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Thread: Hydronic radiant heat stat

  1. #1

    Default Hydronic radiant heat stat

    I installed my "in-floor" heat last fall, and never really go a chance to "let it run' I installed 1 honeywell heat-only stats on each of the 3 zones, my question is.
    Is there a special type of stat required for hydronic radiant heat system?
    and is a slab sensor a must have for it to work correctly?

    it seems that the room temp overshoots when it runs and stat lets the slab get cold when it is,nt running because it take a long time, 3 days it seems for the slab to cool down and the room temp stays at 70 due to forced air heat which means the stat is satisfied that the room is up to temp not letting the in-floor system work at all. Maybe when the outside temp falls more the forced air won't keep the basment as warm and the in-floor will make up the diff. which is what i'm aiming for. Both systems working together to heat as efficiently as possible.

  2. #2
    DIY Junior Member tedfrk's Avatar
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    i dont have much experience with these but we did have to order a specific t-stat for the unit we put in.

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    Master Plumber nhmaster's Avatar
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    We use floor sensors and no room thermostat for the most part. johnson control or ETC digital aquastats. Occasionally Wirsbo floor sensors.

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    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by crater View Post
    Is there a special type of stat required for hydronic radiant heat system?
    and is a slab sensor a must have for it to work correctly?
    In my own experience, the ideal here is to have an air-temp thermostat with a limit sensor embedded in the floor.

    Quote Originally Posted by crater View Post
    it seems that the room temp overshoots when it runs ...
    Through the thermostat and its floor sensor, I have my floor limited to a little over 80*, but you might want a little more or less than that from your floor.

    Quote Originally Posted by crater View Post
    ... and stat lets the slab get cold when it is,nt running ...
    You can solve that by setting your floor stat just a little higher than your stat for the forced air ... then the floor stat's limit in the floor restricts it from overshooting ... and by keeping the floor set a little higher than the forced air, your forced air will only have to make up the difference whenever the heat from the floor is not quite enough.
    Last edited by leejosepho; 10-24-2008 at 03:14 AM.

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    Many electronic thermostats have adjustable modes to define what type of heating system you have. Because of the relatively slow response from in-floor, one set up for say a forced air furnace would not work well and give you a big overshoot. The anticipation needs to respond to the source properly.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member Steven62's Avatar
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    Default I agree with LEEJOSEPHO

    We usually spec a dual temp type stat for these systems, and that way we have lots of adjustment room. Most of my customers report turning down the central heat after installing floor heat.

    If you wish more detailed info P.M. me

  7. #7

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    Stat is a Honeywell CT30A non programmable stat
    so I think it is geared for a forced air system plus it was cheap.
    Big question is the floor sensor, since I did'nt install one when I poured the floor can I drill a hole and install it now? And where in the floor. I have heard it should be near the heat source, but my floor gets real hot there, so I would think it should be place away from where the loops terminate?

    So if you only install floor sensors how do you control room temp? Just by turning the sensor up or down?

  8. #8
    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by crater View Post
    Big question is the floor sensor, since I did'nt install one when I poured the floor can I drill a hole and install it now? And where in the floor. I have heard it should be near the heat source, but my floor gets real hot there, so I would think it should be place away from where the loops terminate?
    I have two areas of floor with embedded heating wire, and my sensors are centered between two wires and about midway along one wall. I do not have any actual experience with hydronics, but it would seem logical to have a sensor in a location where it can read a "typical" or "average" floor temperature. However, putting it near your hottest spot would help assure no overheating anywhere ... and to get a better overall average, you could adjust your stat's floor limit to a little more than you might if the sensor was somewhere else.

    I know of an automobile dealership with hydronic heat in the floor, and I was there doing some work on that building when someone else had to use epoxy to glue the front-end rack to the floor since the hold-downs had been missed prior to pouring the floor and there was no way to later know for sure where they could drill safely. In other words, I would not want to drill into a heated floor without first knowing *precisely* where it would be absolutely safe to do so. If I were you, I would ask some hydronic experts about alternatives. Overall, you only need to be able to get at least a relative reading the stat can use to limit the floor to a temperature you can check with a thermometer temporarily placed under a towel.

    Quote Originally Posted by crater View Post
    So if you only install floor sensors how do you control room temp? Just by turning the sensor up or down?
    I have my room-air-reading floor-heat stat set at 2* above my separate forced-air stat, and I have the floor-heat stat also set to limit the floor at 80* or so. In that way, my floor-heat stat is always trying to use the floor to warm the air in the room, but its limit keeps it from running the floor continually when it cannot actually get the room-air job done ... and then the forced air comes on to assure the actual room temp I want to maintain.
    Last edited by leejosepho; 10-24-2008 at 05:19 PM.

  9. #9

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    1)So you keep your concrete slab approx 80*with slab sensor all the time in heating mode.
    2) You set room stat 2* above forced air stat room temp to gain what?

    So what I can see is that the slab is maxed at 80* and the system will modulate between approx 77*-80*until the room temp reaches the stat setting
    Because there is a slow rise in the floor heat and a fast fall in room temp it would seem like this is a very in-inefficient way to heat

  10. #10
    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by crater View Post
    1)So you keep your concrete slab approx 80*with slab sensor all the time in heating mode.
    2) You set room stat 2* above forced air stat room temp to gain what?
    First, maybe I have not described my floor-heat thermostat well. It is like a regular thermostat in the sense that it is reading room-air temperature and is trying to heat the air in the room to 72* or so, and it does that by sending power to the floor. However, it also has an internal high-limit circuit that reads the sensor in the floor and limits the floor's heat to a little over 80*. So then, and whenever the air in the room is below 72*, that thermostat is continually cycling power to the floor ... and that results in radiant heat from a continually- and consistently-warm floor in a way that is 100% efficient since there is no flame and chimney involved.

