Any advice on how to factor a gas fireplace into a Wrightsoft Manual J model?
Seems like it would reduce the required heat from the central gas forced air system in that room. But not quite sure how to include this in the model.
Any advice is appreciated. This is my second Manual J model.
Ultimate Home Performance
I would not include it. Given that it is likely much less efficient than the gas furnace, you don't want to count on the occupants using it when the outside temperature approaches the design temperature. However, if it is atmospherically vented, you will need to include the flue in the air leakage model.
Most fireplaces a just a hole that lets heated/cooled air out chimney. If the heated hole(fireplace) is use much just add more CFM to vent rate. Most fireplaces are for looks and not a real way to heat the building (eye candy). I find a open fireplace is negative 5% Eff.
I agree, that fireplaces are for looks and not heat. There are those who insist on having a fireplace, especially gas, where the vent is always open. The fireplace needs to be covered with insulation to stop heat/cooling loss. Fireplace Fashion makes an insulated, magnetic fireplace cover to be used when the fireplace is not in use. This stops ;the air flow in and out of the chimney, thus stopping any heat or cooling lost dollars. More information can be found at fireplacefashion.com.
Wrightsoft has an option to include a fireplace in a zone. I would use that option since it complies with the Manual J instructions. If you use some other method (hole or heating source) you should make sure your method complies with Manual J.
What kind of gas fire place? Gas logs... in "generic brick fireplace" with dampers that are left open? Or a sealed combustion fireplace insert? With a pilot or pilotless?
If you've got a high performance envelop - and an efficient sealed gas insert.... they can do quite well in heating one end of the house.... and not be a massive air leak. We have two...in our house.. and it tests at 2.8ACH50.
If the units are cheap - and don't seal up... treat them as a brick fireplace with damper stuck open.
I put a 2 Inch foam cover and painted it black and dropped the heat loss by 3/4. You can not use it a fire place till remove the cover. Most cast iron damper are so bent I can not close. Just say NO to fireplaces and put in the 90% heating system. Over 1/2 the high CO probems I go to are with fireplaces
Sealed combustion fireplace insert. No pilot. I believe it can be hooked up to a room thermostat.
some can get up to 92% with PVC vent. Some use 24 volt thermostat that can be +- 2 dead band with heating thermostat
When you model, add a small additional area -- say less than 10 sq inch of for possible air leaks...(that should be real high)... be safe and model the fireplace at 60%... efficient. It really should be used just occasionally not as the primary heat source...
With an insert - it really isn't the driver for your heat loss in the house.
ACCA Manual J does not consider any heating (BTU gain to a room) from a fireplace. Manual J only allocates infiltration to the home caused by the fireplace's construction quality.
I think that was part of his concern. Its a sealed combustion unit. So the infiltration that might be used for an ordinary fireplace is going to be off the chart. If its a new home with high end sealed combustion fireplace, correctly installed per manufacturers design... the infiltration might be closer to that of a tankless hot water heater.
A. The point is that ACCA Manual J does not model for the Btu gain from fireplaces.
B. Fireplaces are a part of the infiltration load in ACCA Manual J. You have a choice as to construction quality - Tight, Semi-Tight, Average, Semi-Loose and Loose. It is all spelled out in the actual Manual J manual published by ACCA.