Home>Discussions>INSULATION & HVAC>If both furnace sizes are adequate, how many BTUs should I go with?
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If both furnace sizes are adequate, how many BTUs should I go with?

I'm a landlord and my property is in Northern San Diego County. We have received 7 bids from HVAC contractors for replacing the furnace and the ac (they're 30+ years old), and some recommend a 70 KBTU furnace and some recommend 90 KBTU. One contractor told me that either one is adequate (San Diego does not get very cold) but the 90K has the same size and cost as the 70K so he says I should go with the 90 because it will heat the house faster,.

Problem is, this page says that too many furnace BTUs causes energy loss and more wear and tear from excessive cycling on and off.

Also, my (admittedly non-professional) ****** research, along with one contractor telling me that our insulation is excellent, strongly indicates that 70K will be enough.

My question for the professionals is this: Assuming the contractor is correct, and either 70 or 90 will work, is it really better to go with the 70K furnace as the web page indicates, or is the difference really no big deal?

Thanks in advance for any help you can give.

Re: If both furnace sizes are adequate, how many BTUs should I go with?

You should size as close to the design load as possible. i remember once we sized a furnace with 20 % to cover warm ups
but what happen is that we over shot the required temperature the room instead of being 72 it went to 76 . we had to go back in and install a very expensive heat anticipator to shut down 2 degrees before it came up to room temperature.


Re: If both furnace sizes are adequate, how many BTUs should I go with?

First of all let me congratulate you on your research and in submitting a fine article on furnace sizing (Should I Care About an Oversized Furnace?)---the article is well-written and documents many fine points, which in general I feel should be followed.

As the article notes, there has been a LONG HISTORY in the heating industry of installing oversized heating systems, a pervasive motive on the part of some unscrupulous contractors can be that they get a larger markup ($$$) for installing the larger unit; and often it could be because of failure to use easily available on-line heat loss calculators; both reasons are not fair to the homeowner, since they are the ones who have to pay the replacement costs.

I'm by no means an expert in this area, but I feel most people who are in the market for new heating equipment should read the article; there are HEAT LOSS CALCULATORS that do a fairly good job of calculating the amount of BTUs a building needs based on one's geographical location, building materials used, tightness of the building(s), severity of the winter, and numerous other factors; it's also important for the homeowner to keep in mind any expected extensions of the heating system, to be added areas in the future, such as a remodeled cellar, or adding heat to an additional small building, such as heating a garage/workshop in the foreseeable future using zone valves, etc., this would be quite an additional cost if this is not anticipated & factored in when buying your heating system.

Other topics to Google search would be:
Furnace Sizing Calculations
Furnace Sizing Theory
Residential Heat Loss Calculations
Best Residential Gas Furnaces

Dedicated Heating System sites would be:
Heating Help:The Wall


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