How do you size a furnace?
Title says it all ... I have a 1957 house, and am doing a complete-gut remodel. furnace (gas fired, forced-air) looks to be original, and lacks its' nameplate.
Furnaces have come a long way in fifty+ years.
Can anyone point me to a reliable site, so that I may choose a furnace that's not too big?
do your homework on finding a top notch contractor and the rest will fall into place
as they then will do a heat loss / gain calc. and be there after the sale to service and back their work
you could go on the contractot map of this site as a place to start
First, you need to know the winter design for your area; then a manual J room by room heat-loss calculation should be performed. While you're doing it, also do the heat-gain calc for cooing.
There is helpful software that costs very little to use for your home; do a search.
Or, you can pay someone to do the heat-loss calc.
Better yet, is to 'first' have a "Home Energy Efficiency Rater/Audit" performed.
Then do all the high return on investment things to reduce the heat loss so a smaller furnace can be installed.
$49 to register for homeowner use
How do you size a furnace - Let's see. Judging by most everything I see, you put in the biggest furnace that will fit in the physical space. THE VERY BIGGEST!
Almost everything I see is over-sized.
Get a load calculation.
"Hey Lama, hey, how about a little something, you know, for the effort." And he says, "there won't be any money, but when you die, on your deathbed, you will receive total consciousness." So I got that goin' for me, which is nice. - Carl Spackler
Reno is that you?
Renosteinke is an electrician and moderator on the Electrical Contractor Network blog. That is you, I hope.
As stated, a heat loss calc is the only correct way to size it. Once you've done an accurate one, size the furnace based on its OUTput rating to match the calculation. Don't over-size; that's about the worst thing you can do. It's not like sizing an electrical service where adding a little capacity is a good thing. You only need the full capacity on the expected coldest night of the year (aka design temp). At any temp above that, the furnace will be over-sized already.
If the calc puts you on the border line of two furnace sizes, go with the smaller one: all the programs are padded for error by 10-15%.
I'd also recommend that you look at a modulating furnace that varies its firing rate to match the load. You get much better comfort.
Another thing is your duct system: it's gonna need to be analyzed to see if it's sufficient to handle the increased air flow that newer furnaces produce. It will most likely need modification, especially for A/C.
You can choose to do what you want, but you cannot choose the consequences.
Thanks, Bob, for the reply. Yes, that's me on the electrical forum
I agree about not over-sizing stuff; that's been a real issue I've had with the air con guys as well! I also like the 'modulated,' or fancier stuff. Make a regulating burner, use a multi-speed fan, and the thing can run continuously at a quiet, low setting .... much better than having to constantly adjust the TV volume is response to the blower roar!
Now ... here's the challenge .... let me share the data, so you guys can see for yourselves:
1000 sq ft house. 25x35 footprint. Narrow side south. Single story, over a crawl space. 2x4 walls with iffy insulation right now. Many ceiling areas lack any insulation (limited access). NE Arkansas (72315)
Current winter gas bill - heat only - is over $120/mo. House is bleeding heat- to the point where the carport thermometer routinely reads 10 degrees over the yard thermometer.
In the course of the remodel, the walls will be filled with foam. Attic issues will be addressed. "Central" air con will be installed - as three 1-ton mini-splits. Existing gas furnace will be replaced with a new one.
I've been to a few 'load calc' sites, and they consistently come back with a heating load of less then 3 tons. However, nearly every furnace I find is easily twice that size. Central furnaces seem to start at 80,000 btu's.
So ... why bother at all with a central furnace? Three reasons:
1) The same load calcs say I'm pushing the 'heat pump' capabilities of the mini-splits;
2) House already has central gas furnace & ducting; and,
3) I really want the heat to be available when the power fails - which it did last Christmas, for 11 hours!
Hyperheat mini splits will give you fantastic performance even down to zero or below. If your going to e doing mini splits anyways I would look at that option for sure. Your load calc is going to come out pretty tiny. As for the backup heat?? Put in a generac generator and live life
Central furnaces start at about 40,000 BTUs.
I think I have seen a 2 stage 20 40 also
Originally Posted by beenthere
What you need as many have posted is a load cal. To detmine what size a/c you need! Then the contractor should be able to size the furnace to match the load as long as what ever load cal says the home needs to cool.
In other words you need to know the a/c size via load cal to determine what size drive/ton systems needed to move the right amount of air flow for a/c and heat.
Depending on the load of your home you can get as small as 40,000 btu furnace which will have a 2 ton drive. If that matchs what size a/c unit you need and load shows that 40,000 btu input with 80% furnace will provide roughly 32,000 btu of heat.
Without a load cal no one would truly know what size furnace and a/c you need. Example if your home only requires say a (2) ton a/c unit then going with 40,000
Btu furnace with a (2) ton drive should meet your needs for heating and cooling? That is why load cal is so important to sizing not only a/c unit but also heating system.
Multiply the rating by the efficiency? Now, there's a step I had not thought of. Ironically, that might push me toward a less efficient unit - surprise!
Living in this (somewhat dated) house has been a real education. For example, I had never before appreciated just how much heat can leak out the uninsulated floors! Or, for that matter, the importance of attic airflow in the summer.
Generac generator? Please don't tempt me to use the language that comes to mind! Well, generators are another topic entirely, as are folks' expectations.
The place is a real mess right now, but I expect in a few years to have a rather comfy little Hobbit-hole. Getting the HVAC right is an important part of the plan.