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NTI boiler sending heat to furnace when not needed
Hi all, new here, thanks in advance for any help you can provide!
New ish to me townhouse, 8 months or so, first winter (GTA, Canada).
NTI boiler (Tft85) - heating domestic hot water (with tank) and furnace (Airmax Maxair 50ep2)
Thermostat kept at 20c during day, 15c at night. Fan always on in winter. Humidifier installed a couple months ago.
Few days ago came home to 24.5c. Assumed it was from the sun (huge south facing windows warm up my main floor on sunny days). Thought nothing of it.
Last couple of nights I've woken up sweating to 23c+. Yesterday I swapped thermostats (there are two, one in main level that we use, and one downstairs that I keep off. I'm told they can be used in tandem but we don't.). Assumed this would expose a bad thermostat, but no dice.
Last night I turned off the furnace and air handler and now am finally getting around to dealing with this problem.
It seems that the boiler (or valves, not entirely sure how this system regulates) are sending hot water to the furnace when it's not needed. (Because the fan is running and the house is well above the called for temp) The supply water pipes to the furnace are hot as they get when heating, but the stat shouldn't be calling for heat. In the diagnostics on the boiler it shows a demand for domestic hot water, but there is not hot water being used in the house.
Where do I start chasing this? I think by swapping the stats I should have eliminated that as a problem, but should I be starting with a new stat? Should I be calling NTI?
Thanks in advance!