8 posts / 0 new
Last post
flipfriddle
What size furnace

I've hear Richard on the show say over and over to not buy a furnace that is too big or too small for your house. I'm looking to get rid of my oil fired forced air system and switch to natural gas, and I've got one quote right now specing a 66,000BTU 2-stage system for my house. I got the quote pretty quick so I'm concerned they are trying to sell me an over-powered system to cover themselves. My house was built in 1924 and has all original windows (with modern storm windows), and the only insulation is the foam panels under the 1980s era vinyl siding and some poorly done fiberglass in the roof bays. I'm still waiting on another bid so I have something to compare.

Anyone know if this seems like it's in the ballpark?

Thanks for any advice.

Timothy Miller
Re: What size furnace

Howdy, have them show you the equations an input they used to bid.... Some just install the same unit everywhere, or what similar homes have.... Self educate at the manufacturers web pages. Get more bids and be there an see if the windows and rooms are measured and insulation in attic checked an figured into what is needed.

Re: What size furnace

I can tell you that the guy only took measurements in the basement in regards to the installation. He just took my word for the insulation and window quality. I'm getting more bids so we'll see.
Thanks.

keith3267
Re: What size furnace

You show your location as Rochester, NY and from your somewhat limited description of your house, I'd say that if the house is in the 900-1200 sq ft size, this is about the smallest size heater you can get by with.

Check to see what size you have now and if it is about this size and has been adequate for you, then this will probably work. I don't think they make gas heaters much smaller than this except for portable and unvented units.

I actually would expect that a 1200 sq ft house in your location would be looking at a 100,000 btu furnace.

motoguy128
Re: What size furnace

Given limited info, if your house is 1200 sqft, it's probably abotu right, but if it's closer to 900sqft, it probably a little big.

How many supply registers do you have and what size are they?

What was you gas bill in Jan., does you bill how the average daily temeprature? What efficiency is your current furnace 80%, 75%? Standing pilot?

For example, my bill for Feb was 250 therms. Meaning I used 2.5MM BTU's over 30 days. The average daily temperature was 34F and my desing temperature is -2F. I use 70F as my indoor design temperature, that means on average, I only needed 50% of design capacity. So doing a litle math, that's 34777 BTU's. So my home at design temp needs 69444. I'll add 20% to account for increase stack effect at colder temperatures (it was a warm winter) so 83,000 BTU's. Now, I can pick between a 90 or 100k BTU furnace.

Oh... I live in a 3200sqft 2 story home with no insulation and 41 windows, 3 entry doors and full basement. I'll bet most contractors assume I need a 140k BTU furnace... or 2 furnaces.

IN fact I have 2 furnaces and the PO's oversized them badly. They are 100k downstaris and 80k upstairs. The 80k BUT furnace upstairs can recover 5F in less thna 15 minutes when it's 10F outside. It should take it 2-3 hours ot recover if it was sized properly.

I'll eventually replace them with 40k and 60k BTU furnaces and a 2 ton and a 2.5 Ton AC unit. Ya, it has two 3.5Ton AC systems.

I'll put it this way. At 35F outside, a properly sized system in ROchest, NY should be running at least 50% of the time to amintain temperature. Meaning hte time it runnign is equal ot the time it's off.

canuk
Re: What size furnace
flipfriddle wrote:

I've hear Richard on the show say over and over to not buy a furnace that is too big or too small for your house. I'm looking to get rid of my oil fired forced air system and switch to natural gas, and I've got one quote right now specing a 66,000BTU 2-stage system for my house. I got the quote pretty quick so I'm concerned they are trying to sell me an over-powered system to cover themselves. My house was built in 1924 and has all original windows (with modern storm windows), and the only insulation is the foam panels under the 1980s era vinyl siding and some poorly done fiberglass in the roof bays. I'm still waiting on another bid so I have something to compare.

Anyone know if this seems like it's in the ballpark?

Thanks for any advice.

It's impossible to say over the internet as sizing of HVAC just as it would be trying to get quotes over the phone.

However, 66k btu furnace is a small unit considering the less than ideally insulated home.

keith3267 wrote:

You show your location as Rochester, NY and from your somewhat limited description of your house, I'd say that if the house is in the 900-1200 sq ft size, this is about the smallest size heater you can get by with.

Check to see what size you have now and if it is about this size and has been adequate for you, then this will probably work. I don't think they make gas heaters much smaller than this except for portable and unvented units.

I actually would expect that a 1200 sq ft house in your location would be looking at a 100,000 btu furnace.

I pretty much agree with much of this post.

canuk
Re: What size furnace
motoguy128 wrote:

Given limited info, if your house is 1200 sqft, it's probably abotu right, but if it's closer to 900sqft, it probably a little big.

How many supply registers do you have and what size are they?

What was you gas bill in Jan., does you bill how the average daily temeprature? What efficiency is your current furnace 80%, 75%? Standing pilot?

For example, my bill for Feb was 250 therms. Meaning I used 2.5MM BTU's over 30 days. The average daily temperature was 34F and my desing temperature is -2F. I use 70F as my indoor design temperature, that means on average, I only needed 50% of design capacity. So doing a litle math, that's 34777 BTU's. So my home at design temp needs 69444. I'll add 20% to account for increase stack effect at colder temperatures (it was a warm winter) so 83,000 BTU's. Now, I can pick between a 90 or 100k BTU furnace.

Oh... I live in a 3200sqft 2 story home with no insulation and 41 windows, 3 entry doors and full basement. I'll bet most contractors assume I need a 140k BTU furnace... or 2 furnaces.

IN fact I have 2 furnaces and the PO's oversized them badly. They are 100k downstaris and 80k upstairs. The 80k BUT furnace upstairs can recover 5F in less thna 15 minutes when it's 10F outside. It should take it 2-3 hours ot recover if it was sized properly.

I'll eventually replace them with 40k and 60k BTU furnaces and a 2 ton and a 2.5 Ton AC unit. Ya, it has two 3.5Ton AC systems.

I'll put it this way. At 35F outside, a properly sized system in ROchest, NY should be running at least 50% of the time to amintain temperature. Meaning hte time it runnign is equal ot the time it's off.

I have to question much that has been posted here. In particular taking 2-3 hours to recover. If that were the case you either have immense heat loss or a grossly undersized heat system. There's no way your furnace should run 2-3 hours ---- otherwise it will be costing you a fortune.

Sten
Re: What size furnace

The only way to be absolutely certain of the correct size furnace is to make sure the HVAC Contractor does a Load Calculation, if he doesn't move on to another. Make sure you get at least 4 Quotes and make sure you see the Load Calc.

Sponsored Stories

TV Listings

Find TV listings for This Old House and Ask This Old House in your area.