Gas furnace BTU- how much do I need for my home in NJ?

[shabby]

just have the seller give you $5000 back at closing, that should get you a decent unit. you can continue to use the current unit until you've done your research, you might get a few more yrs out of it, why the rush?

Because when it stops working during the coldest day of the winter you'll be asking yourself why didn't you replace it sooner.

 

[EliteRetard]

Because when it stops working during the coldest day of the winter you'll be asking yourself why didn't you replace it sooner.

Or when it does this:

 

[Red Squirrel]

My furnace is 60,000 BTU for a 1100sqft house. I find it slightly under sized but it works, and when I insulate my basement it will probably be ok. Recently got a new patio door and front window installed so that made a small difference in how fast I loose heat as well. So for 2300 sqft I'd probably go with at least 120,000.

Mind you there are lot of other factors like how well insulated it is, how many windows, if the bigger windows face south, etc...

 

[shabby]

My furnace is 60,000 BTU for a 1100sqft house. I find it slightly under sized but it works, and when I insulate my basement it will probably be ok. Recently got a new patio door and front window installed so that made a small difference in how fast I loose heat as well. So for 2300 sqft I'd probably go with at least 120,000.

Mind you there are lot of other factors like how well insulated it is, how many windows, if the bigger windows face south, etc...

Do you live in the northwest territories or in a log cabin? I have 100k btu furnace that runs at 60k capacity in the 1st stage and its enough to heat my 3000sqft house.

 

[Greenman]

Manual J is your friend.

 

[Red Squirrel]

Do you live in the northwest territories or in a log cabin? I have 100k btu furnace that runs at 60k capacity in the 1st stage and its enough to heat my 3000sqft house.

My house is pretty old, so it has sub par insulation, well the insulation is not bad, I think it's mostly R12 batts, but vapour barrier is not installed all that great and seams are not tapped properly or even tied in properly with next sheet, at least from the places I've seen where I took drywall out.

My basement is also not insulated, that's my next project, I want to get that spray foamed. I only really notice the furnace having trouble keeping up when we get -30's and colder but it's not to the point where I'm freezing, I just can't expect to keep the whole house at 23C without having it running non stop. I typically let it drop down to like 13C otherwise it's running way too much. That 75% gas hike a few years back hurt a lot.

 

[shabby]

What's on the outside, siding or brick? If its siding you could still re-insulate when replacing siding, that is if you plan to stay in the house for a long time.

 

[Red Squirrel]

It's brick, so it would be quite the job. It would also be hard to fix vapour barrier from that side. Honestly if I had not repainted the whole house when I moved in, I'd probably just take the drywall down one room at a time and fix it. The vapour barrier is probably the bigger issue than the insulation is. Back then it was actually a new requirement, and it was often sub par. If you don't tape the seams properly and tie it in with everything properly it defeats it's whole purpose.

 

[AlFrugal]

Manual J is your friend.

Easier said than done. I've had two estimates from highly rated companies for a 915 square foot ranch home in southern NJ (5000 annual heating degree days). Both took no interior house measurements and they did not examine the ducts. They both recommended 60K-65K BTU 80% AFUE single stage furnace. Both companies are members of ACCA (the publisher of Manual J).

ACCA publishes a Quality Installation Checklist for homeowners to evaluate heating contractors, as well as a Quality Installation Standard for contractors. Both of the companies I tried (ACCA members) would score a zero on the QI checklist. The checklist includes using a Manual J for sizing. I would very much like to find a heating contractor that adheres to the ACCA QI standard. So far, I have little hope of finding one.

I plan to check out loadcalc.net for a DIY Manual J.

 

[Greenman]

Easier said than done. I've had two estimates from highly rated companies for a 915 square foot ranch home in southern NJ (5000 annual heating degree days). Both took no interior house measurements and they did not examine the ducts. They both recommended 60K-65K BTU 80% AFUE single stage furnace. Both companies are members of ACCA (the publisher of Manual J).

ACCA publishes a Quality Installation Checklist for homeowners to evaluate heating contractors, as well as a Quality Installation Standard for contractors. Both of the companies I tried (ACCA members) would score a zero on the QI checklist. The checklist includes using a Manual J for sizing. I would very much like to find a heating contractor that adheres to the ACCA QI standard. So far, I have little hope of finding one.

