Gas Furnace BTU vs Blower Capacity, etc.

Old 12-31-05, 05:14 PM
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Gas Furnace BTU vs Blower Capacity, etc.

This is my second try at this post.

We have a 20 year old house/gas furnace 4 miles from ocean in north San Diego County. 10 years ago, we added 800SF to a 1700SF house and just extended ducts. The big back room stays cold in winter.

When furnace was started this season, the blower bumps on and off several times after the furnace shuts off waking us up in the night. Tech say it is heater control panel and costs $500 to replace. Also says putting money into 20YO furnace is a waste of money. Also quotes $2300 for a new 80%Efficiency single stage furnace and thermostat.

The original heater is a 75000 BTUH gas furnace put in for the 1500SF. What size should the new heater be? Can the blower on the new heater be oversized (higher CFM and/or static pressure rating) to improve air flow in the extended ducts?

A second company had similar quote for heater and thermostat, except with a breakdown shows $500 for the new installed thermostat. The web shows this Honeywell thermostat available for $67.

Any thoughts on furnace rating, fan rating, need for furnace replacement, reasonable cost for furnace and thermostat replacement.

Is this the kind of job a do-it-yourselfer should attempt?

Thanks for your thoughts.

Old 12-31-05, 07:07 PM
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A few ponderings:

1. If furnaces was sized right to begin with, then it is too small now. May not be that big of an issue though.

2. If the unit is 20 years old it should have a fan limit switch in it that controls fan on and off temps. Look for a silver box with a dial inside of it. The switch at the lowest temp should be fan off temp and could be turned down to help remedy the fan kicking on and off after the furnace has stopped.

3. Is the room that stays cold an add on with three outside walls? If so you are gonna have a hard time keeping it the same temp as the rest of the house, and you should get a small electric or oil filled heater to plug in the wall for a little extra heat out there.

4. When the ducts were extended were the extra run taken off the end of the trunk, or were they ran back close to the unit? Although not the proper way to do it, if they were run back close to the unit it might keep you from having to redo your ductwork to get proper airflow to the rooms these ducts feed if you do upsize your unit.

Just some things to think about.
Old 01-01-06, 05:27 AM
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Your idea of a "reasonable cost" may be different from mine.

Everyone has to make money, yet HVAC contractors continually have their prices driven down by consumers shopping the internet.

The consumer shopping the web doesn't have the equipment to fabricate the sheet metal needed to do the job, the expertise to wire it up properly or the insurance burdens that the HVAC company has, yet the same consumer seems to know what "too much" is.

No one wants to pay for a Manual J, yet they complain when the equipment doesn't heat or cool the home properly.

No one wants to pay for the Manual D, yet they complain that there isn't any air coming out of this register.

Builders only care about stuff visible outside of the drywall, so who cares about the crappy HVAC install because it only has to last a year.

Maybe the company with the $500 t-stat has a really competitive price on the furnace and zings you on the "extras". The practice is no different than the rustproofing/fabric protection price structure on a new car. Do you think the $150 extended warranty on that new TV actually costs the store that amount of money?

If the rest of their bid is OK, and you want them to do the work, then delete that $500 tstat from the bid and get your own to install after they are done.

How often do you hear someone say that the doctor on the internet will do the heart transplant for less than you will, so how 'bout beating that bid? Is there a DIY option for this?

If you stop and think about it for a minute, why does the "fan control board" need to be replaced if the fan is coming on? Hmmmmmmmmm... Maybe someone is trying to sell a new furnace instead of fixing it.

If the furnace has a standing pilot, there is no "fan control board", but it does have a fan limit switch. A bad one will cause the problem you are describing.

If the rest of the furnace is in good shape, $500 is 75% less than the cost to replace it. If they are quoting $500 to change a limit switch, you might want a second opinion.

Unless the problem is obvious, like a cracked heat exchanger, you should always get a second opinion when someone starts talking replacement.

Unless you size the new furnace and ductwork to the home, and pay for the changes needed, it isn't going to work right.

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