Two finalists, two different BTUs?

[tikimonster]

With the help of the great info I've gotten from this site, I have my bids down to two. Both are Lennex units, the high-efficiency variable speed furnace (G61V), with slightly different AC units (one using R22). Otherwise, most of the differences are minor: 1 year warranty on labor vs 5 year, whether I need a condensate pump or can use the floor drain (previous owner put in a non-code shower stall using the floor drain as the shower floor! No exhaust vent, either.)

My big problem is that the furnace sizing is different between the two bids, and they do not seem that close! I think the difference is sizing is due to our finished basement. It gets quite chilly, especially in the winter. I mentioned this to both reps, and one is going to put in two cold air returns, the other says it won't do any good.

The rep who will put in the returns is going with a 45,000 BTU unit, the other rep is sizing a 70,000 BTU unit!
Now to me, that seems like a pretty big variation. I've read here that oversizing a high efficieny furnace is bad and awful, so I'm concerned.

Here are the stats: I live in a 1960 ranch (aka, the shoebox) in MN. There's over 16 inches of blown in insulation in the attic. There is insulation in the walls,but heaven knows how much. Half of the basement is finished and the other half (split the long way) is laundry, mechanicals, and a half-bath with the "shower".

In the finished half, there are two vents in the basement ceiling, and one lone return. The other (unfinished) side has only two vents, no returns. Other than the basement, the house is pretty comfortable, the bedroom walls gets cold in the dead of winter, but the room itself is comfortable.

I checked the estimates that didn't make the cut, the size is either not calculated (sizing to my existing unit, or the "I've seen a thousand of these homes, and they always take...") or fall right in between the two bids. Granted, I had a total of 6 bids, so there isn't a lot to average. I'm going to try the HVAC calc, but I'm concerned that I can't address the comfort variables in the program, and I think that is the source of my problem.

So, experts, what do you think? Is the difference between 45,000 and 70,000 not as much of a difference as it seems to me? Or is one of these guys not doing his homework correctly? I want to be comfortable downstairs, but don't want to bake upstairs or oversize my furnace, either.
Any advice you can give will certainly help me to make the right choice!

[Christheheatingdude]

Returns in the basement will make a world of difference. Especially if they are place near floor level. I would insist on that. Are the Lennox 2 stage variable speed units?
It would help comfort levels if they were. Being able to run the fan on constant at a slower speed will help the comfort level in the home.

I would recommend you doing your own heat loss. With out all the specs on your home it is hard for us to know which contractor is correct on the sizing.

[BaldLoonie]

My experience sez that dealers quoting smaller equipment are the ones sizing right. Those who are guessing, guess big to be safe.

HVAC-Calc will tell you and if you have any questions, e-mail Don or ask us.

[mayguy]

Not knowing how many sq ft the house is.. I agree the bald on the sizing.. but go ahead and down load the program and see what you get.

I would suggest a two stage t-stat on that two stage furance.. You will for sure get the comfort in your home..

I am in MN as wel, I have it.. Our basement is the same temp as upstairs.

[tikimonster]

Thanks for the responses! The total finished square footage is just under 1450, including the basement. Yes, both furnaces are two stage variable speed, the only difference I can find is in the model numbers. One is G61MPV36045 and the other G61MPV36B-070.

I'l try the HVAC calc program and see what I get. Thanks for the great, prompt responses! You people are the BEST!!

[Jultzya]

My vote goes to the smaller equipment.

[docholiday]

Me too but better know for sure either way.

[dash]

Sizung,either could be right,but like others I'd think the smaller is the accurate one.What size do you currently have and how much does it run on a cold day/night??



Returns,simple test ,with the furnace running,does opening the door to the basement cause a noticable increase in supply air to the basement?If yes returns will help,if no it's a lack of suppy air.If the door is always open and the basement is cold,it's lack of supply air.


Location of the return high or low ,though a popular rule of thumb,will make very little difference in comfort in the room.

[2hot2coolme]

Go with a 60,000 2 stage, then everything should be roses.

[Christheheatingdude]

Originally posted by dash
Sizung,either could be right,but like others I'd think the smaller is the accurate one.What size do you currently have and how much does it run on a cold day/night??



Returns,simple test ,with the furnace running,does opening the door to the basement cause a noticable increase in supply air to the basement?If yes returns will help,if no it's a lack of suppy air.If the door is always open and the basement is cold,it's lack of supply air.


Location of the return high or low ,though a popular rule of thumb,will make very little difference in comfort in the room.

I beg to differ on your last point. It is not a rule of thumb. It is experience. Something I have seen time and time again. High returns in a basment do little to help when the system is heating. In a northern climate, such as where I work, we are trying to get our supplys low to floor, and our returns as well. This makes a tremendous difference in the comfort level. It also works well when the supplies are in the cieling.

But if the returns are high as well, you do not get the circulation of air, and it tends to just pull the air from the vents back to the furnace.

Also, I do not understand why you are all saying that he shold go with the smaller furnace. I am not trying to start a war here, but you too are going by experience. Saying that the smaller furnace is probably right. Each home needs a proper heat loss performed on it, to know for sure. I don't think anyone should steer a homeowner in one direction unless they know what the numbers are.


[Edited by Christheheatingdude on 10-11-2005 at 05:23 PM]

[tikimonster]

Thanks for all the great advice. I seldom keep the door open to the basement, in the winter all the heat comes upstairs, leaving me with a warm kitchen, warm stairs, and a cold basement. The heating vents are in the ceiling, and the single return is low. If I compare it to upstairs, a slightly smaller total area broken into three rooms instead of one has a total of 4 returns versus the 1 return in the basement. The contractor who will put in the two additional returns will place them low.

I ran the HVAC calc program last night. I made a few guesses, but the program was very easy to use and was quite doofus-resistant. I came up with a total heat loss of just over 46,000 BTUs. (I knew there was a good reason for that 16" of insulation in the attic! Without it, I calc at about 55,000 BTUs)

Cooling gets me a 1.5 ton on the program, all of the contractors sized the AC to a 2 ton (I'm "on the cusp" between 1.5 and 2). I think the difference between the program and estimates is site variables, such as the asphalt driveway next to the house. That big black area on the south side of the house can sure heat things up! No trees on that side, either.

So I called contractor A back (who sized at 45,000 BTU with 2 returns) and asked a few more questions. I got the change I needed on the condensate pump (having one put in, I don't trust an inspector to approve the drain to the floor drain/shower!) This Friday he comes over to measure for the plenum, etc, and I'll sign the deal! In another week, I get warm and comfortable!

Thanks to all of you, without the great advise I would have probably gone with the lowest bidder. Instead I'm paying a bit more but getting the right equipment from an installer I trust. I'll try to get some before and after pictures, my antiques should cause a good laugh!