Relationship between setpoint and differential
Two years ago, with much help from this forum, I replaced a 100 gallon commercial HWH with an 80 gallon Phoenix NG modcon water heater from Heat Transfer Products. This unit heats water for one side of the house and services the radiant floor heating for the whole house (7K sq ft) here in No. CA.
During a recent cold snap (temps down to 26 degrees...REALLY cold for us), the radiant heating wasn't able to maintain the thermostat setpoints in rooms set at 69 degrees (dropped to 66 overnight), despite having half the house's thermostats set at 50 degrees (closed rooms we only use when we have house guests). The installer told me I'd just have to move the setpoint on the Phoenix higher during cold weather. It's already at 145 degrees and the differential is at the factory default of 7 degrees. Next Christmas, when we have a full house of visitors demanding warm rooms AND hot showers, I'm concerned that even with the setpoint at 180 degrees I couldn't keep the house warm and/or deliver DHW. There's a mixing value to prevent scalding so my biggest concern is whether this unit can simply meet the demand.
When cold weather sets in, will it be better for me to a) raise the setpoint or b) lower the differential or c) some combination of the two? Thanks for any suggestions you can offer.
Here's some additional info, Jim...
I didn't find any indication in the Phoenix manual regarding installation of an outdoor reset control so I'll check with the manufacturer on that. Regarding the radiant system, the floors are limestone or w/w carpeting over gypcrete or concrete slab. And, as I understand it, the heat exchanger thermostatically limits the radiant output to 85 degrees, i.e, no danger of overheating. But I don't know if that limit can be adjusted or what the system implications of doing so would be.
Can you help me understand the implications of raising the boiler's setpoint vs. lowering its differential?