    Quote Originally Posted by crater View Post
    So what I can see is that the slab is maxed at 80* and the system will modulate between approx 77*-80*until the room temp reaches the stat setting
    Because there is a slow rise in the floor heat and a fast fall in room temp it would seem like this is a very in-inefficient way to heat
    A "slow rise in the floor heat and a fast fall in room temp" is a matter of relative responsiveness and not of outright efficiency. Efficiency is determined by how much energy is actually lost during a given process.

    A room-air thermostat with a floor-limit sensor will hold your floor within a range between its air setting and its floor setting, and your forced-air system can be set to fill in whenever the hydronic system needs a little help.

    I have two floor areas with heating wire. One is concrete and one is wood, and I have their thermostats set to keep those floors warm in order to provide as much 100%-efficient floor-limited heat as they can ... and my less-efficient-but-more-responsive forced air system makes up the difference whenever necessary.

  11. #11
    Moderator & Master Plumber hj's Avatar
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    Default heat

    Your problem is caused by trying to mix two vastly different systems in the same area. Radiant heat is an accumulative process. It heats slowly, but the mass retains the heat for an extended time. Hot air heats quickly and cools down just as quickly. So the radiant system runs a long time and then shuts down for an extended period of time, while the hot air is going on and off continually. You really have to decide which one will be the primary source of heating, and then set the other one to a lower temperature so it only operates when the primary one is being overwhelmed by excessive heat loss. Normally, because of response time, the radiant would be the main heat source.

  12. #12
    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by hj View Post
    Radiant heat is an accumulative process. It heats slowly, but the mass retains the heat for an extended time. Hot air heats quickly and cools down just as quickly ...
    Yes, and one of the adjustments people have to make while getting accustomed to radiant heat is letting go of the sudden comfort that can be felt from a burst of warm air. Compared to forced air, radiant heat is more like a low simmer essentially heating everything other than the air.

  13. #13

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    ok I get the gist now.
    A room-air thermostat with a floor-limit sensor will hold your floor within a range between its air setting and its floor setting, and your forced-air system can be set to fill in whenever the hydronic system needs a little help
    And it makes sence to do exactly that, now will drilling a hole in the concrete and inserting the sensor work, or will there be to much heat loss for the sensor to be acurate?

    Also my forced air is GEO Thermal so one of the burning questions is: Is running the combination of the GEO and the radiant as or just as efficient as just heating with the GEO?

    NOTE: my GEO is not the heat source for the radiant. The radiant has a seperate 15KW (50k btu) electric boiler as it's heat source.
    Last edited by crater; 10-27-2008 at 07:00 PM.

  14. #14
    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by crater View Post
    now will drilling a hole in the concrete and inserting the sensor work ...
    The sensor should be embedded (cemented) in the floor, but drilling into a floor that has cast-in plumbing is a very risky thing to do unless you have some absolute way of knowing you are not going to hit anything you do not want to ruin.

    Quote Originally Posted by crater View Post
    ... or will there be to much heat loss for the sensor to be acurate?
    As long as no air can get to the sensor, it should provide an accurate reading. To make them easily replaceable, I have mine pushed into plastic tubing cast into the floor ... but I do not know the best way for you to install one after-the-fact. If I knew for absolute certain that I could cut a groove about an inch deep without hitting any piping, I would probably do that and and use some thinset to bury a couple of sensors (one as a spare for later, if ever necessary) a couple of feet out onto the heated floor.

    Quote Originally Posted by crater View Post
    Also my forced air is GEO Thermal so one of the burning questions is: Is running the combination of the GEO and the radiant as or just as efficient as just heating with the GEO?

    NOTE: my GEO is not the heat source for the radiant. The radiant has a seperate 15KW (50k btu) electric boiler as it's heat source.
    I know nothing at all about GEO, but I do know electric heat (such as from your boiler) is 100% efficient since the system does not carry or send any heat out of the house.

  15. #15

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    Ok I believe I can get a hole or groove or something in the concrete without hitting a tube thru my IR thermastat also I built it so I took alot of pictures of it but that does'nt help with measurements and exact location of tubing therfore I think IR thermastat is the way to go. I'll just take readings as the floor warms up there seems to be a pretty big temp difference directly over the tubing until the floor is up to temp, then it gets harder to find them.

    So I'll want to get a Stat that has an internal high-limit circuit that reads the sensor in the floor? Can I ask my plumbing supply place for just that and they'll know what I'm talking about?

    Also with 3 zones I would assume that I would need 3 Stats and 3 floor sensors? 1 for each zone

    And is the floor sensor supplied with the stat or is it like a Johnson Control that sends a low voltage signal to the stat?

    Sorry about all the questions but last year it cost me $700.00 for 1 months electric bill trying to get it up and running and never really did work. Now this fall after some re-plumbing and starting it before it was -30d it seems to work but just needs some tweeking. Things I was too scared to try last year expecting another hugh electric bill if I kept trying.

    Oh yea I do know that the electric is 100% efficient, That is what sold me but damn electric is expensive in our area. Plus our Electric Co. does not offer a reduced rate for electric heat, just a bigger rebate if yo go all electric. And with the recovery rate of the electric boiler I am beginning to wonder if it was a good decision for my application.
    Last edited by crater; 10-28-2008 at 06:42 PM.

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