better to oversize or undersize?

[green1]

I have decided on a Carrier Infinity for my old (1897) home in Oregon. I will buy the load calc software and do the calc for my home since none of the contractors did. The reason I will do one myself is because the 3 BTU estimates I received for the Carrier were 60,000, 80,000 and 100,000. (my current furnace a York 120,000 is apparently too large for my (approx) 1600 foot home.)

My question is: Is there a general rule of thumb about whether it is better to slightly oversize or undersize the furnace. I dont want something too big for my house, on the other hand I dont want to be cold. (Money is not the issue here). The contractor who is suggesting the 60,000 was adament that 80,000 would be too big, and as a result would be bad for the equipment, cause it to cycle on and off more and be a fire hazard. The guy who said 100,000 BTU responded to my concerns about the large variation in BTU estimates by saying he thinks 80,000 would work.

Any thoughts would be greatly appreciated!

[BaldLoonie]

Personally I'd undersize. The only time an undersized furnace is a problem is on the coldest morning of the year. The rest of the season it's fine.

Cycling is inefficient. Uses more gas, harder on the equipment. But an oversized furnace isn't a fire hazard. That's a new one!

Around here, most 1600 sq ft homes would get a 60K 90% furnace and we size to -10 out. Do you get that cold? How well insulated are you? We're normally at least R19 ceilings and R11 walls. Be interesting to see what your calcs show.

[Irishmist]

best to PROPERLY size!

[2hot2coolme]

It's better to be a little undersized than oversized, You'll get a little longer run time, but it's better than shortcycling.

[beenthere]

After you do the calc, think 2 stage.

[green1]

Around here, most 1600 sq ft homes would get a 60K 90% furnace and we size to -10 out. Do you get that cold? How well insulated are you? We're normally at least R19 ceilings and R11 walls. Be interesting to see what your calcs show. [/B][/QUOTE]

The temps get into the 20's but usually the winters are pretty mild (30's - 40's). I will know more about how well insulated on Wednesday after an "energy audit" from Oregon Energy Trust. The insulation in the attic looks lumpy and black. Most likely the walls in the house were insulated before I bought it 15 years ago when they remodeled. There is one small bathroom that does not feel insulated. Finished basement has new double pane windows, the rest of the house has older single pane (wood).

One of the reasons the contractor said he was suggesting 60,000 was because of the small size of ducts in my house. Does that make sense?

Thanks!

[green1]

Originally posted by beenthere
After you do the calc, think 2 stage.

can you explain what you mean by this? I do understand that at first the furnace will kick on at a higher level and then a lower level.

Thank you.

[2hot2coolme]

One of the reasons the contractor said he was suggesting 60,000 was because of the small size of ducts in my house. Does that make sense?

Thanks! [/B][/QUOTE]

Yes that makes sense, if the ductwork is too small then the furnace will shut down on high limit, that's even worse than short cycling.
Hopefully your ductwork is sized properly for your shack.

[green1]

Hopefully your ductwork is sized properly for your shack. [/B][/QUOTE]

do you think the contractors would have mentioned this if the duct work is not a good size (none did)? and, would the problems be short cycling or something else?

[beenthere]

It will run at a lower btu first then go to high later, best used with a 2 stage stat.

If your calc ends up being between sizes, you can go to the larger furnace, and with 2 stage you won't get the short cycling.

[cem-bsee]

put on storm windows!! almost as good as dual- pane, besides, they protect the glazing!

All 15 of my 1974 dbl hung windows which had storms had good glazing -- the 4 in the gar took 1 week to reglaze, replace the broken glass caused by me taking out the old glazing, & painting.

BTW, you could NOT give me a vinyl windows & install them! The ones used by Habitat sagged in the middle when shimmed just at the corners.

[mrand]

Originally posted by cem-bsee
put on storm windows!! almost as good as dual- pane, besides, they protect the glazing!

All 15 of my 1974 dbl hung windows which had storms had good glazing -- the 4 in the gar took 1 week to reglaze, replace the broken glass caused by me taking out the old glazing, & painting.

BTW, you could NOT give me a vinyl windows & install them! The ones used by Habitat sagged in the middle when shimmed just at the corners.

Sorry for dragging on an off-topic discussion: how old were the vinyl windows? I've "heard" (i.e. not seen research papers first hand) that vinyl's have gotten a lot better in recent years. We're considering replacing some/most/all of ours.


Back to the topic at hand: after doing a manual J, the A/C firm bidding to replace my system says that I'm between a 3.5 and 4 ton unit rather than the 5 ton that I have now. It surprised me a little that he didn't seem to have a preference on which way to round.

Marc

[amickracing]

Manual J is a bit on the oversized side if you follow thier numbers.

Which in my mind means that if they oversize a lil bit (which I don't think is too much), but then you put in some fudge factor when crunching the numbers, and then when you pick the unit say you can choose beween a 2.5 or 3 ton, and go big, then you're way over sized.

For me, if I ran the numbers as best I could with manual J, I'd go with the unit that was closest. Say you needed 65,000 btu, I'd go with a 60,000 unit, not an 80,000.

Just my 2 cents anyway.

[dan sw fl]

Originally posted by green1
I have decided on a Carrier Infinity for my old (1897) home in Oregon. I will buy the load calc software and do the calc for my home since none of the contractors did.
(my current furnace a York 120,000 is apparently too large for my (approx) 1600 foot home.)

The guy who said 100,000 BTU responded to my concerns about the large variation in BTU estimates by saying he thinks 80,000 would work.

Any thoughts would be greatly appreciated!

Calculation is not even close unless you KNOW the infiltration rate based on YOUR 1897 house.

So How can you decide whether a system
is OVER or Under sized?

Energy Audit from Oregon Energy Trust
should address this issue.

[green1]

So How can you decide whether a system
is OVER or Under sized?

Energy Audit from Oregon Energy Trust
should address this issue.


Funny you should mention them - I have a schedulled appointment already.

By the way - I did HVAC-Calc and came up with around 40,000 BTU heat loss. The software says add 20% although they also suggest a smaller percentage if you are concerned about conservatio - which I am.

question1: if I got a 40,000 load calc- Im assuming I can go with the 60,000 Carrier Infinity - right?

question 2: What percentage would you add to the 40,000 BTU figure if conserving was important for you?

Thanks again for everyones comments and advice.

[dan sw fl]

Originally posted by green1
Around here, most 1600 sq ft homes would get a 60K 90% furnace and we size to -10 out. Do you get that cold? How well insulated are you? We're normally at least R19 ceilings and R11 walls. Be interesting to see what your calcs show.

I will know more about how well insulated on Wednesday after an "energy audit" from Oregon Energy Trust.
Thanks! [/B][/QUOTE]

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