I plan to check out loadcalc.net for a DIY Manual J.

"Both took no interior house measurements and they did not examine the ducts."
That right there is why they would score a zero on the checklist. What they're doing is an educated guess, and as long as they oversize the system, it will work no matter what they overlooked.
There is a big difference between a system that will get the job done, and one that's efficient. A properly sized system with the correct air flow to each room will result in a more comfortable home, and reduced energy bills. Designing that system takes some effort and thought, and a lot of HVAC company's don't want to do it. They want to get in and get out. So they oversize the system and often use the existing duct's without even checking them for leakage. I've run across houses with new equipment and ducts completely opened in the crawl space and attic.

Right now I'm remodeling a house in CA, I've added 650 square of living space, and I'm reducing the furnace size from 70k btu's to 45k btu's. The place will be just over 1900 sqft when complete. It will also be very comfortable and energy use will be less than it was before the addition.

 

[Joe C]

Easier said than done. I've had two estimates from highly rated companies for a 915 square foot ranch home in southern NJ (5000 annual heating degree days). Both took no interior house measurements and they did not examine the ducts. They both recommended 60K-65K BTU 80% AFUE single stage furnace. Both companies are members of ACCA (the publisher of Manual J).

ACCA publishes a Quality Installation Checklist for homeowners to evaluate heating contractors, as well as a Quality Installation Standard for contractors. Both of the companies I tried (ACCA members) would score a zero on the QI checklist. The checklist includes using a Manual J for sizing. I would very much like to find a heating contractor that adheres to the ACCA QI standard. So far, I have little hope of finding one.

I plan to check out loadcalc.net for a DIY Manual J.

Try here for DIY Manual J. I used the Slant Fin program on my house and discovered my boiler was almost twice as big as needed to heat the space. Wasted $$ every time it runs.

http://www.doityourself.com/forum/b...646-slant-fin-taco-heat-loss-calculators.html

 

[rh71]

"Both took no interior house measurements and they did not examine the ducts."
That right there is why they would score a zero on the checklist. What they're doing is an educated guess, and as long as they oversize the system, it will work no matter what they overlooked.
There is a big difference between a system that will get the job done, and one that's efficient. A properly sized system with the correct air flow to each room will result in a more comfortable home, and reduced energy bills. Designing that system takes some effort and thought, and a lot of HVAC company's don't want to do it. They want to get in and get out. So they oversize the system and often use the existing duct's without even checking them for leakage. I've run across houses with new equipment and ducts completely opened in the crawl space and attic.

Right now I'm remodeling a house in CA, I've added 650 square of living space, and I'm reducing the furnace size from 70k btu's to 45k btu's. The place will be just over 1900 sqft when complete. It will also be very comfortable and energy use will be less than it was before the addition.

I'm trying to understand something - why have it sized for EXACTLY what you need? You just mentioned expanding the home yourself, so it's not out of the question that it may happen for others down the road. There's also the issue of degradation over the years - more heat loss within the home.

We purchase many things every day and we never aim to purchase "just enough". If you have natural gas, it isn't expensive and neither is the electric it takes to run the system. Lastly, these boilers jump from 80,000 BTU to 100,000 BTU for the next level up so getting it exact may not even matter.

For the money you're spending on getting a company to measure you, you're losing any perceivable savings anyway. I realize people treat this like a science but in practical terms it provides no real benefit because of negligible cost differences.

 

[Joe C]

I'm trying to understand something - why have it sized for EXACTLY what you need? You just mentioned expanding the home yourself, so it's not out of the question that it may happen for others down the road. There's also the issue of degradation over the years - more heat loss within the home.

We purchase many things every day and we never aim to purchase "just enough". If you have natural gas, it isn't expensive and neither is the electric it takes to run the system. Lastly, these boilers jump from 80,000 BTU to 100,000 BTU for the next level up so getting it exact may not even matter.

For the money you're spending on getting a company to measure you, you're losing any perceivable savings anyway. I realize people treat this like a science but in practical terms it provides no real benefit because of negligible cost differences.

