How do I properly match a new gas furnace to an existing A/C unit?

January 17, 2010 8:02 AM

HVAC befuddlement - Does anyone understand the consequences of having a blower on your furnace that is undersized for the A/C unit you have?

I live in a house with a gas furnace that needs to be replaced (busted heat exchanger, leaking carbon monoxide). The furnace is way overpowered for the size of the house -- 130,000 btu whereas our needs are about 80,000 btu. An 80k btu furnace would give us a "3 ton blower," but because our current A/C unit is 4 tons, I'm told, this will be a mismatch.

What is confusing me is the precise consequences of the mismatch. One contractor tells me that the problem will just be inefficiency, because the blower won't be big enough to move the cold air around. The other contractor, though, tells me the consequences will be much worse -- "freeze up" of the A/C unit. So the second contractor's proposal is to get a 100k btu furnace with a 4-ton blower which will avoid that problem, even if the furnace will be a little too big for our house.

The thing is, we don't use the A/C more than about a week out of each year, versus the furnace for several months in the winter. So it's more important to me to have an efficient furnace. But I don't want to risk "freezing up" the A/C.

Has anyone encountered similar problems in trying to match a furnace and A/C unit? What did you do?

posted by chinston to home & garden (12 answers total)

The thing to know is that there are 3 components to your typical residential furnace/AC system and you can mix-and-match to a limited extent.
1. The condenser that sits outside. Sounds like you have a 4-ton unit, which is quite large for a single residence.
2. The furnace and blower, which come together. It won't hurt to have one that's slightly oversized, it just means your furnace will cycle on and off more.
3. The evaporator, or cooling coil, which attaches to the furnace (usually on top). The compressed refrigerant runs through here, evaporates from liquid to gas (the phase change takes a lot of heat) and then runs back to your condenser.

The danger comes when your cooling coil is too big compared to your blower. What happens is the warm air moves across the cooling coil and loses heat to it, so the cooling coil gets warmed and the air gets cooled. If not enough air is moving, the coil gets colder and colder, and can actually freeze. This is a common problem in HVAC systems that are not designed right; when it happens you can get permanent damage to the coil. Pipes can burst and spill refrigerant all over. It is definitely something to take seriously.
posted by beandip at 9:45 AM on January 17, 2010

(on preview beandip covers some of this above)

I am not a HVAC contractor. I had an air conditioner that froze up and I had a long conversation with the guy who came to fix it. The following contains intentional misstatements of the actual thermodynamics that are involved to simplify the explanation.

It is my understanding that having a blower that is undersized for your air conditioner MAY cause your A/C unit to freeze up. This is because the coils that are inside that air conditioner rely on having a high enough volume of warm air flowing across them to keep them from freezing. This can also happen with any air conditioner when the air filter is not changed, because this blocks the flow of warm air across the coils (this is what happened to me). This is most likely to happen when the temperature outside is hot or the house itself is hot inside and the AC is turned on set at a colder temperature, because the thermostat is telling the AC unit to keep cooling the air as it has not yet reached the right temperature and there is not enough air flowing across the coils to carry away the cold that is being generated. Any moisture that is in the air will freeze on the coils and if this goes on long enough the AC unit will eventually freeze solid. If this goes on long enough, there may be damage to the AC unit.

If you are only using your AC during the hottest weeks of the year, this would actually be the time that it is most likely to be a problem. You can probably avoid having this be a problem by setting the AC on a warmer temperature and keeping the AC turned on so it is running only intermittently.
posted by jefeweiss at 9:51 AM on January 17, 2010

I agree with others; the issue is that the undersized blower might not move enough air across the coils to keep them from dropping below 32F. If that happens, you can start to get ice buildup which at best will stop your AC from working and at worst might damage the coils.

Whether you'll get ice buildup depends on a lot of factors. The duty cycle of the AC (how much time it's on versus how much time it's off), the humidity (more humid = faster ice buildup if the coils do get too cold), and the temperature in your house are all issues.

