Thermostat with Remote Bulb Help

Discussion in 'HVAC Heating & Cooling' started by statjunk, Jan 26, 2008.

  1. statjunk

    statjunk DIY Senior Member

    Messages:
    542
    Hey guys,

    I'm looking to buy a thermostat with a remote bulb. The bulb needs to embedded in the concrete. The run on the bulb is only about 5'. This will be the thermostat for a boiler system that will serve as a floor warming system.

    The thermostat needs to be at least slightly attractive. Nothing overly fancy but no elongated dial units. Any ideas on where I might find one of these?

    I've checked Graingers and several online sources and I've yet to find anything that fits my description.

    Thanks

    Tom
  2. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    I have used the Ranco ETC-111000. It is an electronic control unit and the sensor cable is 8 ft, but you can add many ft of cable to it because the additional resistance doesn't affect the measurement. You can route the sensor and power wires out the back so you don't have ugly wires exposed on the wall.

    If the boiler is using a 24 Volt system you can get the ETC-112000 model from Grainger and use low voltage power for the electronics.

    I would put a tube in the concrete so the probe could be put in after pouring and removed for repair or replacement.

    If you have access to the bottom of the floor you can curve the tube a bit so the open end is exposed and the other end is embedded where you want it in the concrete. The sensor is 1/4" diameter and about 2" song so a 3/8" ID tube will be big enough.

    For heat-transfer reasons you will get the best accuracy if you put the tube in so that maybe 2 ft of tube is embedded in the concrete and located approximately equidistant from any pair of heating elements (wires or tubes).

    If you search on the famous auction site for RANCO ETC-111000 Microprocessor Temperature Control you will find some for sale.
    Last edited: Jan 26, 2008
  3. statjunk

    statjunk DIY Senior Member

    Messages:
    542
    Bob,

    Thanks for the reply. I looked that unit up on the famous site and I'm not sure that's what I'm looking for. Let me expain the situation better. The house is a slab house. Exising with radiant floor heating in the slab. What I'm looking to do is mount a thermostat on the wall with a temperature measuring device that runs down the wall and that I can slip right behind a piece of trim. Drill a hole in the concrete and imbed the probe into the slab. Thereby the thermostat will not measure the air temperature but the floor temperature and call for the boiler to cycle when the temperature drops so many degrees.

    Does this description help?

    Thanks

    Tom
  4. leejosepho

    leejosepho DIY scratch-pad engineer

    In this list of various models of thermostats, "A" means air temperature is measured and controlled, "F" means floor temperature is measured and controlled, and "AF" means air temperature is controlled but limited by a maximum floor temperature.

    It sounds to me like you are looking for something like a TH113-F that will read the floor temperature and signal your boiler's 24-volt control circuit when heat is needed. An "AF" model would do essentially the same (as in my own embedded-wire system) by attempting to heat the floor up to an adjustable point no greater than 104* as long as the ambient air temperature is below the demand setting. I do not know the maximum floor temperature for the TH-113.

    http://www.aubetech.com/products/list.php?noLangue=2&noFamille=1&app=7
  5. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    The thermostat will measure the temperature of the probe, whether it is in air or concrete. The description of how I would put the probe in a new floor is to minimize any differences between the concrete and the sensor. Where that is not possilbe, one does the best that can be done.

    I would put a little grease in the hole to provide good heat transfer from concrete to probe, and plug the hole at the top to keep junk out.

    The dead band (range betwen on and off) is adjustable, and you will want to keep the setting small because the temperature of the concrete will change slowly.
  6. Raucina

    Raucina Previous member

    Messages:
    515
    Search Graingers for "line voltage thermostat" and you find what you seek. There are a few with 8' probes.

    I have done several radiant in slab systems, and would never consider reading the slab temp. I use the Honeywell -very simple- line voltage units with dual bulb technology, and control the pump directly for simplicity. I make sure the unit is placed out of drafts and sun and centralized to the home main living area. With a bit of zone tuning, soon you will never touch the unit again except to bump up or down a few degrees.

