Boiler for Radiant Floor Heating - Need Advice

[Statjunk]

Hey guys,

I have a slab house that has copper tubing that runs through the slab to heat the house. I spent about a month installing all the duct work and a new forced air system. I was told that no matter what the house will feel cold because the slab will always be colder than the house. Is that true?

So some folks have suggested that I convert the boiler over to a floor warming system. I've been told by multiple sources that I need to have sensors installed an inch into the slab in at least a couple of places in the house. I'm not looking to install the new boiler or do much of anything with that job but I'd like to install the wire for the sensors because I'm about to blow insulation into the attic.

My question is what do the sensors look like? Can I just run low voltage wire to two places in the house?

Any advice would be appreciated because I was planning on putting the insulation in there this weekend. I'd like to have this out of the way. It's going to be cold soon.

Thanks

Tom

 

[hj]

?

You start by saying you have heating pipes in the floor, then you have a forced air system, then you need sensors (I am not sure for what) in the floor, and now you want to put them in because you are putting insulation in the attic? Very confusing.

 

[Sponsor]

 

[Statjunk]

HJ,

Sorry for the confusion. Hopefully this will clarify.

I installed the forced air system so that I could retire the boiler system. Now folks are telling me that I likely can't do that.

So I need to convert the current boiler into a floor warming system which means I can no longer use a thermostat mounted on a wall. I have to imbed sensors in the concrete slab (1" I believe) to detect the slab temperature.

I need to know what kind of wire and where in the house to run it so that the floor warming system will know to kick on to warm the floor.

So what kind of wire and what are ideal locations to put it.

Thanks

Tom

 

[Jadnashua]

Not necessarily true...you do need a thermostat that is smart enough to understand the dynamics of a slab (big thermal heat sink). Many of the radiant floor heating systems don't require sensors in the floor and just rely on a room thermostat. The system I have (Wirsbo or Uphonor, not sure which name they use now on their heating stuff) works this way. Those thermostats are designed to control the circulator and a zone valve. You need to adjust the water so that it doesn't get too hot.

 

[Geniescience]

ditto. you got good advice so far. Fom here, and from the other people commenting.

When one part of your body is always colder than the rest, you may eventually get used to it. Visitors won't, and I have found that they will speak up and comment on the unusual sense they get from cold feet in a warm room. They don't like it.

A cold slab might ALWAYS be chilly when you are trying to stay warm. Tell us more about your climate (a big factor), your slab (also important to know about), and your house's construction (can provide significant background information).

The notion of comfort is always related to the amount of heat being lost in general. When the amount of heat being lost in ONE direction is far far greater than the amount of heat lost in another direction, you have an uncomfortable environment.

Since you already have a floor made to transmit heat, use it.

How you regulate the ON-OFF cycling is not a big deal. Obviously, you would like to have that conclusively decided upon, before laying down a finished floor, so that later you do not have to redo the floor just to get a thermostat into it. When the thermostat (or its probe, its sensor) is positioned inside the slab you have more accurate readings of whether the temperature is increasing or decreasing, in the slab and in the air -- and this helps $100 thermostats calculate whether to decrease or increase the temperature, since they can extrapolate based on the trend in the readings. Useful when the temperature is dropping fast and you want more heat to compensate for that, and now, not a halfday later. And not to overheat too. Manual adjustment means you overshoot and undershoot often when outdoor temperatures change. Having a probe / sensor in the wall may be good enough, but... Right now I don't think it will be possible for anyone to guarantee that you will get satisfaction, although it IS possible, and maybe even likely.

Need to know more first before commenting more.

The sensor is part of the thermostat mechanism. In floor heat applications, it is at the end of a wire that goes down into the floor. In most thermostats it is inside the thermostat device, so you never see it.


david

 

[Statjunk]

Guys,

I'm slightly more confused. LOL.

Not sure what you guys are asking for but here goes. Things about the house:

5" thick slab

Some of the radiant heat is below the slab some is in the slab

2800 square feet

2 zones and two thermostats

Not sure what else to tell you guys.

Are you guys telling me that it's possible to get a thermostat that will control the boiler as a floor warming system simply by detecting the air temp in the house through a fancy thermostat mounted on the wall?

