Hydronic radiant heat stat

Discussion in 'HVAC Heating & Cooling' started by crater, Oct 23, 2008.

  1. crater

    crater New Member

    Messages:
    49
    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. tedfrk

    tedfrk New Member

    Messages:
    16
    Location:
    New Jersey
    i dont have much experience with these but we did have to order a specific t-stat for the unit we put in.
  3. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    We use floor sensors and no room thermostat for the most part. johnson control or ETC digital aquastats. Occasionally Wirsbo floor sensors.
  4. leejosepho

    leejosepho DIY scratch-pad engineer

    In my own experience, the ideal here is to have an air-temp thermostat with a limit sensor embedded in the floor.

    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.

    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: Oct 24, 2008
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,134
    Location:
    New England
    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.
  6. Steven62

    Steven62 New Member

    Messages:
    9
    Location:
    Missouri
    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. crater

    crater New Member

    Messages:
    49
    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. leejosepho

    leejosepho DIY scratch-pad engineer

    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.

    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: Oct 24, 2008
  9. crater

    crater New Member

    Messages:
    49
    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. leejosepho

    leejosepho DIY scratch-pad engineer

    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.

    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. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,822
    Location:
    Cave Creek, Arizona
    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. leejosepho

    leejosepho DIY scratch-pad engineer

    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. crater

    crater New Member

    Messages:
    49
    ok I get the gist now.
    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: Oct 27, 2008
  14. leejosepho

    leejosepho DIY scratch-pad engineer

    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.

    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.

    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. crater

    crater New Member

    Messages:
    49
    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: Oct 28, 2008
  16. leejosepho

    leejosepho DIY scratch-pad engineer

    I do not know anything about your plumbing supply place, but I am using one of these:

    http://www.aubetech.com/products/produitsDetails.php?noProduit=133&noLangue=2

    Click on "hydronic" on the left, and look for an "AF" stat, meaning it reads both "air" and "floor" temperature.

    Yes, as far as I know.

    My only experience is with the Aube stats, and the sensors come with them.

    If I were you, I would just turn the system on with a low boiler (water temp) setting (such as possibly 100*) and let it run for a couple of days in one or more zones so the floor temp can spread and stabilize ... then watch to see how much or how often the boiler runs to maintain whatever temperature has been achieved ... and you can read that temperature by placing a thermometer under a piece of foil and covered with a folded towel (or some other kind of insulation) in the middle of the floor. Since your electricity is so expensive, maybe you should just try to hold the floor around 80* and let your forced air do the rest.

    Again, I would begin with an output temp of about 100*, and with a low (turn-on) setting just a few degrees below that ... then decide about raising both after a couple of days. The idea here is like placing small sticks on a cooking fire to maintain a desire temperature rather than just tossing in another log ... then decide whether you can afford to purchase more sticks.
  17. Bill Arden

    Bill Arden Computer Programmer

    Messages:
    584
    Location:
    MN, USA
    Then I would just heat the slab a small amount since a ground source heat pump is 300% to 500% efficient. (COP 3-5 compared to electric heat)

    I'm not sure why you need a in-floor sensor. you could just measure the temperature of the return pipe and the electric boiler also has a temperature limit you could set.

    in-floor sensors are mainly needed for in floor heating wires.
  18. crater

    crater New Member

    Messages:
    49
    OK so heat the slab to 80* (the point where thermal mass will radiate heat)
    then let the Geo take care of the air temp rise? So I'd have a higher electric bill to have warm feet but not necessarily a warm room (due to the radiant).

    Measuring the return temp. would assume that the liquid was the same temp as the thermal mass would'nt it?

    IE: If return is = or > 80* then system shut off OR if return is < or = 79* then system stay on

    Is the aquastat the limit selector you are mentioning? It limits how hot the boiler will get before modulating.
  19. Bill Arden

    Bill Arden Computer Programmer

    Messages:
    584
    Location:
    MN, USA
    I'm not sure where it would be located, but the boiler must have a temperature sensor already to measure the water temperature.

    if you have an aquastat, then yes, that is what you would use.
  20. crater

    crater New Member

    Messages:
    49
    With a 30* Delta-T, I would think I would need to have the Aquastat set at 110* to maintain the thermal mass(concrete floor) at 80*
    So I could regulate floor temp at the return line. Setting return line stat to be satisfied at 80* and if I wanted to heat with the floor then set a air temp stat at say 72* then if the floor was 80* but the room was 70 system would be running and if the room temp was 73* but the floor temp dropped below 80* the system would start and heat the floor back up to 80* but would't that require a constant flow to get a real floor temp. not just a drop in temp at the sensor?
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