    If I was in Alaska, I would consider monitoring outside air temp with that probe to get a lead on a cold spell, and to turn the pump on early.
  7. statjunk

    statjunk DIY Senior Member

    Messages:
    542
    Raucina,

    Above you say that you would never consider reading the slab temp. Then you go on to say that you line voltage units with dual bulb technology and control the pump directly.

    Isn't that what I'm attempting to do? Just allowing the user to set the temperature and then the pump will turn on when the slab lowers in temperature by so many degrees?

    Tom

    P.S. Thanks to all that have offered input. Greatly appreciated.
  8. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    The difficulty with controlling slab temperature to achieve a comfort level in the house is that there is not a fixed relationship between slab temperature and comfort temperature in the house.

    Assume that you can set and control the temperature at precisely what you want. However, the slab temperature necessary to transfer enough heat to the house depends on how cold it is outside. If it is 50 degrees outside then it may be necessary to have the slab 5 degrees above the comfort temperature of the house. But if it is 10 degrees outside it may be necessary to have the slab 20 degrees above the comfort temperature to keep the house comfortable.

    That means that there is no single temperature that you can control the slab at to keep the house comfortable.

    Because the slab has so much mass, and therefore stores a lot of heat, a warm slab will still have a lot of heat to give up when the room temperature reaches the comfort range. You could get a lot of overshoot on the high side after the slab temperature control shuts off.

    Control systems generally have means to anticipate demands by measuring rate of change of the error between setpoint and measured value, and to correct for the lag that exists when there are storage elements such as a massive slab. Those systems are usually not available in residential systems and the temperature control range in a home is usually acceptable.
  9. Raucina

    Raucina Previous member

    Messages:
    515
    Ditto to Bobnh.... Read the temperature of the mass of the interior of the building - air/walls/ chairs - [wall mounted radiant thermostat] and control your circulator pump with that. Fine tune heating by throttling zones flow rates.

    There is no reliable connection between slab temp and comfort level. I keep some cheap thermometers taped to the out and in manifolds to see what the temperature differential is through the loop. That gives me the slab temp, but I cant say that I have anything special to do with that knowledge.

    A large differential tells you that its cold outside.... but unless you are caged in, a finger on the window pane, or a step outside works just as well.

    If you built a house with large thermal mass, lots of cement and stone and insulation on the outside of that, you wont need any special radiant floor controls other than a simple thermostat and some adjustable ball valves on each zone.

    If you have a thin stick home with single pane windows in Fairbanks, you will need some very special controls and piping to keep warm.
  10. statjunk

    statjunk DIY Senior Member

    Messages:
    542
    Ok, I'm not completely confused here but I'm not exactly comfortable.

    I'd like to bring you guys in the loop to see if you can offer me some advice. I installed a forced air furnance in a slab house. It works great and can easily warm the house on it's own. The issue is that the floor stays cold no matter what. The house has a very old boiler system with radiant tube heating in the slab. I need to replace the boiler. I won't be doing the boiler replacing that I'm leaving to a professional sometime next year when I have the money. The issue I have sitting in front of right now is that I need to lay the wood floors down in the house. I can't do that if I need to imbedd sensors into the slab to sense the temperature of the slab.

    Any advice on this front?

    Also I understand that the issues of the slab having a large mass and the outside temperature etc.... Won't the outside temperature cause the slab to cool thereby causing the thermostat to call for the boiler to cycle more often? Doesn't the sensor in the slab basically solve this problem?

    I'm trying to make buck on this place and do a good job. Do you have any other recommendations as to how I might configure the boiler system?

    Thanks

    Tom
  11. Raucina

    Raucina Previous member

    Messages:
    515
    I would first pressure test the in slab pipes. Then I would quick rig a circulating pump to the water heater and push some hot water through for a few days or weeks, assuming you have a water heater that has some extra capacity.

