Can I upgrade my furnace?

I just put an addition on, and the inspector let me know that my five year old furnace is too small. It’s a 70,000 btu, 80%, so that makes it about a 50,000 btu overall output. According to the HVAC sub that was here today. I need 75, according to the inspector. Do I need a whole new furnace? If I do, how much does a furnace cost? I see various models on line, but no prices, and the btu’s are given as a range, not an exact amount.

Any HVAC guys out there that can help me out a bit. I also put in a high efficiency wood burning stove to fight against the minnesota winters, so I’m certain I’ll be toasty warm this winter even if I don’t put in a new furnace.

I doubt you can upgrade it directly. You might be able to install an additional one but that probably wouldn’t be worth it. New furnances always seem to cost about $5000 around here with installation. That is how much mine cost and my house is very old and fairly big. It works great.

If you have a sufficient supply of wood, your new stove may take up the slack. on the other hand, it wouldn’t hurt to have a large enough furnace for those times when you don’t feel like tending the fire.
There are a few things about your situation that I don’t know which could affect the installation costs of a new furnace, for exmple, is your new addition ducted into the existing furnace? If so, then installation costs should be considerably lower. You might also take this opportunity to upgrde to a more efficient furnace. Your current furnace uses 70,000 btuH of gas and delivers 56,000 btuH of heat. A new 90+%, 85,000 btuH furnace would give you over 76,000 btuH of heat.
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I did a little (very little) searching and found this listing at Grainger - 80,000 btuH input, 75,000 btuH output, 94.3% efficiency, retail cost $967.00. And Grainger is not the cheapest place around.

I know nothing about building codes or labor rates in Minnesota, but if you know that the furnace costs under $1000, you should be able to swing a reasonable price for installation. And the increased efficiency will help offset the cost over the next few years if gas prices continue to rise. There might even be some rebates available from your gas supplier for buying a high-efficiency furnace.

Can you spend a smaller amount of money having your home inspected for leaks and cracks and greatly improve all the insulation?

To tell the truth, I’ve never heard of mandatory minimum furnace sizes.

Before I worked for TSA, I was an HVAC installer with my Dad’s company. I would say you’re gonna need a new furnace, the whole new unit installed. Prices vary and you’re probably not gonna find it online, because most places are wholesale to the trade only and of course that means there is gonna be a markup. I don’t know if Home Depot sells them, but I doubt it. Out here our supplier is R.E. Michel Inc.

BTW, the price I quoted from Grainger is the retail price. Anyone can walk in off the street and buy it for that amount. An HVAC dealer with an account there will pay from 15 to 25% less than retail, and almost certainly be able to beat that price at any number of wholesale dealers. As I mentioned, Grainger is not known for their low prices.

You could install supplementary heat in your addition, such as electric baseboard heat, in-wall gas heat, radiant floor heat, etc. and thus avoid the need to upgrade your furnace.

But those might be less cost-efficient than a new, larger furnace. Still, they might be worthwhile as a stop-gap measure. You could use them for 5-7 years, until it is time to replace your current furnace.