Help on optimizing outdoor reset curve?

nuclianba

Homeowner question:
Im in a new (to me) house and just replaced an ancient boiler with a TT mod con unit. Starting to turn on heat for our first time and not sure what I should expect in terms of time to heat etc (my previous home was forced air, so this is my first radiant system). The system is radiant slab on grade, installed in 1953, so copper lines with wide spacing (looks from my IR cam to be around 18" OC).

The system has been on for 8 hours now and the temperature on the thermostat hasn't budged (setpoint of 68, reading 66) and the floors feel barely warm (wood flooring, the bathroom tile feels moderately warm...I wouldnt even know the wood feels warm except the house has a small addition and you can feel the point in the floor where the radiant ends and the addition, which does not have a radiant slab, starts). Boiler is reporting its running the water at 106.

Are there recommendations on how to set/optimize the outdoor reset curve (for what its worth, im in the san francisco bay area)? I also have a nest thermostat which tries to adjust when it calls for heat based on the time it takes to warm the slab etc...should i still try using that, or is the best mode to set and forget, essentially, at one temperature and let the boiler do the modulation?

Thanks for helping a newbie!

Leon82

It will take a while for the slab to come up to temp And effect the thermostat.
Let it run for a couple days. If it's not raining the temp you can add 5 degrees to the curve and see what it does

Leon82

Ditch the nest. The recovery time for the slab is too high for it to work

Ironman

The NEST is the wrong stat for a modulating boiler; it works counter-productive to the boiler's logic. It's also the wrong stat for a radiant slab. I'd recommend a Tekar 518 which is made specifically for a radiant floor.

As far as your ODR curve goes: the only proper way to get it close is with an ACCURATE heat loss calculation matched to what output the floor is capable of in btus per sq. ft.

If we knew the load for your house, the boiler, pump and piping size, we might be able to get you in the ball park and you would have to play with it from there.

Also: you don't want to use any setback with a radiant slab; it has very slow response and setting the stat back will only be more counter-productive. Put the Tekmar on it, set it and forget it.

Rich_49

Nests are for birds . Get rid of it .

nuclianba

haha ok no nest. The house had the original thermostat from 1953...just a mercury switch, so i wanted to change that.

The boiler is a triangle tube CC125H. Pump is a Grundfos UPS26-99FC set on low speed. The header looks to be 1.25" copper and there are 6 returns, all 1/2" that merge in to a single 1.25" return. The whole house is one heating zone, ~1500 sq ft on-grade slab. House is mid-century modern, so lots of windows (all double pane/argon filled) and no attic (flat roof with beams/TNG). Any thoughts on the best ODR curve to start with is most appreciated.

On the thermostat: Currently I have a single zone relay next to the TT boiler, with the two wires connected to the thermostat jumpers on the TT. If I replace my thermostat with the Tekmar, im assuming it uses the same 24v hot/signal/common 3-wire setup from the relay box. I see the Tekmar talks about using PWM to control the boiler...I'm used to PWM meaning very very fast switching, like for controlling an LED brightness or something...Im assuming here the PWM cycle time is on the order of 15+ minutes to keep the boiler from short cycling? its not turning on/off thousands of times a second...right?