Is it OK for boiler to run constantly?

[Maine Way]

I have installed a new MOD CON boiler and I've set the system temp to 115 degrees, which gives me a EWT of 109degrees. At this setting it condenses abundantly and uses fuel efficiently. But, it needs to run constantly.

The system is only partly running. The one zone that is connected is a radiant floor comprising 70 per cent of the downstairs, and nothing upstairs. The remodel is still underway, and the one zone is keeping the whole house at 50 degrees, and the rooms it serves directly at 62 degrees. Outside temps have been about 25 degrees.

I expect once the rest of the radiation is connected, it will satisfy thermostats more readily, although this is just a guess.

Is it better to use a higher system temp to allow the boiler to cycle, or maximize fuel use efficiency with lowest temp to get the job done?

SHAWN

 

[Jadnashua]

Each time you start/stop the thing, it wastes some energy. If it can run continuously, it should be most efficient, and things tend to break when turning on/off, so just on should be fine. Comfort is highest when it's running continuously rather than bouncing up/down.

 

[Sponsor]

 

[Maine Way]

Thanks Jim! Very helpful.

 

[Dana]

What Jim said. Not only are long low temp burns are more efficient- it's easier on the equipment overall. Cycling on/off adds thermal mechanical stresses due to the large and rapid temperature swings when re-firing, and every ignition cycle uses up another tiny hunk of the ignition subcomponent service life. Firing 5x/day is WAY easier on the equipment than firing 5x/hour. The idealized setup is when the thermostats are (almost) NEVER satisfied, but the indoor temperature doesn't lose ground, when the boiler burns at a nearly 100% duty cycle, modulating the fire up/down in response to outdoor temperature using the boilers "outdoor reset" function.

Since you are currently running only part of the total system radiation (and at a very modest 62F zone temp to boot) it's possible that even 5+ burns/hour could be construed as "running constantly" if the system is taking forever to satisfy the thermostat, where the pumps are running constantly, yet the burner is cycling on/off Hopefully that is not the case here(?). Observe the boiler operation closely, and verify that the burns are in fact constant (or at least 3 minutes/burn, fewer than 5 burns/hour) during the call for heat. Modulation ranges aren't infinite- there is a minimum firing rate, and to prevent cycling the radiation needs to be emitting heat at the same rate that the boiler is with an oversized boiler it's common to see cycling (even short-cycling) when only one zone is calling for heat. If you are seeing sub-2 minute burns, 10+ burns per hour it's time to crank up the boiler temp 5-10F, whatever it takes to reduce the total number of burns, and lengthen the length of the burns.

With enough thermal mass in the radiation (is it a radiant slab?) the burn times will often be long enough that it doesn't matter- the total number of burns/day can still be low enough to not really matter (for both efficiency & longevity).