Thanks

Tom

 

[Jadnashua]

The Uphonor pex radiant heating system uses a wall mounted theromstat to control the heat. Now, they may also have one that uses an in-floor sensor, but the one I have in my house does not. Whether the pex is embedded in concrete or clip in panels still use the same thermostat. Seems to work well in my situation. I've got pex in a wood and tile floor.

 

[Statjunk]

My only concern though is that I'll have a forced air system. So ideally the forced air system will run and keep the house warm and the boiler will just take the chill out. So I imagine that the boiler won't run if the furnace is keeping the house warm.

Is this correct?

Tom

 

[CHH]

It seems to me that you're on the right track stat. Wall thermostat to control the forced air system and thermostats/sensors in the slab to control the radiant/slab warming system. You'll need a sensor for each zone just like you used to have a thermostat for each zone. Low voltage wire should be fine for the sensors but the safest way to be sure is to find a few sensors and look at the specs.

I suppose a few of us, like jad, wonder why you would relegate the radiant system to just warming the floor but that really isn't a pertinent question at this point.

 

[Jadnashua]

A floor warming system vs a radiant heating system would likely require different controls. A floor warming system would need an in-floor sensor, since all you are trying to do is keep the floor warm. This could even be used in the cooling season to slightly warm the floor without heating the room much. A radiant heating system is fine with monitoring the room temperature, and doesn't need an in-floor sensor and would not provide any heat at all in the summer, if that was desired. Systems like electric infloor heating mats typically use an infloor sensor. A radiant heat system (from what I know) doesn't.

I have a similar system in my house, I generally keep the forced air (also used for cooling) thermostat set so it doesn't normally turn on and the radiant flooring system keeps the house warm. If for some reason, it failed, forced air system would take over. I'm using a water/air heat exchanger which still requires the boiler to run, but it is a separate zone and circulator, so it's not 100% backup since it still requires the boiler.

 

[Alternety]

You say some of the radiant pipe is in the slab and some is below it. That is very odd. How old is the house? Do you know if there is a moisture barrier and insulation below the slab? If it is old there probably is not.

Cooper pipe embedded in concrete is problematic. It used to be used but concrete has a tendency to eat the copper. Getting pinhole leaks in or under the slab gets to be real annoying.

If the slab is not insulated you are going to have significant heat loss to the ground (and no this does not make it a heat storage "reservoir"). Your ground temperature (a function of location/climate) will always be less than a heated room. This will cause more heat migration to the ground. Conductivity of the underlying soil will be the determining factor. Without knowing what is under there is is hard to actually quantify it. The places where the tubing is under the slab will be the worst. Embedding it in the slab at least gives you a usable but expensive split of heating ground and slab.

I suggest you get someone to do some numbers for you and get an idea what it will cost in energy to do what you want with the floor. To get it right they will probably have to monitor operation for a while for the parts where the pipe is not in the slab.

If you have heating bills for a year or two while the radiant heating was being used, you could get some idea. Do a heat loss on the structure as it was and another with your upgrades. Boiler and furnace efficiency will affect the results for cost.

Have you thought about some carpet with a nice thick pad.

If you are going to continue with the radiant you should have someone in and do a pressure test before you go to the trouble of replacing the boiler.

Just a note on floor temps. In a properly operating radiant system the floors will not feel warm to the feet. Not as cold, but not warm.

Coordinating the radiant and hot air will also be an issue. As someone else noted, the slab is a huge heating flywheel. Running it from a hot air thermostat is likely to cause temp swings. A separate thermostat for the floor will get confused. There are thermostats that have both slab and air sensors. I am not sure if they are available other than as parto of a wqhole heating control system. I used slab sensors and air sensors where I have large areas of slab exposed to sunlight.

 

[Statjunk]

I know some of the copper is in the slab and some below it because I cut through the floor at one point to install some drainage lines. I pressure tested the system by increasing the pressure up to 27 lbs and watched over the course of two weeks. The pressure dropped very slowly over that period by about 2lbs. It ended up stabalizing around 23 lbs after about 6 weeks. Hasn't moved since.

So I believe that means no major leaks.

The old boiler is hog. I mean a dirty hog that needs a .22 behind the ear. In February it cost me $879 to heat the house. After finding that out I turned it down about 7 degrees, wore jackets and it was still in the high $300s for the rest of the winter months. The current boiler is 250,000 btu with what I was told an in the 30% range efficiency.

