Please post the brands and model numbers of the furnaces you are considering. Also indicate your location and the size of your townhouse. I want to make sure you are not mixing up input and output sizes.

If you can hear the furnace operating, then you should be able to tell if it short cycling. Furnaces, whether they are old or new, do not shut down for 2 hours. The thermostat tells the furnace when to turn on and off. Unlike an AC condenser, there is usually no lock out mechanism on a gas furnace. It can cycle on and off constantly.

Good Morning Mike, Considering Carrier 58STA070-08 and 58STA045-08. My home has 1450 livable Sq. Ft., 2 Stories, homes on either side of mine and I live in the Zone 4, Moist (A) section of the country. I should further clarify that I'm concerned about short cycling if the furnace is oversized. It is my understanding that an oversized furnace may short cycle. Short cycling can overheat the heat exchanger. Today's furnaces have relatively sophisticated circuit boards. One function of circuit board/computer is to communicate with the temperature switch. To protect an overheated HX from cracking, the circuit board will shut down the furnace until the HX cools to a safe temperature. My understanding is that if the HX overheats 3x within a certain period time the circuit board automatically shuts the furnace down for a minimum of 2 hours. Would appreciate your thoughts. Let me know if you need additional info. As an engineer, but not an HVAC engineer, I have researched this to the 9s, ask a lot of questions and perhaps now I am bleary eyed. May I ask your background? Looking forward to your response.

I am a homeowner with an interest in HVAC. I own Carrier equipment so I am most familiar with it, but I would not say I am an expert.

The load calculation seems on the low side. However you butt against two houses so that does reduce the load. What is the closest major city? I would like to look up the winter outdoor design temperature. I will also assume you set your thermostat to 72 degrees in the winter.

I reviewed the product data document for the Carrier 58STA. You have the correct output sizes. Assuming the load calculation is correct, then the 58STA045 would be sufficient. Even if one of your neighbors was not living in their house, they would have to keep the house to at least 50 degrees in order for the plumbing not to freeze. I don't agree with the contractor's logic for choosing the bigger furnace.

I have not been able to find anything in the Carrier installation for product manuals that talks about the furnace shutting down for a minimum of 2 hours if it over heats. There is an over heat safety shut down. It should go off if air intake or exhaust is blocked and will continue to do so every time the furnace fires until the problem is corrected.

If you have other information please share it. I would like to understand how this would actually work.

If the load calculation was done by a competent person, was
done to a known, recognized and up to date standard with a result that indicates
32.5MBtu, then you need a furnace that is the next size up from 32.5MBtu that
outputs 32.5MBtu, probably ~40-MBtu input and 37MBtu output unless something
smaller is available.

I’ll go out on a limb and guess that if you’re looking at
single stage 80% efficiency then you’re not likely in a location that’s heating
dominated. I would also suggest that if air-conditioning is also required that
you confirm that your duct work is of a sufficient size and the furnace you’re
considering is the proper match to both heating & AC cfm required in both modes.

My background is in geothermal design, installation and
drilling/groundloop consultation, as well as hybrid systems involving all manor
of space conditioning.

IMPO

SR

I live in the Zone 4, Moist (A) section of the country.

HVAC equipment is location dependent it is not a 'one size fits all' application. You being an engineer I am surprised with your response. If you are on the run, (the lamb) as it were you do realize who ever is looking for you they have your exact location?

So the question I ask myself is 'why are we less than them?'

Heat exchangers are built to withstand the heat of the furnace. There are controls employed to shut them off if they over heat.

Typically if a system is going to over heat it is related to the lack of return air, supply air blockage, improper gas pressure setting or a combination there of... The lack of return air or supply air blockage corresponds with design of the system or in some cases improper maintenance or lack there of.

If you don't size the furnace properly, it can be more of a problem when it comes time to cool the structure. Improperly sized furnace for the AC will give you far worse problems to deal with in a moist maybe humid part of the country.

Short cycling typically does not occur with furnace... unless you live in a tent with no insulation causing vast fluctuations on the control that is what causes the system to turn on to begin with.

Most of the controls of a gas furnace are to prevent the thing from catching fire and burning your house to the ground. If there is a over heating problem many manufacturers will lock out the furnace under such scenarios. It will continue to do so until the problem(s) are corrected.

Stop worrying about a cracked heat exchanger, they are quite durable.

Size the furnace to run properly with the AC system. Short cycling the AC is a completely different subject.

If your furnace short cycles it's because your home is not properly insulated, or there is a safety control of the furnace telling you to get the thing serviced.

I am not an engineer. I am a Texas Licensed HVAC contractor that specializes in HVAC repairs, retrofits, new system design and installation from single speed AC systems up to full variable speed systems as well as zoning for primarily the residential HVAC market.

I work on and repair all HVAC brands. I service the Katy, Texas area and some surrounding areas. To schedule a service appointment: call 832-475-6895.

what if anything are you doing about AC?...the size needed for AC could impact the size of your furnace as far as blower rating required.

IMO

While it is true that you don't want your furnace to short-cycle, all the reasons you have been given are wrong. A lot of on/off can cut the life of the appliance or increase repair frequency in addition to lowering efficiency. This is not a modulating/condensing furnace, but with those furnaces, you lose even more of the efficiency that you pay dearly for in higher equipment costs. The comments of others here indicate that the 35k furnace should be sufficient. If you wanted to go a little higher, other brands may have something that falls between 35 and 50. That is probably another way of saying what fsq4cw was getting at. If it is humid when it is warm in your part of the country, you will be more comfortable with a variable-speed blower with the proper controls.