Two scenarios. Which is better?

cesc

Member
Sep 3, 2017
8
New Jersey
Scenario 1: A house with a fireplace insert on the first floor and two heating zones where zone 1 is the first floor and zone 2 is the upstairs where the bedrooms are. Zone's 1 thermostat is set to 62. Zone's 2 thermostat is set to 64. It's a cold night, you're sleeping, the fire starts to die down. At 1 AM, zone 2 calls for heat and the boiler and zone 2's circulator run for 30 minutes. At 2 AM, zone 1 calls for heat and the boiler and zone 1's circulator run for 30 minutes. Throughout the night, this keeps happening: one zone calls for heat, the boiler and circulator for that zone run for a while, stop and some time later the other zone calls for heat, which makes the boiler/circulator run for a while again.

Scenario 2: Same setup as scenario 1 except that when one zone calls for heat, the circulator in the other zone also turns on. Example: it's 1 AM, zone 2 calls for heat and the boiler and zone 2's circulator run for 30 minutes. At the same time that zone 2 called for heat, zone 1's circulator also turns on and runs until zone 2 stops calling for heat.

My question: which scenario is better in terms of saving on oil and keeping your house warmer overall and why? Please don't change the scenarios when you answer.

My thinking: since the boiler is already running when one zone calls for heat, leveraging that work to heat the other zone at the same time is more efficient than heating each zone individually. In other words, I think that by doing this you end up spending less oil and keeping the house warmer overall.

Curious to read what you guys think about this! Thanks.
 

cesc

Member
Sep 3, 2017
8
New Jersey
Not seeing where there would be much difference.
Looking at the 1 to 2:30 AM time period as described in the scenarios, the total running time for the boiler and circulator is 1 hour (half hour for zone 1, half hour for zone 2). My theory is that with scenario 2, the overall total running time will be less because it will take zone 1 less time to come up to temperature when it calls for heat given that it piggybacked on zone 2.
 

maple1

Minister of Fire
Sep 15, 2011
10,357
Nova Scotia
Yes, but during that 'combined run time', there will be twice as much heat drawn from the boiler with 2 zones drawing vs. 1. So theoretically the boiler would need to run more, if it was sending more heat to loads. It should cycle off & on while heating - would just be a difference in how much cycling it does.

The basics are - the house will draw heat from the zones as it needs it. And the boiler will make enough heat to satisfy the house. If one scenario keeps the boiler hotter for longer, that might make for more standby heat up the chimney - all other heat generated should make its way into the house envelope. But IMO that would be the only heat that could be saved, and it might not be any if your boiler is kept at the same temps all the time whether zones are calling or not. Aside from cold start boilers, most are maintaining a constant boiler temp.

No idea on your boiler or settings - there might be easy savings to be had by simply changing them.