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hvac sizing

John Dirks Jr

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The pics are of my house. It's 1200 sq ft on the main level with a full basement that is 1/2 finished.

I'm planning in the not too distant future to replace the HVAC systems. The old Sears AC unit is a 2 ton and the old Mueller naturally drafted furnace lists 120k btu. Both of these systems are still in working order and meet our comfort expectations.

In sizing and choosing new systems, what should I consider? What type of gas furnace and what size do you think would be in the proper range?

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The old climatrols were so wildly inefficient, the actual output was probably about 80,000 btus, and that's still probably too much.

When I replaced mine, I did the calcs and came up with 132000 for a drafy old house with minimal insulation that's almost 3 times the size of your house, in a substantially colder climate.

You'll probably end up with a calc that's 75,000-80,000 tops.

But, every house is different. Do the numbers.

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Get one of those online load calc programs and run it.

Your furnace is too large.

I don't know about sizing air conditioners in your region.

I like the size of my AC unit at 2 ton. It runs long periods and does not cycle very much. Maybe that's why its 39 years old and still running. I can measure air temps at the registers in the 40's. I recently put a low return in the basement and a high return on the main level and that helped the entire system tremendously. Things are more even throughout the whole house now.

Frankly, the whole system is still working well but I guess age does make it a ticking time bomb. If I don't see energy savings with investment of a new system, it will be disappointing.

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Better to undersize your furnace slightly than oversize it. Ideally, on the coldest day of the year, it should be running almost continuously.

I'm with you on that.

Even though my windows are old, I got good insulation in the attic and the brick veneer (on all sides) doesn't leak much air. We do ok for an older house and it doesn't take much air to fill up the main floor where we spend most of our time. No vaulted ceilings to waste heat on either.

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Yes, please do the math. Then, tell us.

Regarding price, if 70,000 is the right size, and the installation is competent, it would be a decent bargain. I'd want to know specific model and serial #'s before I purchased, and I'd want to know how they were going to install it.

You can find small cheap Lennox Cat I furnaces in a box for around a grand to $1200, sometimes less. Same with AC; maybe $900-1000 in the box. The AC might be an old 10 SEER unit; if it is, I wouldn't want it.

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Here's a calc for the AC. 2 ton is more than enough.

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BTW, how efficient do you think the 50 year old Mueller Climatrol is? 50-60% maybe? It has never struggled to keep up even on the coldest days. If its 60% and listed at 120k then that comes out to 72k actual usable heat for the living space. We never even came close to struggling with it on the coldest day.

So, if a new Lennox 90% efficient is listed at 70k, that would come out to 63k usable. I bet that would do it. It might be a tad under sized in the coldest weather but that's not frequent. Even then, new and more efficient windows could be coming soon too.

So based on my experience of living for and extended time with what I have now, the way my house acts and what my family needs to feel comfortable, that too can help me decide what I need as good as any calculator, can't it?

I'll go look up some heat calcs now.

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This might explain why my current furnace cycles, even in the coldest weather. This quicky calc includes the zip code too so I suppose it gives a value to the average climate in my area. This and all the other reading I've done says 70k is more than enough. Even a two stage on low speed would keep up.

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Another consideration is geothermal. What are your thoughts on that? I understand the greater initial cost but if I can get it back in 5 years or so it might be worth it. 30% tax credit on the installation.

The only thing is, today's high efficiency gas furnaces are hard to beat so recovering the initial cost could take longer than expected.

What are considerations for geothermal for a house like mine in my climate?

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Around here a geothermal heat pump runs around $18,000 ? last I checked ? a big expensive unless it is the last house you plan on living in.

Another consideration if you?re really geared toward saving on utility cost is the blower motor used. Remember the efficiency rating (i.e.92%) is that of the fuel, not taking into consideration the efficiency of the blower. There are multi speed and variable speed blowers that automatically adjust depending on what is being called for. Just need to raise the temp 1 degree the blower runs on a lower setting so as not to uses as much energy. There are also gas valves that will run on higher / lower setting depending on what is being called for. Of course these bells and whistles and to the cost ? but it is something to look into.

When I had my equipment replaced a few years back I went w/ a 16 seer heat pump with a 80% gas furnace as the e-heat. Variable speed motor, dual gas valve, 6? filter.

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I wouldn't mess with geothermal or even heat pumps on a house the size of John's. Do the math and install it to commercial standards. Keep it simple, inexpensive and aim for an efficiency that's average or somewhat above average for today's new construction. It'll have a long, relatively trouble free service life, great comfort level and will give you a maximum return on your investment.


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You might want to check out the Mitsubishi ductless heat pump systems. No leaky ductwork and you can control each indoor unit individually.

"Mr. Slim."

This website has some prices. You'd want a tri-zone, I think, to take advantage of the design.

http://www.e-comfortusa.com/index.php?m ... =47&page=3

I've seen a couple of them and I think they are pretty cool heaters. [:)]

One advantage is that you can leave your old furnace in place for a backup, so it's an easy transition to the new system.

A new company up here has just come out with a huge heat pump unit that looks like a garden shed. It uses a heat exchanger to heat domestic water when you run the AC. It also has a self contained NG generator built in for power outages. Price will be thru the roof, I imagine.

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Yes Id aim for the 70-75,000btu range 90plus furnace with a good 13 seer condensor or heat pump.

warrantys tend to be generally the same between most brands in my experiance,since most of the components are only made by a few companys .

The install makes the majority of the differance in how well it will work/how long it will last

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