A properly done Manual J will give a conservative estimate on BTU loss during the COLDEST expected day of the year. So buy default the system will be slightly oversized. In my part of NJ for example, the coldest expected temp is 0 degrees F. So here you design a system to maintain the desired indoor temp for those days it's zero outside, any other time the equipment could be considered oversized. The goal is to replace heat at the same rate as the building loses heat. If you throw to many BTU's into the space to fast you'll get temperature swings that can make the occupants feel uncomfortable. It's similar to being stopped at a red light and when it turns green you smash your foot into the accelerator just to get to the next light a block away as opposed to gently pushing the accelerator to get there. Replace the lost heat slowly.

Before a Manual J is done, you should "tighten the envelope" first, caulk holes, repair/replace drafty windows and doors, add insulation to the attic etc. Drafts will make any system work harder and make the space feel colder.

 

[AlFrugal]

I'm trying to understand something - why have it sized for EXACTLY what you need? You just mentioned expanding the home yourself, so it's not out of the question that it may happen for others down the road. There's also the issue of degradation over the years - more heat loss within the home.

We purchase many things every day and we never aim to purchase "just enough". If you have natural gas, it isn't expensive and neither is the electric it takes to run the system. Lastly, these boilers jump from 80,000 BTU to 100,000 BTU for the next level up so getting it exact may not even matter.

For the money you're spending on getting a company to measure you, you're losing any perceivable savings anyway. I realize people treat this like a science but in practical terms it provides no real benefit because of negligible cost differences.

The consumer guides to furnace selection say that an oversized furnace results in "short cycling". They say short cycling is bad for several reasons including but not limited to being uneconomical. In my case you are correct that the potential saving is small. The 25-year old furnace in my 915-square foot house is 94K BTU. My total gas consumption for the past eleven months is 632 therms at a cost of $448. This includes space-heating, water heating, gas oven and cooktop, and gas clothes dryer. This eleven month period had an above average number of heating degree-days for my location. I've been advised that a professional Manual J sizing would cost me $150-$300.

As Joe C mentions, short-cycling produces uncomfortable heating. An article I saw on the web mentions another drawback of short-cycling. To attain higher efficiency (AFUE), the heat-exchangers in modern furnaces are made of thinner metal than in older furnaces. Short cycling causes repeated thermal expansion and contraction which can eventually ruin these thinner heat exchangers via metal fatigue.

I just completed running loadcalc.net. The result was 22214 total BTU's heating. Thus, a 44K BTU 80% AFUE furnace would suffice. One of the two HVAC contractors who gave me estimates (and who recommended a 60K BTU furnace) told me that his computer program (which he didn't name) told him my house needed 24K BTU. (He took no detailed measurements of my house.) He said he would only install a 44K BTU furnace in a condo where there are adjacent units, not in a house with four walls.

Joe C: Thanks for your links to Slant-Fin and Taco Manual J software. I browsed one of the user manuals and viewed the Taco video. My PC's are Linux only (without the Wine MS-Windows emulator ) so I can't run either of those .exe files.

Joe C: In addition to the "envelope-tightening" measures you listed, I've read that duct sealing should also be done. Leaky ducts can lose up to 30% of heating energy. I'd like to have a duct leakage test performed but I expect I'll have a tough time finding someone to do it.

 

[Greenman]

I'm trying to understand something - why have it sized for EXACTLY what you need? You just mentioned expanding the home yourself, so it's not out of the question that it may happen for others down the road. There's also the issue of degradation over the years - more heat loss within the home.

We purchase many things every day and we never aim to purchase "just enough". If you have natural gas, it isn't expensive and neither is the electric it takes to run the system. Lastly, these boilers jump from 80,000 BTU to 100,000 BTU for the next level up so getting it exact may not even matter.

For the money you're spending on getting a company to measure you, you're losing any perceivable savings anyway. I realize people treat this like a science but in practical terms it provides no real benefit because of negligible cost differences.

Rather than looking at as just enough, look at it as exactly right. You size a system for the job it needs to do, over sizing means temperature swings and a constantly cycling system. That's inefficient, and uncomfortable.
I build the addition tight, and upgrade the existing house as much as possible, then size the system to work with those conditions. Heat loss is figured for each room, and ducts are sized accordingly. The resulting system operates at peak efficiency, uses less energy than before I built the addition, and if more comfortable.
If someone down the road decides to add more footage to the house, they'll have to upgrade the furnace. For the house I'm working on, it will never happen because there isn't anywhere else to add on, it's as big as it can be.