    See what you think - see what your heat losses are, and you can still keep the hot air unit going. If you dont have any slab edge perimeter insulation, you may have a rather inefficient set up. Also, upon start up expect to take some time to heat the mass just below the slab as well. If you just want to get the chill out of the slab, I would not bother with a boiler when a water heater will do. Perhaps you can figure out the zones and only heat the critical barefoot area....
  12. statjunk

    statjunk DIY Senior Member

    Messages:
    542
    Raucina,

    I've already pressure tested the pipes and they are good to go. Are you claiming that I could replace my boiler with a hot water heater to keep the slab warm on a 2800 square foot house in Michigan?

    The two pumps that operate the zones are brand new. If this is possible I want to go this route.

    Would using a hot water heater need thermostats? I would think not. However would a hot water tank be able to keep that amount of water hot enough to warm the furthest points of the house?

    I'd assume that I do not have any permiter sealers of any kind.

    I need to heat the whole thing. Can't just pick and choose because the whole house is hardwood.

    Tom
  13. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    From your description I infer that your slab is on the earth. If it doesn't have very good insulation between slab and earth you are going to spend a LOT of money keeping it warm.

    If you look at the design recommendations for "radiant" heating of slabs they specify substantial insulation, above the water table, with the heating coils in the top 1/3 of the slab. Those features are necessary to keep from warming the earth with your $energy$.

    When my house was built in 1965 and oil was $0.13 per gallon, the lower level was heated with the usual baseboard convectors but the pipes connecting the baseboard convectors were run under the slab without insulation. The length of pipe under the slab exceeded the length of the convectors and the heat loss through the intimate contact of pipe to soil was greater than the heat delivered to the house. I have abandoned that system because of the great heat loss combined with $3 per gallon fuel oil.

    You might be able to effectively insulate the floor with carpet over foam padding, which you would not want to do if the floor is heated because that would prevent the heat from coming up and would drive more heat into the earth.

    Unless you can determine the insulation and construction characteristics of the slab/heating system you risk investing a lot in a boiler that will prove to be uneconomical to operate.
  14. Raucina

    Raucina Previous member

    Messages:
    515
    Michigan without under slab or at least perimeter insulation, plus hardwood on top spells a pretty dark future for your plans. You dont need the slab sensors but if you did they can be installed with or without the wood in situ.

    I have been using a water heater called the Polaris, its claimed at 96% or more efficient, nat gas or propane. I think they go to 200,000 btu's and work great on radiant. You should determine your heating needs in BTU's and have a look at water heaters that way.

    If you have a good forced air system, you are probably better off staying with it. Your action of installing the hardwood will make foot warmth sensation much nicer. You can use an engineered flooring and add extra layers of foam insulation also.

    You might use electric heating "blankets" in bathrooms and sitting areas just under the wood, and spot use them with wall switches. I see electricity heading toward being cheaper a fuel than oil very soon, and already so in above base level useage.

    I grew up in a house in Wisconsin with radiant and the only warm place was laying on your stomach on the floor just over a pipe. Probably there is a good reason your system was shut down.

    How many zones do you have? have you traced their locations? If one zone was master bath-bedroom, you could warm just that area with your existing gas water heater on a part time basis.

    Anyway. if you nix the radiant idea, be sure to get some insulation and a very serious vapor barrier under the wood. You may achieve most of the comfort level you seek that way. All this assumes slab on grade.
  15. statjunk

    statjunk DIY Senior Member

    Messages:
    542
    Wow. Talk about a situation. I'm damned if I do and damned if I don't. I don't want to bring a boiler specialist in here because honestly I don't trust a lot of the trades in Michigan right now. They are so desperate for work that they'll do anything. The other day I had a contractor tell me that it was ok to put up cultured stone in the current weather as long as it was put in during day and it didn't get below 30 till dark. I said thank you and sent him away.

    I may have to ponder this one a bit longer before I'm all set. The good news is I have some time about 3 weeks.

