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Furnace Sizing


dpopgky
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I just had a routine service call on a 5 year old Trane Furnance installed in our 2800 sq ft home. Tech says that our heating and cooling is grossly oversized for our home. Has a 120 BTU furnance and 5 T AC unit. He suggested replacing both units to the correct size and is working up an estimate on costs? Do not have specifics today, but can get them. This just occurred yesterday 2/8/10. System is working but had issues in Dec with High Limit Switch trips when weather got cold.

Mike posted that downsizing burner/etc would be a cheaper fix than a complete system changeout.???

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Hi,

Yeah, I theorized that I thought it might be possible, I didn't say that I know it for a fact. It was an idea, since I see so many of them that seem to be the identical furnace with different BTU outputs. I don't know if it can be done or not.

It's a 5-year old furnace? Is the house also 5 years old or did you have a furnace in an older home replaced? If you had the furnace replaced, who sized it and why haven't you called that person to ask why he or she installed the wrong sized furnace and AC in your home?

ONE TEAM - ONE FIGHT!!!

Mike

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Hi Gary,

If I understand this correctly, your furnace was shutting down on high limit in December, so you hired a 'Tech' to offer a remedial course of action a few days ago. His advise was to replace the furnace cuz it's too big. [:-bigeyes

Is work that scarce that trades have to resort to that?

My first advise to you, DO NOT sign anything until we have this little discussion. The size of the furnace has NOTHING to do with high limit switches.

The size of a furnace is determined by calculating heat loss of a building. It considers such things as size of house, type of construction, insulation levels in walls and ceiling, type and location of windows-sliders, sealed or casement, house orientation and exposure and the list goes on. Once we know how much heat the box looses then we can determine the amount of heat required to keep the box at an acceptable level at a specific outdoor temperature.

Or

We can rely on historical experience and make an educated guess as to size.

So for us to opine on your furnace size is absolute folly, we neither have the information nor the experience to advise you.

In my market, I would expect to see two furnaces in a home that size. The main floor/basement around 90MBTU and top floor 75MBTU with the possibility of a small radiant system for the basement floor.

The high limit switch is a safety device that protects the furnace from running wild. It shuts off the gas valve generally around 200 degree F to prevent the furnace from overheating and causing a possible fire. It has NOTHING to do with size of furnace or house.

Assuming the safety switch is functioning properly, the causes are generally a blocked passage in the throw and return loops in the form of:

Blocked returns- undersized openings, carpet or flooring in the return, convoluted pathways etc.

Dirty air filter

Contaminated A/C coil (evaporator inside the furnace)

Blocked heat runs

Closed register dampers

From my experience I'll bet the A/C coil is plugged with drywall dust.

Here is my recommendation to you.

1. Determine if the switch is in fact ok. Find a hole in the heat plenum near the appliance and insert a probe cooking thermometer, turn on the furnace and observe the temperature as it rises. If the furnace shuts down on high limit (200 degree F) then the switch is doing its job otherwise get back to us with your observation.

2. Search out the cause of the blockage from the list above. There is a product that is specifically designed to clean A/C coils. It comes in a large spray can that dissolves crud without scrubbing the coil. If you want to go one step further you can also buy a special 'comb' to loosen difficult residue. However, DO NOT touch the coil with your fingers, it damages easily.

If you don't feel confident with that, hire it out.

3. Repeat No. 1 and hopefully that will make the switch happy.

Good Luck,

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Hi Gary,

If I understand this correctly, your furnace was shutting down on high limit in December, so you hired a 'Tech' to offer a remedial course of action a few days ago. His advise was to replace the furnace cuz it's too big. [:-bigeyes

Is work that scarce that trades have to resort to that?

My first advise to you, DO NOT sign anything until we have this little discussion. The size of the furnace has NOTHING to do with high limit switches.

The size of a furnace is determined by calculating heat loss of a building. It considers such things as size of house, type of construction, insulation levels in walls and ceiling, type and location of windows-sliders, sealed or casement, house orientation and exposure and the list goes on. Once we know how much heat the box looses then we can determine the amount of heat required to keep the box at an acceptable level at a specific outdoor temperature.

Or

We can rely on historical experience and make an educated guess as to size.

So for us to opine on your furnace size is absolute folly, we neither have the information nor the experience to advise you.

In my market, I would expect to see two furnaces in a home that size. The main floor/basement around 90MBTU and top floor 75MBTU with the possibility of a small radiant system for the basement floor.

The high limit switch is a safety device that protects the furnace from running wild. It shuts off the gas valve generally around 200 degree F to prevent the furnace from overheating and causing a possible fire. It has NOTHING to do with size of furnace or house.

Assuming the safety switch is functioning properly, the causes are generally a blocked passage in the throw and return loops in the form of:

Blocked returns- undersized openings, carpet or flooring in the return, convoluted pathways etc.

Dirty air filter

Contaminated A/C coil (evaporator inside the furnace)

Blocked heat runs

Closed register dampers

From my experience I'll bet the A/C coil is plugged with drywall dust.

Here is my recommendation to you.

1. Determine if the switch is in fact ok. Find a hole in the heat plenum near the appliance and insert a probe cooking thermometer, turn on the furnace and observe the temperature as it rises. If the furnace shuts down on high limit (200 degree F) then the switch is doing its job otherwise get back to us with your observation.

2. Search out the cause of the blockage from the list above. There is a product that is specifically designed to clean A/C coils. It comes in a large spray can that dissolves crud without scrubbing the coil. If you want to go one step further you can also buy a special 'comb' to loosen difficult residue. However, DO NOT touch the coil with your fingers, it damages easily.

If you don't feel confident with that, hire it out.

3. Repeat No. 1 and hopefully that will make the switch happy.

Good Luck,

You are dead on, unsurprisingly. I read the other part of this thread that was split or deleted earlier, and my first thought was that the furnace was UNDERsized, not grossly oversized.

The only thing I would add is that the ductwork should be checked. There may not be enough supply runs and, as you said, the return trunks may not be large enough.

There could be other issues that need addressing, as well. But like Rob said, the schmuck who advised you to replace the entire system might be a little lacking in integrity.

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Agreed,

He should show how he determined that it was "grossly oversized". The rule of thumb used in Code check for an old house with poor insulation is 55 per square foot (154,000 Btu/Hr for this house) and for a new house with good insulation it's 35 per square foot (98,000 Btu/Hr for this house).

That's a rule of thumb only and it only tells you whether you are somewhere in the ballpark. It doesn't take into account the amount of glazing or anything else and it should be a starting point only. If they guy was looking at a newer house with good insulation, using the rule of thumb, he might have decided that this furnace was roughly 120% of the size that was actually needed; but at that point he needs to do a complete calc as described by Rob above.

I still think the place to start is the guy that installed it in the first place. Find out how the size was determined.

ONE TEAM - ONE FIGHT!!!

Mike

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I don't want to leave folks thinking that I'm suggesting the size of the ducting is inadequate. What I'm referring to here is the method in which the return was channeled through the framing, particularly to the second floor.

Second floor returns are problematic and can sometimes take a lot of jogs and turns that are unseen until the walls are opened.

While I'm on the subject Gary, you might want to test the returns on the second floor with a sheet of toilet tissue.

Get back to us with your results.

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