Best Thermostat / Sensor Combination to Avoid Overshoot

blobby

Hi:

This spring I finished construction on a single story 1600 sq ft vacation house with a concrete slab floor with radiant heat throughout. The heat for the radiant (and hot water) is provided by a Bosch Greenstar on-demand propane boiler. The contractor installed a basic thermostat to control that system. There is also a heat pump which is controlled by a Nest thermostat which can handle all the actual heating and cooling needs.

Our idea was that the radiant system could simply be used to take the edge off the cold floor, but when we tried it a few times this spring (by setting the radiant system's thermostat to 1 degree higher than the current temperature) the temperature overshot by a considerable margin and we'd wake up sweltering in a 95 degree house. I should note that the house is pretty well insulated.

So, obviously it would have been better to consider all this before construction, but given where we are today, can anyone recommend a thermostat / slab sensor combination which might be able to help with this issue? Bonus points for it being wifi enabled, since an ideal solution would allow us to switch the radiant on before we arrive this winter. I may also try setting the radiant water temperature to as low as possible, but I gather that the real problem is the thermal mass of all that concrete continuing to release heat well after the system switches off.

I'm not totally wed to the Nest for the heat pump, and if both systems could be controlled in an intelligent manner by one thermostat I'd be happy. Thanks in advance for any advice!

Blobby

kcopp

Using radiant as a "take the edge off" is not going to work. Especially w/ a slab. The heating curve is much too "flat" to do that. It will take a long time to heat up and a long time to cool off. The Greenstar FW200 control that comes w/ the boiler has a lot of features that will allow you to program the run up time so that over shooting should not happen. Did your installer go into the program and set it up? Did you install the outdoor sensor? How do you control the water temp to the slab and what temp do you have that set to? Ideally you could set the boiler w/o any mixing set up at all....just was not sure if you had any other higher temp zones.

blobby

Thanks for the reply. I did not get the FW200 control, although I do have a wall hung unit; the one I have is the ZWB 35-3. So there's no outdoor sensor, just a basic thermostat like any you'd pick up at Home Depot. This may well have been the first of these units our HVAC guy had installed; it's a rural area, and it took him a while to get it figured out. The water temperature is set by a dial on the unit; the lowest level is 95 degrees, and that's what it's set to. There's no other zones, it's all the same.

So, would you say my best bet would be leave the Nest controlling the heat pump, so that I could get some heat in the place before we arrive, then switch to the FW 200 controlled radiant system to maintain the temperature while we're there? (Assuming I buy the FW 200, despite its lack of remote access.) Regardless, I'm going to get hold of the FW 200 manual & read through it to see what it can do.

Again, I really appreciate your suggestions!

Blobby

Jamie Hall

As @kcopp says, using the radiant to take the edge off isn't going to work. A radiant slab can take days -- literally -- to change temperature significantly. In my view, a better approach to this arrangement would be to use the outdoor reset to control the water temperature going to the slab, and a slab sensor set to something reasonable -- like 65 F, perhaps -- to control the circulation (which, ideally if the outdoor reset is right, will be on all the time!). Then leave the thing alone.

You may find that you have to mix the Greenstar's output down to reach the proper temperature for the slab circulation -- it will probably be only 80 or so, even on super cold days, Your contractor should be able to help you with proper pumps and valves for that.

blobby

I guess it depends on the water temperature you pump through it - when we first tried it we had set it to 150 degrees or so and it got the slab warm in a couple of hours! Of course on reflection yeah, you're going to overshoot if you do that...

Thanks to @kcopp I came across the Bosch CT 100 which seems like a wifi version of the FW 200 which may be just what I'm after. Maybe between that & your idea of mixing down the hot water to get to 80 degrees or so we can have a comfortable slab underfoot this winter!

Yours,
Blobby

SWEI

I would be rather surprised if the boiler shipped without an outdoor air temp sensor. A modulating boiler pretty much requires one in order to comply with the 2012 DOE rules.

kcopp

As the OP says it was the installers 1st Greenstar. $20 says he just didn't install it because he didn't understand how to.... fair enough. I bet he has the package still.
I would ask him for it.
If you want it I have one still hanging around from a past install that you can have.

blobby

Great point, I'll get in touch with him & see if he has it. I'm still tempted by that CT 100 though, it sure would be nice to be able to see what it's doing from my house.

Thanks so much for all your words of wisdom!

Blobby