Helping a friend: Replacing a Furnace


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Old 02-08-16, 09:16 AM
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Helping a friend: Replacing a Furnace

Hi all

A friend of mine had an issue with the heating system not running unless the 'fan' switch was set to 'ON' instead of 'AUTO'

We swapped out the stat, but it did same thing. So, they called an HVAC company and the tech said they needed to replace the circuit board, but he also noticed the inside of the (older) furnace was 'starting to crack' and is recommending they just replace the furnace (board was quoted at almost $1k)

I am more familiar with my hydronic system than any HVAC systems (much thanks to this site!). I know, for boilers, its recommended to do a 'heat loss assessment' before replacing a boiler. Does that translate to forced hot air? I tried searching this forum, but didnt see that pop up--I'm guessing it isnt as important. Only asking to make sure our friends dont buy more furnace than they need!

Thanks in advance for any advice!
K
 
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Old 02-08-16, 09:30 AM
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I'd do a little more investigating on the "starting to crack" issue and maybe look around for a replacement board yourself, they are relatively simple to install.

In answer to the heat loss, yes it should be done for any type of heating system.
 
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Old 02-08-16, 10:21 AM
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Can't comment much on the furnace repair without a make and model number.
I'm curious what he saw that was cracked. The heat exchanger can crack but is not easily seen from outside.

Did he fire the furnace up to the point where the burner was running and then turned the fan on?
 
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Old 02-08-16, 10:51 AM
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I wasnt there. They mentioned using a camera...

They are pretty much determined to replace the furnace... I was just wondering if furnaces (like boilers) are almost always oversized and if thats a problem or not..

Also, if heat loss assessment is important (which it seems like it is, based on previous response)...
 
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Old 02-08-16, 01:28 PM
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Yes, proper heat loss calculation is just as necessary with a forced air system as with a hydronic system. And yes, forced air furnaces are oversized as often and often more so than are boilers.

A "builder's grade" forced air furnace will generally have a useful lifespan of about 20 years, more or less. Many furnaces still in use should have been replaced before now. With my own "builder's grade" furnace the gas burners were totally shot by 18 years. I could have replaced just the burners, at a cost of about $600, but I would have still had an 18 year old heat exchanger, 18 year old blower and motor, 18 year old gas valve and control board and further, I could have replaced the entire furnace (not including labor) for between $700 and $800 so spending the $600 on just the burners seemed to me to be economically insane.

So I went almost to the other end of the spectrum. I had a new, 80% AFUE two-stage, variable speed blower furnace installed. The cost (2005) was about $2800 complete and I have never regretted the decision for a single second. My gas bills dropped about 10% during the heating season, the noise level during operation dropped dramatically and the comfort level risen appreciably.

Now that isn't to say than my furnace story went off without any problems, there were a few problems and the most glaring was that NONE of the contractors I interviewed were willing to do a full-blown manual J (manual as in book) heat loss calculation but they ALL wanted to know the BTU/hour rating of the existing furnace. All of them also asked if I had any concerns about how well the house was heated, any cold area or the like. I finally asked the estimator for the company I eventually hired why no one was willing to do the calculation and he told me because it was much easier to simply go by the existing size and ask a few questions regarding comfort and any energy saving measures that had been taken. What he didn't openly tell me was that heating contractors were still scared to death of having a customer come back a year later stating that their new furnace would not keep the house warm. The result is that that heating contractors will STILL routinely oversize the furnace.

There's more that I can add if you are interested.
 
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Old 02-08-16, 02:32 PM
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great info (as always!), Furd.

I am interested-- in the spirit of intellectual interest.

I havent heard from them since this am, so I assume the new furnace is already installed (dont know if they got the heat loss or not). I suppose if its oversized, the issue will be that their utility bills will just be higher (but they will likely offset that with something thats likely more efficient as its newer)..
 
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Old 02-08-16, 09:03 PM
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I think I have told this story at least a half-dozen times but as long as my fingers hold out I'll do it again.

My original furnace had a 60 kBTU input rating. Because of some experimentation on my part (which I do NOT recommend to anyone without a background in heating equipment and combustion systems) I knew for a fact that my furnace was significantly oversize. Unfortunately, at that time smaller furnaces that had the features I desired (two stage burner and variable speed blower) simply didn't exist. The smallest unit was a Lennox G60 rated at 70 kBTU input.

Now one of the features of two stage burners is that the sizing of the furnace becomes a bit less critical. My existing furnace, if AFUE tests and numbers had been in use when it was installed, would have probably been in the mid to high 60% to maybe mid 70% range. I'll use 70% to account for the damaged burners and probable crudded up heat exchanger. That means that I had a BTU/hour output (from the furnace to the ductwork) of about 42 kBTUs/hour. This amount of heat was sufficient to keep the furnace from ever running for extended periods of time yet also keep the house comfortable. I DID have temperature swings of a few degrees when I first moved there but relocating the thermostat helped immensely. Still, I would have preferred a bit longer "on" times to keep the air moving and reduce stratification throughout the entire house.

But, the new furnace, being about 11% larger output would normally have thrown my desire for longer burn times out the window. Going larger was just counter intuitive to overall comfort and energy savings. THAT is why I specified a two stage burner. With the two stage furnace the first stage is about 60% of the maximum firing rate so the output from the larger furnace (the one I contracted for) was going to be outputting around 33-34 kBTUs/hour which was about 20% LESS than the old furnace output. This was exactly what I wanted and I would still have the increased output of the second stage if I ever needed it.

But, (lots of buts in this story) my old furnace had what is called in the industry a "B" case, a standard measure of the width of the furnace. The new furnace had an "A" case and that meant more changes to the ductwork. So, unknown to me and obviously without my permission, the salesman wrote the installation order for the next size larger furnace, a 90 kBTU/hour model in a "B" case. Well, it sure went in easy, the guys started at about 8 AM and were finished, including clean up and packing my old furnace in their truck, by noon. After the installers left I went out to inspect the job and that is when I discovered that instead of having a new furnace about 11% larger than the original I had a furnace that was 50% larger than the original. Yeah, I was super p***ed off.

When I finally got the salesman on the phone he tried to weasel out by telling me that he had given me an "upgrade" at no additional cost. That with this furnace I would never go to the second stage burner operation. I told him that of course it wouldn't, the old furnace was oversized and now I had a furnace that was 50% bigger than that old OVERSIZED model.

Well, eventually he understood my position and I got the correct furnace. Since then the ONLY time the furnace has gone to the second stage is when I have purposely moved the thermostat setting high enough to cause it. Even during unusually cold periods (for my area) the furnace cycles on and off on the first stage only maintaining the temperature easily. In effect it means that I heat my 1550 square foot one-story home with only average insulation and poor air sealing with a furnace having an output rating of about 34,000 BTU/hour AND it only runs about half the time. Or to look at it a slightly different way, I reduced the size of the original furnace by almost 30% and actually gained comfort. Just think what I could do if I closed up some of the air leakage and improved the insulation!
 
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Old 02-09-16, 01:59 PM
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this is great! Thanks for taking the time, Furd. I was trying to articulate why 'right sizing' is important on a furnace, and it never dawned on me that continuous air movement was a consideration.
 
 

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