I'd talk to your HVAC person and see if you can get a 80,000 BTU furnace but get the bigger blower put in. You really don't want an oversized furnace, so I wouldn't go up to the 130k BTU model in any event, but you do want a bigger blower if you can get it. (Which I doubt would be worthwhile.) If your HVAC person won't do this, I'd check around, or ask about different brands/models of furnaces.

In particular, some furnace/blower units that are designed to be used in conjunction with heat pumps have bigger blowers on them, because heat pumps require a greater number of air changes per hour to heat/cool a house. Even if you don't have a heat pump, a "heat pump ready" furnace might have a bigger blower for its BTU rating than a standard one.

I recently got a new furnace and was in a similar situation. We paid a bit extra for the furnace, and got one that has a big blower with multi-speed capability. It can really move air when it wants to, but most of the time works at a reduced speed. (I'm not entirely clear on how it decides what speed to use; I think it has something to do with the temperature change you're requesting. E.g. going from 60F to 68F = high blower. But 67 to 68 = low blower.) Anyway, if you got something similar, you might be able to get your HVAC person to program it to always use the higher blower speed while in AC mode. That would probably keep the coils from icing.

My recommendation is, in general, not to try to cut corners; dealing with frozen-up coils is a real PITA and it's worth a few hundred dollars extra to avoid the possibility.
posted by Kadin2048 at 10:35 AM on January 17, 2010

Clarification: Strike "(Which I doubt would be worthwhile)" from the third paragraph. It refers to something I removed during editing, and doesn't make sense. I very much do think that the bigger blower would be worthwhile; I don't think that getting a bigger furnace than you need (in terms of BTUs) would be a good idea, though.
posted by Kadin2048 at 10:37 AM on January 17, 2010

I'm confused by the freeze-up theory. Not that I'm challenging anyone, I'm sure you know more than I do about home HVAC. I am experienced with large AC units, though, and at least on the ones I've worked on, freeze-up would have to involve a lot of safety feature failures. I would guess that the problem would just be excessive cycling of the machine - it might trip off before it had cooled the room to the target temp, because it would be in a race condition between that and a low refrigerant temp shutdown.

At worst, low flow over the evaporator would simulate a no-load situation and cause a unit shutdown on low refrigerant temp. (Actually, mine had a low flow sensor that tripped the machine, but say it didn't.) Before that, the unit would try to keep the refrigerant temp up by cutting down on the coolant flow to the condenser (i.e. stop the fan on the unit outside, I'm guessing - mine used a water coolant system.) Are home units just too simple to have any of those features?
posted by ctmf at 10:43 AM on January 17, 2010

ctmf: I think the difference is that domestic AC systems are stupid, and in many cases (the ones I've owned) just don't have those cutoffs.

The system that I had the most freezup problems with had exactly two states: on and off. When it was on, the compressor ran and pumped coolant into the condenser. It did this without any regard for the temperature in the condenser, be it 50F or 30F, or whether the condenser was covered in a solid block of ice. Although I assume there were basic thermal cutoffs in the compressor motor, it certainly didn't have anything that stopped it from over-cooling the condenser. The "turn on" and "turn off" actions were governed by the thermostat in the house, based on the ambient air temperature, and the only thing that prevented a destructive on/off loop was a built-in gap between the "turn on" and "turn off" temperatures. (I.e., when set to 70F, it would turn the AC on at something around 71 or 72, and then cool it down to 69ish, so that the actual temperature sort of averaged out to the setpoint. This wasn't user adjustable and seemed to mostly be a result of slop in the thermostat mechanism, which was a bi-metallic strip and a mercury switch. I assume modern digital units simulate this behavior in software.)

The major sign that the coils had frozen up was noticing that the house was getting progressively hotter even though the blower was running and (not particularly cold) air was coming out of the vents. The solution was to shut everything off, wait 4 or 5 hours (or shut the AC off and just run the blower) so that the ice would melt off, and then restart it, slowly edging the temperature setting down so that the compressor wouldn't run long enough to let the coils ice up.

