Decent free online heat loss calculator

epmiller

Somehow I recently misplaced my heat loss info that I used for sizing heat emitters for a room and rather than spending time looking for it I just went on line and searched for "heat loss calculator". The resulting list gave me some WILDLY different recommendations of what was needed. Some I didn't bother finishing because of all the fine technicalities they wanted and others gave me no way to make even ballpark inputs for things like wall insulation and windows. I mean really, "no insulation, mediocre insulation, very well insulated…" I couldn't even differentiate between 4" and 6" walls. One even told me I needed twenty some thousand BTU of radiation for one bedroom. If anyone can point me to a reliable free online calculator I would appreciate it. More than likely there will be others that will use it too. Meanwhile I have to go hunt for the SlantFin app I had years ago and see if it's still around.

hot_rod

I think Burnham, US Boiler has a very basic calculator

I use Siggys HeatLoadPro. There is a free demo that may give you enough info? Simple to use, allows you to build various walls, etc. It is a very affordable program :)

https://www.hydronicpros.com/shop-online

GroundUp

Builditsolar.com has a free calculator that takes some getting used to, but is usually within 5% of the professional programs when the appropriate values have been entered. Slant/Fin was well known to pad their numbers by sometimes up to 25%, so be aware with this one that it's going to look low in comparison. I have used it many times and it hasn't let me down yet.

jesmed1

I've also used the free builditsolar.com calculator and have found it in good agreement with our actual heat loss.

EBEBRATT-Ed

I still have the Slant Finn CD. Had to get an external CD drive to use it on a newer computer. Haven't used it in a long time but I thought it was accurate enough. I don't feel it was oversized.

DCContrarian

The free calculators are worth exactly what you paid for them.

Houses are like snowflakes, every one is unique. There is no such thing as a "normal" or "typical" house.

The best method is to use your fuel use history, as outlined in this article:

https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

dabrakeman

Yes but he is looking for emitter size for a single room.

epmiller

@DCContrarian This actually is a whole house heating job but fuel use numbers are useless because of the addition of a complete second floor and the previous owner mostly heated the place with a coal stove that was situated right in front of the heat pump air return. No other fossil fuel used. I put floor radiant in the second floor during construction with mini-split heat pumps for AC, haven't needed them for heating but it's there. Two years later I'm now adding staple up radiant heat to the first floor because the owner doesn't like the cool air movement from the original heat pump when it's cold outside. It's not a fantastic duct/distribution system and the far end of the house is always cold. I have to put panel radiators in a couple rooms and I wanted to size them correctly with available water temperatures below 130°. This is definitely a one-of-a-kind job.

Thanks everyone else. I actually used that builditsolar.com website and liked it but I was wondering if there was anything else I had missed. I'm trying to retire so I don't want to invest in any major software but people keep asking me if I can just do one more job for them and it's hard to say no when I actually enjoy the work. I'll keep watching here if someone comes up with any other good calculators.