I need some tech input on a new furnance and ac unit for the house. I had two different companies come out and give me a quote for replacement equipment. I currently have a 100,000 btu furnance and 5 ton ac unit. The house is a two story 3,000 square ft home. One company said I need a 110,000btu furnance and 5 ton ac unit, the other guy said I need a 90,000btu furnance and 2.5 ton ac. He said to go with smaller furnance because my 100,000 unit is short cycling, because it is too big for the house. He also said it is more efficient to go smaller with the ac unit and have it run longer for more even cooling between the upstairs and downstairs. There is a $2,500 cost difference between the quotes. What is the correct size units for my home? Confused after talking to these contractors. Please clear things up for me. What's more cost effective for fuel and electricity? Thanks

Likewise, I'd like an honest opinion on my deal:
1650 SF, currently electric forced-air heat.
Power goes out on occasion during winter.
Local generator guru says 1st priority is switch to gas heat.
I have natural gas.
One of the things I wonder about is if there's no electricity, how does a gas furnace light off?
(As you can tell, I'm pretty ignorant on the topic )

Apologies for the hijack, hope there's a HVAC Stu here...

I can only share my experience.

My Mom needed a new furnace last year for the downstairs and upstairs - 2 apartments. Bottom significantly bigger than the upstairs. Furnaces were from around 1967. Heating guy says the furnaces were oversized so we split the difference between the calculations and what was installed. The results were very good. Asking around found out that in the past the calculations were not very accurate so everyone oversized furnaces.

Oversized furnaces are not good. My Mother in Law's gas furnace is oversized and short cycles. It is not pleasant heating because of it.

On the AC side, 5 ton seems too big and 2 1/2 ton seems too small. I'm in Cincinnati 2 story 1900 square feet and have a tight house and have a 2 1/2 ton heat pump. Seems about right.

I would lean towards the smaller units.......... Maybe 3 ton on the AC.

FWIW, the old rule of thumb is 1 ton of cooling for every 600 square feet. So, 5 tons for a 3000 square foot house is about right. Short-cycling is bad but undersized is no good either...the unit will run a very long time to keep things cool/warm.

Get a 3rd company to give you an estimate and go with whatever 2/3 say who have physically seen your setup and house.

Too big furnace won't let the furnace run long enough to get it's peak performance, too small a furnace will over work it.
Too big of air conditioner will cool but not run long enough to de humidify properly so it will cycle more. Too small of air conditioner will have to run longer to cool properly.

When we had our air conditioner replaced close to 20 years ago it was a smaller unit then the old one and we were worried a bit about it but we've never had any problems.

Go with the one that did a Manual J calculation (which I assume none of them did or they would explain it to you).

Here is a presentation I found real quick that appears to explain it: https://www.energy.gov/sites/prod/files/2013/12/f5/webinar_hvac_calculatingloads_20110428.pdf

basically, you calculate the the loss through the walls (based upon the insulation, windows, size of the external walls), roof, etc... the area the house is in (the need for heating and cooling based upon outside temperatures), and it will calculate the number of BTU of heating and cooling you need; then you can size up a "hair" from there to account for the non-seasonal days, or when the load changes quickly.

Rules of thumb work, but it is very easy to get it wrong because they do not take into account many important factors, such as the condition of the house (insulation, etc...) or area in the country (which obviously makes a difference).

Having a 3th party coming out today. Let's see which way the pendelum will swing. Bigger or smaller?

Being an engineer, this is what I did. I worked with contractor while we were remodeling the house. We had the house extremely well insulated, R60 in attic, R-15 (limited to existing walls) to R-23 in walls. House is 2600 sqft. Ended up with a 90,000 btu high efficiency unit with variable speed blowers. The furnace automatically slows the blower or speeds it up depending on thermostat set pint and temperature. I did a similar calculation for the garage when i added heating system (radiant floor heat). I ended up with 60,000 BTU water heater which is a tad undersize for cold winter months based on the calculations. But for a garage, it works well. Garage is 2x6 wall so R-21 in walls and R-60 in ceiling rafters and about 1600 sqft room space.

For A/C we have a 2.5 ton unit. And I think thermally its fine. The distribution system for the furnace/ a/C isn't optimal so the two front upstairs rooms don't get adequate cooling in the summer. I may install some booster fans. Its not an issue with heat, because heat rises and the upstairs room get a lot of residual heat up from the stairway.

I have become somewhat of an educated consumer in this field ...

Don’t have to worry tooo much about heat down here but AC is a must. I like the idea of slightly undersized!

Gets the humidity out of there bester

I can give you my opinion and what I did.
In house #1 I went with what they told me and was sorry.Every time I had a party the air couldn't keep up and would get quite warm inside and the furnace would shut off and come right back on if it was cold out.
House #2 I went with an air conditioner 1/2 ton larger and furnace next size larger and they both worked out great.They would run long enough but not to long.I was very happy with going a little larger.

So the 3th party contractor came out and ran all the numbers to todays standards. For the furnance a 90,000 btu two stage was recommended over the 90,000 variable speed model. A calculated 1% in efficiency did not warrant a $500 cost increase over the 2 stage model. The a/c required per calculation was a 3 ton two stage air conditioner with a 4 ton A coil. This is the most effective size calculated for a 2,800 square ft 2 story log home. The old calculations required a 5 ton a/c with a 600 square ft per ton calculation. Which to use is still in my gut after your responses?

I have a variable speed setup. It was worth the extra money to me not to get woke up when the fan kicks on. Both the air handler fan and the compressor fan smoothly ramp up to speed quietly. Whereas the old setup the fan would kick on, the compressor was really loud, sounded like someone kicked it to life.

My HVAC is in what amounts to a closet in the middle of the house and shares a wall with the master bedroom.

I like the third proposal far better than the others.

I really like variable speed fans, variable speed compressors and multistage furnaces.

However you are doing the right thing evaluating the cost effectiveness.

Sometimes a 95% unit has too long of a payback over a 90% However sound levels are important and I would pay $500 extra for the varible speed fan without a second thought.

You still need the generator for the blower, thermostat, etc. Switching the furnace to gas will take a 5-15kw load off of your emergency power needs. You could likely get away with 10-12kw gen instead of a 20-40kw set. Thats essentially 2-3cyl territory until you get to 20kw and the 4 cylnders enter the lineup.