Need some thermostat (possibly controls) advice

ThatF_inGuyInNY

The saga continues as I move forward with my system redesign.

Background;

Adding underfloor radiant to my first floor only (heat transfer plates, fastened between joists). LoopCad revealed that this was not sufficient to fully cover heat loss at design temp. So I am keeping my cast iron rads as supplemental heat. Right now, in the design, I have all 7 rads (total, between upstairs and downstairs) on a single zone, but was planning to add TRVs to all of them.

Ideally, I would like the 1st floor rads to act as supplemental heat only, allowing the floor to cover the heat loss down to a certain outdoor temp, and only kicking on when the floor cant cover it.

two big questions;

1. should I split up the rads into two zones, one for upstairs (where they will be the only heat source) and one for downstairs where they will be supplemental.

2. How do I control them via thermostat? Ive been finding a lot of the tstat documentation not great. I see that heat pumps stats often have a aux/backup heat function. I wonder if I can use this to my advantage (can backup heat be driven by outdoor temp?). Or I can use two tstats, but that doesnt necessarily solve the problem of holding back the supplemental rads based on outdoor temp.

Alternatively, I can just set the downstairs TRVs to a temp thats colder than the radiant floor tstat set temp. This is the plan right now, but Im having last minute doubts that it will perform the way I want.

clammy

You could use a 2 stage thermostat 1st stage in floor 2 Nd stage rads will require a isolation relay and some wiring and separating of 1 st and 2 floor rads . Or the basic which would run your in floor and rads as one zone setting your trv on first floor lower then stat set point and use a single stage stat . I use the later on my system w trv on panel rads and all running at about 140 at 5 odt ,and according to pseg I’m in the top teir eff lol my house was built in the late 40 s. It may require a little playing around w radiant temps and rad temps especially if using a cast iron boiler but if a mod con I would run it all as low a temp as possible . If cast I would think of some type of low temp return protection if a mod con then no worries except lowest firing rate and short cycling.
Peace and good luck clammy

Ironman

As Clammy mentioned, do you have a cast iron boiler or a mod/con? The control strategy will be different since a cast iron boiler must be setup to maintain 140* minimum temperature. That’s too hot for a radiant floor. A mod/con can be run at the coolest temperature needed to heat the structure.

Beyond that, a simple radiant thermostat controlling the floor with the TRVs set a little lower should work fine.

If you want to ensure that the rads don’t heat until absolutely necessary, you could use an ETC sensing outdoor temperature to hold them off until needed.

I don’t see that as really necessary, and should the radiant floor have a failure, the rads wouldn’t kick on until you reached that outdoor temperature.

ThatF_inGuyInNY

Ironman said:

As Clammy mentioned, do you have a cast iron boiler or a mod/con? The control strategy will be different since a cast iron boiler must be setup to maintain 140* minimum temperature. That’s too hot for a radiant floor. A mod/con can be run at the coolest temperature needed to heat the structure.

Beyond that, a simple radiant thermostat controlling the floor with the TRVs set a little lower should work fine.

If you want to ensure that the rads don’t heat until absolutely necessary, you could use an ETC sensing outdoor temperature to hold them off until needed.

I don’t see that as really necessary, and should the radiant floor have a failure, the rads wouldn’t kick on until you reached that outdoor temperature.

Should have specified; conventional oil fired boiler however im installing a VSI system w/ outdoor reset to mix down for both the rads and the floor (proportional mix down to the floor with a "dumb" mixing valve)

clammy

Just wondering if vsi is justified usually it for larger radiant system or ones that zoned and w pumps but to each his own . I think for simplicity I would use a taco I series valve odr and return sensor for the radiant and a simple danfoss Thermic valve for low temp protection for the boiler which will cover just about any scenario which may occur ,a ecm td pump for the radiant would be nice and one for your high temp radiators of course ecm for the radiators and being your using trv s a vs pump so no dead heading or a pressure differential valve . I would also think of adding a magnetic dirt separator to help those ecm pumps and remove and magnite better safe then replacing a expensive pump cause nothing cheap any more . Are you piping the boiler into a hydronic separator or using primary secondary piping w a 2 temp system a small buffer tank possibly a 3 or 4 pipe piping arrangement may negate any short cycling while operating solely for the radiant . Mod cons w low turn down rates truly shine w a properly designed layout there’s usually zero short cycling but w a cast iron and zero turn down a buffer tank may level the short cycling. Peace and good luck clammy

ThatF_inGuyInNY

clammy said:

Just wondering if vsi is justified usually it for larger radiant system or ones that zoned and w pumps but to each his own . I think for simplicity I would use a taco I series valve odr and return sensor for the radiant and a simple danfoss Thermic valve for low temp protection for the boiler which will cover just about any scenario which may occur ,a ecm td pump for the radiant would be nice and one for your high temp radiators of course ecm for the radiators and being your using trv s a vs pump so no dead heading or a pressure differential valve . I would also think of adding a magnetic dirt separator to help those ecm pumps and remove and magnite better safe then replacing a expensive pump cause nothing cheap any more . Are you piping the boiler into a hydronic separator or using primary secondary piping w a 2 temp system a small buffer tank possibly a 3 or 4 pipe piping arrangement may negate any short cycling while operating solely for the radiant . Mod cons w low turn down rates truly shine w a properly designed layout there’s usually zero short cycling but w a cast iron and zero turn down a buffer tank may level the short cycling. Peace and good luck clammy

I had it all worked up as a 4way system. Then the i-series went backorder, and I figured a VSI system would be easier to service in the future (007s are easy to come by). The PC705-2 integrates well with a taco switching relay and serves my DHW priority. Piping is primary secondary with a VSI bridge. check the pic, im open to feedback