    Tom
  16. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,765
    Location:
    New England
    With two systems operating, it could be fairly complex control system. Wood flooring can't have excessive temperatures, or you'll mess it up - extreme size changes with seasons and potential cracking. Ideally, you'd have either an outside reset which would adjust the firing temperature based on the anticipated load (most efficient) or a modulated mixing valve. Combining an outside reset with a modulating boiler would allow it to run contantly, but at exactly the temperature you need - this is the most energy efficient and relates to comfort as well. A slab has a fair amount of thermal mass, so on off cycles aren't too bad, but constant flow and heat levels is good. Modulating boilers are more efficient as they are run at less than their maximum output since you've got the same heat exchanger trying to extract a smaller amount of heat - they can approach 100%.

    The problem is that at a -10-degree day, the water temperature needs to be hotter than it would at say 40-degrees. If you don't modulate it, the ultimate floor temp could be hotter than you want, but the average temp might still allow it to be comfortable. It could give the floor fits, especialy at the beginning of the loop. Hopefully, the loops you have are fairly small, otherwise, by the time the watter gets to the end, it could be quite cold, and you won't get much advantage - it won't be even.
  17. Raucina

    Raucina Previous member

    Messages:
    515
    And you have one more burden: determining how much footage of pipe is under the cement. If your system is old enough to be copper, you may have several times less surface area than what would be installed today with PEX. Those systems were often unable to keep up on cold days and many were disconnected. Builders started being cheap a few thousand years ago.

    I worked on a system with new pex over the existing slab, then we poured mortar over that and troweled the finish surface to a texture and stained it. Works great except you must raise all your doors and etc. for the height loss. You can even add a thermal break under the new assembly. Not easy unless you are gutting the house.

    Again, you lose a lot of radiation by adding wood over it all. If you insist, use the thinnest wood system you can find.
  18. statjunk

    statjunk DIY Senior Member

    Messages:
    542
    I'm going to be using an engineered floor. So I'm hoping the plywood construction of the flooring will hold up.

    Jim,

    I think I understand what your saying. If I use a water heater to heat the water in the pipes I may run into extreme temperature swings. Is that what you were saying? So am I back to a boiler system?

    I check the prices and I can get an 80% unit for about $1900 including tax.

    Tom
  19. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,765
    Location:
    New England
    It's more comfortable if you can maintain a constant temp. Now, the slab having a lot of mass, won't change fast. On a cold day, you'd be losing more heat, so ideally, you'd want the incoming water hotter, but there's a limit, especially on a wood floor, it can't be very hot or the wood will have problems. The incoming water at the beginning of the loop can't be so hot it exceeds the safe max floor temp (and a long loop may mean it is too cold at the end to do much). If the supply is fixed temp, it won't keep up on a cold day, or it will need to run hotter. Running it hotter for a short time to limit the slab temp (so your average can be nice) eventually runs up against a wall, you can't make it hot enough. Plus, turning a boiler on and off (short cycling it) is very inefficient and shortens the life. A modulating boiler (the one I have can go from 20-100% of max output) adjusts to the needs of the building. Mine also has an external sensor that along with the smarts in the boiler, decides what temperature to make the water to maximize the comfort and efficiency.

    Considering that your forced air may be the primary heat, and all you want is the floor warmer than it is on the current slab, any warming would be nice, so you don't need to get too sophisticated.

    Life's a series of challenges and compromises...
  20. statjunk

    statjunk DIY Senior Member

    Messages:
    542
    I heated the house all of last year with the boiler. My bills were very high though. So I feel like a reasonable temperature would keep the floor warm. I think I might need to call in a professional to get some opinions but it brings me back to the point I made above.

    Jim,

    How different is the pricing on a modulating boiler?

    I was told by my buddy who was in the business a long time ago that based on the description of my place and the intended purpose that I'd be fine with a 70,000 btu unit running at 80% efficient. Any idea what the equivalent modulating boiler would run?

    Thanks

    Tom
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