Mechanical thermostat?

[JosephP]

While helping do some basic "handyman" stuff at a family member's office they asked if I could figure out why their radiant heat system has not been working.

It is PEX in a concrete slab heated by a 40 gallon gas fire water heater.

Started at control box and worked out...to find that the programmable thermostat had dead batteries!  Replaced those, relit the pilot light and it was up and running!

Since the thermostat is only acting as a 'switch' with the R and W wires it isn't getting power.  With that, those dead batteries shut the whole thing down (and it has been a few years!) (they do have gas forced air heat as well...).

I'm thinking there are 2 options to improve this for the long-term and looking for thoughts:

Option 1)  Install a mechanical thermostat.  Something like this Honeywell (I even have an old mercury switch one that I took out of the house...but for less than $20 it makes a lot more sense to just but new!).  Obviously, no battery to die.  But it is not programable.  I'm thinking you probably just want this system on at a consistent temp...right?  It takes so long to change the heat of the mass - do we really want to let it cool back down for a few hours just to spend a few hours using extra energy to bring it back up to temp as opposed to just maintaining it?

Option 2)  Run another wire from the transformer into "C" on the existing thermostat.  Access is easy, so not a problem if it is really the better option.  I assume this will mean the batteries are just back up and the power from the transformer will keep the thermostat working?

[Splinter Puller]

Option 3)  hang a sign in the mechanical room that says, if the heat does not work try replacing the darn battery in the thermostat... it will be flashing "Battery Low" for a few month before failing.


Mine needs a new batter every year or two.  I seem to remember just fine.

[daddo]

You could install a manual stat along side the programmable one and set it 5 degrees cooler, then if the programmable one quits, the manual one will operate to keep the space from getting too cold. Just a thought.

Programmable is nice, but day to day in my opinion saves you nothing when having to raise the temp and heat the space and the objects within right back up after only a few hours- especially if the heat loss is so much quicker than the heat gain. If the space is to be empty for a few days or weeks, then it may be practical.

[JosephP]

It will only be empty on weekends... Can still continue with fluxuating the forced air units.

[blackhat]

It's called battery day. It can be any day of the year but it's the same day every year. You replace the battery in thermostats, smoke detectors, garage door opener and whatever else on that day. Every year.

[JosephP]

I understand battery day but it is not my place. Looking for a way to help them in case it battery does not get changed.

Is running power to thermostat a bad idea? It would cost about $4 in wire and take 15 minutes.

Likewise is a mechanical thermostat a bad idea? That would also take 15 minutes but the thermostat is more than triple the cost at a whopping $13.

[blackhat]

Not sure if the existing stat can use system power or not. If the batteries leak, they can trash the stat anyway. A mech stat is probably the best choice. Programmable is a waste on high mass radiant like a slab.

[TDKPE]

What blackhat said, especially as it pertains to high thermal mass systems.  Like my in-laws' slab house up here.  No point in setting it back overnight or whenever as the floor stays warm for a day, and takes forever to heat the place back up when it's running.

I like things simple when simple is adequate for the task.  Give them your old Honeywell hockey puck with the mercury in glass switch.  My parents' house has one that was installed in the 50's, when the garage was converted to a room, and it's still working just fine.  Or they can buy a new mechanical thermostat.  Put it in parallel if you want, but I wouldn't bother - I'd just toss (or save) the existing.

[srv52761]

....
Option 1)  Install a mechanical thermostat.  ....
  I'm thinking you probably just want this system on at a consistent temp...right?  It takes so long to change the heat of the mass - do we really want to let it cool back down for a few hours just to spend a few hours using extra energy to bring it back up to temp as opposed to just maintaining it?

You will save energy and money by setting it back.  However, as was mentioned, you may end up sacrificing comfort in the morning, or the evening for that savings...

In theory, the laws of physics will apply as always. You should be able to figure the mass of the slab, the surface area, the difference in temp between the slab and the air at the occupied set point ,and then compute  rate of heat transfer (or more likely you will just use the thermostat it has now to turn it off at some time and monitor its change to see how  the temperature degrades, then turn it back on in the morning and measure how it rebounds.) 
   In theory, there will always be a time you can turn the stat down and a time to turn it up to maintain a comfortable temperature.

 There are a number of factors that will influence how it performs: the mass of the slab, the possible solar gain, the insulation of the envelope, occupancy, movement of people and air, ground temperature ...  

   I help with the energy management in our school district, and we spend an inordinate amount of time just recording measurements.
   So really, the easiest way to find out would be to try it by using the current stat in manual mode and recording data.   With that information you can make an informed decision.  

By the way, that may mean that you upgrade to a smarter thermostat.  They have some designed to account for the response times and idiosyncrasies of radiant slab systems.

[JosephP]

Thanks all!

I'll replace the thermostat with the mechanical.  I was leaning pretty hard that way...just wanted to make sure there weren't red flags.

This has become more complicated than hoped...

The system has not been run for a few years.  I think that has caused some problems.

The water heater won't stay lit.  I replaced the thermocouple.  No joy.  Will replace the water heater's thermostat.  I also wanted to replace the thermal switch in the water heater (while I had it torn apart and the heater wasn't staying lit...I figured it would be $10-15, so might as well try), but that is not sold separately - have to replace the whole burner assembly.

Also, on the water heater:  After the expansion tank, there is a T with a 12" pipe and a pressure relief valve on top of that.  Problem is that while the T is copper, upright pipe is black iron.  LOTS of corrosion.  That will get replaced.  I'm wondering what it looks like inside there.

The pump was whining pretty bad after it ran for a bit.  I'm guessing there is a lot of sediment in there (and possibly a blocked T on the water heater).  I bought a new pump, but we'll see if replacing that T and flushing the system lets them keep the current pump.  I can't help but think the whining pump just build up pressure trying to pull water through a plugged T.  We'll see.