Temperature setting on new mod-con boiler/water heater
With help from contributors to this forum, I've had an HTP Phoenix mod-con boiler water heater installed but am now a bit confused about what settings make sense. The unit heats both DHW and a hydronic heating system. I've set the thermostat on the boiler to 130 degrees and the factory setting on the differential is 7 degrees. But I've noticed that the upper tank temperature has dropped as low as 113 degrees and the boiler is still not running at full tilt. The fan speed at that temperature was only 490 RPM while I've seen it run as high as 700 when it's going full bore.
Is it reasonable that there should be a drop of 17 degrees from the setpoint yet the boiler is still not running at top speed to bring the temp back into range? I assume this may have something to do with balancing precise temperature maintenance with energy efficiency. But since 120 degrees is recommended as the minimum temperature for sterilizing dishes, it appears that I may have to either lower the differential or raise the boiler's thermostat to 140-145 to ensure 120 as a minimum. There's a mixing valve on the DHW to prevent scalding but having the higher thermostat setting will compromise my energy efficiency.
Here's some more info, Jim...
The Phoenix is like a boiler with an indirect tank but it's all one unit; the boiler's combustion chamber and heat exchanger are inside the tank with the boiler control unit mounted on the side of the tank (more info here: http://www.htproducts.com/phoenix.html). My radiant heating system pulls water from the tank and uses a heat exchanger to heat the PEX tubing water to about 85 degrees. So far as I know, there's no outside reset such as you have on your system. The radiant is simply controlled with thermostats in each room, turning the flow on or off as needed. The thermostat on the boiler responds to the temperature sensor at the top of the tank and that's the temperature that's displayed on the control panel (although you can also have it tell you the temp at the bottom of the tank as well as fan speed, ionization current and several other readings that I don't fully understand).
I just learned that I have to go out of town tomorrow morning and won't be back until December 10 so I apologize if I can't respond until then.
Thanks for any help you can give me in the meantime.
Thanks for the explanation, Jim.
I'll look to see if there appears to be a check valve in the DHW system. As to the capacity of the system, I live in northern California with minimum temps in the high 20's. It remains to be seen whether this new system will work for this house. It was installed this summer and we haven't yet been through a winter. In addition, we have 14 radiant thermostats and keep a number of the rooms at 50 degrees unless we have house guests. We haven't yet had house guests during the cold weather and that'll be the acid test of this system.
I'm still interested in learning if anyone can answer my question about lowering the setpoint of the boiler during the winter when we're on vacation. Anyone?