Ok guys the million dollar question. If I decided to go the winter without replacing the boiler and not using the current one, what are the chances of my water lines freezing over with the forced air system heating the place?

Thanks

Tom

 

[Geniescience]

a good teacher can bring students to a higher level of understanding. We are just people sharing what we know. It may be good stuff, that you can use.

As alternety pointed out, you have a weird system. I didn't want to touch that topic, as I had enough to say already just covering the basic stuff.

Your system may be a bad one. I can't believe that a properly designed system would put pipes under the slab, into the earth.

It is not going to freeze under the house either in the slab or in the ground under the slab.


dAvid

 

[Statjunk]

David,

Knowing that it won't freeze I'd like to just drop the low voltage lines for the sensors and ride out the winter without it and see how it goes.

Thanks

Tom

 

[Alternety]

Another idea. You have not said what fuel you use. If it is gas or propane, and you have not bought the furnace yet (from you posts I think you have) you could update the boiler and use a coil in the air conditioning system to generate forced air heat. An indirect water heater would also by a useful upgrade to save fuel costs. It uses a boiler loop to a highly insulated storage tank.

If some sort of gas, you could get a condensing boiler that can reach 95% or more combustion efficiency. There are some condensing boilers for oil but not as many as for gas.

If you go this way you could continue to heat with the floor until/if there is a problem with the pipes.

A you describe the tube placement it feels like a bad economy move to keep heating the slab.

Your best bang for the buck is probably upgrading insulation, windows, and air leaks in the house.

Before getting either a boiler you should do a heat loss for the house. It is probably unwise to simply use the capacity of the original boiler. Particularly in older installations, it was real popular to install a great big boiler rather than actually doing the loss calculations. Oversizing is bad for both heating and cooling because it creates more wear and tear than necessary, reduces comfort, and lowers efficiency.

If you want to play with numbers, I think Slantfin has a free program.

There are a whole lot of factors in properly designing a system. You probably won't gain the necessary knowledge from posting like this.

 

[Jadnashua]

A properly designed heating (or cooling, too) system runs continuously at the max design load. I put in a Buderus condensing boiler that modulates its output. It can go from 20% to 100%, and is more efficient when running at less than 100% (where it is nearly 95%!). I use it to heat my tank, as a zone in my air handler, and for radiant floor heating. I've only got 1100 sq ft, but my average level gas bill is $63/month, and that includes a stove, dryer, and barbeque grill. The rates for gas have gone up, and my bill still went down after it was installed.

As stated more than once, you need to do or have done a heat load analysis as installing too large of a heating system will lead to premature wear, and much more rapid cycling along with less comfort and efficiency.

 

[Statjunk]

Thanks a lot guys.

Yes the Furnace is already installed and operational. I paid a local heating and cooling place to size the unit to my place. I'm in the process of putting in new windows and insulation as we speak. So that should improve the house tremendosouly. I also took down about a 1/3 of the walls in the house and new insulation was placed in there as well. I'm going to bring the attic space up to R38 so that should solve most of the air leakage problems. Eventually I'll be getting new doors too.

I spoke with a boiler guy last night and told him the situation he said to go ahead and run 4-wire to the thermostat locations and to hold off putting down flooring in those areas. He claims that later on it would be easy to drop the wire down to the baseboard and drill a hole in the concrete for the sensor. So today that will be my project. It is going to be extremely difficult to run the wire into the walls because my electrical ladder track runs right over the two sections of wall that I need to get into.

Sucks to be me.

Tom

 

[Jadnashua]

A common misconception is that insulation stops air infiltration. While maybe true for certain types of insulation (properly packed cellulose or sprayed in foam), this is not true for most of the fiberglass stuff. It acts like a big filter if there are air leaks, but does not stop them. You only get the designed insulation factor when they are in still air; i.e., no air leaks.

 

[Alternety]

I believe if you try to warm the floor to the point you notice it, the floor will be the dominant heat source. Your ability to control two simultaneous heat systems will be seriously screwed.

To stop air infiltration (a serious loss mechanism) you need to foam or caulk openings around all penetrations of the heating envelope. This includes areas at the top of walls that may leak.