This never seemed to actually damage anything, although I was warned that allowing too much ice to accumulate on the condenser could cause the coils to break, since they're soft copper and not built to withstand the weight of all the ice.

A low-coolant-temperature cutoff would have been a nice feature, but I'm pretty sure even my current AC system doesn't have one. (Although maybe I'm wrong; I haven't had it freeze up on me so who knows.)
posted by Kadin2048 at 11:36 AM on January 17, 2010

I agree with everything Kadin2048 said above. The system that I had did not have any controls that would prevent the cold coils inside from forming ice. The system would still run even when the part of the AC that was inside the ductwork near the blower was encased in a solid block of ice. The only way that I knew that anything was wrong was that the system was on and it was only putting out a small amount of warm air.

The problem in Chinston's case is that the blower is always sold as part of the furnace. My opinion based on my experience would be that the freezing problem is something that can be managed by knowing how the system works, and that it might be worthwhile if the AC is something that is only used for a week or two in the summer. The problem is that it's really just an opinion, and I'm not especially qualified to say for sure that it is really the case.
posted by jefeweiss at 12:16 PM on January 17, 2010

To throw my $.02 in, have you considered replacing the AC coil too? How old is it? Ask how much that would add to the job. It's probably less expensive than you think, and it'll let you get the properly sized furnace.

Don't forget that furnace blowers use electricity, and using an over-sized blower all winter will cost you on your electric bill.
posted by dalesd at 4:12 PM on January 17, 2010

Get a dual speed blower.
posted by gjc at 4:29 PM on January 17, 2010

"The other contractor, though, tells me the consequences will be much worse -- 'freeze up' of the A/C unit."

This is a possibility, impossible to say how likely without knowing the specific details of your coil and even then could be tough to tell with certainty. If I was your technician I'd error on the side of caution.

beandip writes "This is a common problem in HVAC systems that are not designed right; when it happens you can get permanent damage to the coil. Pipes can burst and spill refrigerant all over. It is definitely something to take seriously."

An icing coil will not result in a refrigerant leak or even damage to the coil. Unlike water freezing in a bucket the ice produced on the coil is generated in molecule thick layers. So no damaging liquid to solid expansion takes place. In an extreme case where everything was just rightwrong you might be able to damage a compressor from liquid being delivered to the suction side. Still so unlikely that I can't remember ever seeing a case and extreme icing is actually pretty common as it's what happens when you either don't change your furnace filter or don't put a new one in when the old one is taken out.

From your clarification here's what I'd do:

1. Get a properly sized (for your heating load) furnace with a variable speed blower able to keep your coil clear. Probably going to cost you a bit more as you'll either have something non standard or be paying for premium equipment
2. If that is too costly just run the unit as is; you may not experience any icing depending on your specific conditions. Make sure to install a new furnace filter at the begining of A/C season to minimize restrictive flow.
3. If that doesn't work and you get icing, and considering you only use A/C a week out of the year, buy a programmable A/C thermostat with as many programs per day as you can. Mine has six which would allow for9-12 hours of A/C use. Use these to automatically turn your A/C off and on during the day letting it run for few hours and then off for a hour. Keep the fan running continuously. This will let any frost which builds up on the coil melt and drain away at the expense of higher temperature variation.
4. If that doesn't work (because there aren't enough on/off periods to cover your usage) there are timers you could wire into the signal wires going to the condenser (the box outside) that would allow you to set as many as 24 on/off cycles. The timer is around $100 and it's something that would take a tech less than an hour to Install.

posted by Mitheral at 5:23 PM on January 17, 2010 [1 favorite]

Thank you all for the helpful information. This has given me some basic understanding of the situation that I can use to talk to the contractors -- much appreciated.
posted by chinston at 6:41 PM on January 17, 2010

Incidentally, by talking to a few different contractors, I was able to get a properly sized furnace (btu-wise) for my house that also could be fitted with a blower that was properly matched to my current A/C unit. So in the end I didn't have to compromise. Thanks again.
posted by chinston at 7:02 PM on February 12, 2010