to prevent extended stagnation.

    I feel more comfortable with a separate system, either use that tank and isolate with a plate HX, or consider a Combi boiler or the boiler specific options.

    A heat load calculation should be the first step, followed by a design indicating operating temperatures required, etc.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
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