Heat loss calculation - what about setbacks?

bslne

If I understand correctly, a heat loss calculation as used for sizing a heating system is just looking at maintaining a steady state. What about a house where the residents want large setback at night or for vacation? Is there a good way to determine the additional BTUH required to bring the space up to daytime temperature in a certain amount of time?

Hot_water_fan

You'd oversize it by a bit, which is likely going to happen naturally since the appliance isn't going to be exactly the right size. Remember, the heat loss is on design day - so while a 100% perfectly sized system would struggle to come back from a setback, it would just be on the coldest days. So if you’re 25% oversized on a day with 90% of the heat loss, you’ll have ample capacity. A deep vacation setback might be more challenging. A WiFi enabled thermostat would probably be the most comfortable option.

Jamie Hall

Just to add to @Hot_water_fan 's comment -- it depends a lot on how fast you want to recover. If you are talking a large setback -- say 10 degrees -- and recovering every morning, then the usual slight oversize will probably do. It will take a couple of hours (a sort of ballpark is around 5 degrees per hour for the average sort of residential system). On the other hand, if you are looking at a larger setback, either plan on spending a day or so getting the place back up to temperature or have a way of starting the recovery early.

There are methods for computing the recovery time and heat required, but they require knowing a good bit more about the structure and its contents than one usually has available -- and unless one is working on a high value or industrial application, not worth the effort involved.

hot_rod

recovery depends a lot on the heating system. Forced air is quick. Concrete slab not so much.

The Steam Whisperer

hot_rod said:

recovery depends a lot on the heating system. Forced air is quick. Concrete slab not so much.

oh course with "quick" forced air, the air will be 70F, but everything will still be cold. For churches, I usually figure about 30% extra capacity for recovery... that keeps it slow enough so the objects get a chance to warm up. Like already said, for 90% of the heating season, you'll have about 60% extra capacity to recover with, so its not really a big issue. Besides if your structure drops 10F overnight during a typical winter day, then the structure probably needs some thermal upgrades.