Extremely short cycles -- heat loss calculation for new boiler

ineffable

My boiler is a 151,000 BTU Weil McLain WTGO-4 (oil and hot water, located in CT). After adding insulation to much of the house, insulating pipes, sealing drafts, installing networked thermostats to synchronize calls for heat, installing a smart circulator and new Hydrolevel, and switching to an indirect tank and cold start, it is extremely short-cycling--anywhere from 2 to 60 seconds once every ten minutes in winter temperatures, while keeping the house at 60-65°F and the water at 140° F. I.E., the boiler is running at most 25% of the time, and more frequently <10%. The burner cuts off at 190°; the boiler temperature gets up to ca. 216°.

The boiler is leaking, so I would like to replace it with a Buderus G115. A heat loss calculation yields 74,000 BTU at an indoor temperature of 55° F or 89,000 at 65° F, when it is 0° outdoors. With milder winters becoming the norm, during the uncommon cold snap, when it's zero out, it's nearly always night--and the thermostats set back to 55. It seems odd to pair a daytime indoor temperature with a nighttime outdoor one. It is also hard to square a heat loss of 74,000 BTU with a 151,000 BTU boiler that runs well less than half of the time.

I am not sure what size Buderus to buy (74,000 or 95,000). Any advice would be appreciated!

F

Jamie Hall

74.000. Bigger is not better...

hot_rod

Keep in mind the boiler BTU may be input not output. 95,000 x 87% = would be closer to 82,000 actual output. Use the DOE number. Also, someone may want to keep the home warmer than 65F someday
So size it to a more common 70F at design. It's common as we age to want to have warmer, 72- 74F indoor. So don't shave it too close to the load number.

Tinman

55 degrees is a very low indoor design temperature.

EBEBRATT-Ed

@ineffable what @Steve Minnich said is correct. You should use 70 deg indoor temp for heating. If your getting permits the local CT building dept may ask for this information (not that its never not fudged) but you will sell the place someday

Also check your radiation against your heat loss calculation. Run the water at the lowest temp your baseboard will allow ....you will save fuel You don't want the boiler up to 216

Jamie Hall

At least in my community (and years ago, when I was the villain), the heating system must be shown to be able to maintain 70 inside with the design temperature outside to obtain a certificate of occupancy -- and therefore a mortgage or whatever.

Not, as @EBEBRATT-Ed said, that it's never fudged.. but...

D107

Does recovery from planned night setback factor into a heat loss calc? If you base your heat loss (let's say 50Kbtu) on 70º day temp, set back 5º at night on a design day how much higher will your heat loss be each morning?

Inliner311

You might have a setting wrong in your thermostat. It could either be cycles per hour or mininium on time. It could be a combination of the both. You seem to have it set at 6 cycles per hour when you should have it at 1, 2, or 3 per hour depending on what feels best. Also you will want to check if you have a setting for mininium on time. This is the mininium amount of time the boiler will need to be on before the thermostat will call for heat. So if you have it set for 5 minutes the thermostat will wait till it thinks you need 5 minutes or more of heat to call for heat to the boiler between cycles. It might skip a couple cycles to get to that point.

Jamie Hall

D107 said:

Does recovery from planned night setback factor into a heat loss calc? If you base your heat loss (let's say 50Kbtu) on 70º day temp, set back 5º at night on a design day how much higher will your heat loss be each morning?

No, it doesn't. The extra fuel required to bring the space back up isn't related to the outdoor temperature -- the heat loss to outdoors will be the same for a given indoor temperature, whether it is steady or rising. Where the energy goes is into raising the air -- and all the contents of the house and the walls etc. -- back up to the upper temperature. And that can be a quite surprising amount of fuel...