Outside unit not running in the winter
January 15, 2011 6:04 AM   Subscribe

[HVAC Filter]Is there a way to harness the power of my outside unit to force more and faster air through my gas furnace?

The kuanes family has moved into a house that uses gas via the hot water heater for heat. There is an outside fan unit that only comes on when the A/C is used in the summer. I'm assuming that the air for the heat is only forced through with a small fan within the furnace. The outside unit is *NEVER* used for forcing air when the heat is on. The furnace fan doesn't seem to be pushing the heated air very well. Is there any way to use the outside unit to push the heated air with more force? I have no HVAC experience, but am willing to pay someone to hook this up if it's doable.
posted by kuanes to Home & Garden (8 answers total)
Your question is a bit confusing. You initially state that your heat is from the hot water heater... Hot water heat systems typically are radiant and don't utilize any type of forced air.

The "outside fan" is probably the fan on your AC compressor, that has nothing to do with the air handling unit (which is probably INSIDE) that moves the for cooling.
posted by HuronBob at 6:20 AM on January 15, 2011 [1 favorite]

This is a schematic of a typical AC/heating system. As you can see there are two separate and totally unrelated fans. The one outside in the condenser unit is there to blow air across the coils to cool the refrigerant which has just been compressed, which is how heat is moved from indoors to outdoors. It has nothing to do with moving air through ducts, nor can it be made to do so. When the AC runs the air is forced through the ducts with the exact same fan that forces heated air through the ducts in winter.
posted by Rhomboid at 6:51 AM on January 15, 2011

The same indoor fan is used to push both heated and cooled air through your house.

If you feel that the airflow is not what it could be, check to make sure that your furnace filter is clean (or simply replace it). Make sure that all furnace vents and air returns are open and unobstructed.
posted by davey_darling at 7:06 AM on January 15, 2011

No. But I used to think the same thing. It seems like there would be a big duct between the indoor and outdoor unit. But there isn't.

It is just a couple of pipes. The way it works is exactly like those cans of "air" that you use to dust out computers and fax machines. (In many cases, the gas is even the exact same chemical!) You know how when you blow for more than a couple of seconds the can gets cold? Well, it uses that effect to cool the air. Instead of being in a can, the gas is inside a radiator kind of thing that the air is blown over.

The difference is, instead of the gas escaping into the air, it gets piped back to the outside, and the compressor outside turns it back into a liquid and pipes it back into the house cooling unit.

Anyway, the same deal is probably going on with your heating "side" of the furnace. The hot water is pumped through a radiator in the furnace (technically, an air handler) where it warms the air.

A couple of things can be going on.

1- The fan motor is going bad. Listen to the furnace when it is running for heat. Is it humming? The 50 or 60 hertz hum that electric devices sometimes have? If you can hear any hum at all over the noise of the moving air, the motor is almost for sure bad.

2- The air filter is clogged and preventing air from flowing. Needs to be changed more often than you think, especially if you have a dusty environment.

3- The radiator things are clogged with dust. This might be a do it yourself task, if the furnace/air-handler has panels that you can unscrew. You might be able to vaccuum them out, or brush them off with a moderately stiff brush. (Go WITH the grain of the tiny fins! If the fins get bent over, they will have to be carefully straightened.) But more likely, it would need to be disassembled and cleaned by an HVAC person.

4- It might just be designed that way- since it is using warm water instead of fire to heat the air, it might purposely be running slowly so the air captures more heat off the water.

5- Inside some of the ducts there may be dampers. These are valves used to balance the pressure in the ducts. If they are closed, open them. They are supposed to be like water and gas valves, where the handle indicates whether they are open or not. If the handle is pointing parallel to the duct, it is open. Perpendicular, closed.

6- The ducts are clogged. This would be hard to do, they are usually pretty large. But mice/rats/squirrels can clog one up in no time.
posted by gjc at 7:10 AM on January 15, 2011

Just thought of another thing. Are you concerned because it just doesn't seem right, or because you aren't getting warm enough?

If you aren't getting warm enough, it is possible that the water temperature is set too low. What is probably going on is that there is a burner that heats water. It sends that water to the furnace, where it releases heat to the air. And it also sends water to the water tank, where it releases heat to the water in the tank.

(I'm almost certain the two circuits of water are separate- I don't think it is code to run potable water through a furnace. You have to use a water to water heat exchanger, which is just a coil of tubes inside the water tank that warms the water.)

(Another thought is that there are multiple systems going on- there is radiant heat in one area of the home, and forced air in others. Or it is radiant all over, and the "furnace" just runs to circulate air.)

Anyhow, if it is set up that way, the water temperature for the burner ought to be fairy high- in the 160-190 range. And thermostats and valves are used to keep the water in the tank at the right (cooler) temperature.

So anyway, if everything checks out (by a professional HVAC person), and you still aren't feeling as warm as you want, one option would be to have the system partially converted to a heat pump system. This would likely mean ripping out the external unit and some stuff in the mechanical room. What a heat pump is, is an air conditioning system that can be run backwards. Because the "work" that an air conditioner does is actually to move heat from one side of the system to the other. In the summer, you remove heat from the inside to the outside. With a heat pump, it can do the opposite and run heat from the outside to the inside. Adding something like this would make the system more efficient (from a dollars per btu of heat perspective), and also give the hot water heating system a little boost and make it more comfortable.

Another solution would be to just upgrade to a more powerful fan. Not sure if this would help anything, but at least the air would be flowing the way you want.
posted by gjc at 7:32 AM on January 15, 2011

The furnace fan pushes the heated air through my ductwork just fine. It just doesn't seem powerful enough (or as powerful as the A/C). Our 12-foot vaulted ceiling in 1/2 of the house don't help much either.

Thanks for all of your answers. I'm not sure I'm ready to have the system converted to a heat pump system, as this sounds expensive.
posted by kuanes at 8:49 AM on January 15, 2011

If it is always slow for heat and fast for AC, the furnace or thermostat might be programmed to run the fan at half speed during the winter on purpose. Maybe to keep it from feeling drafty?
posted by gjc at 9:06 AM on January 15, 2011

Wait, what?

If you have ducts, that's a forced air system. Do you also have radiators? That is possible.

If so they're likely two different systems. Older homes that started with just radiators for heat could have had an AC system added to them using ducts. The heat in that situation comes from hot water being pumped around through the radiators. Convection from them heats the rooms. In that case the ducts wouldn't be doing much to help with a heating situation. They'd just be moving air around, not contributing to heating it.

The outside compressor fan would only run when cooling is needed. And that works by pumping refrigerant around, from the outside compressor to an exchanger inside the duct assembly. It takes heat from inside the house and dissipates it outside.

Unless you're in a very mild climate heat pumps are often something to be avoided.

So what is it you really have there?
posted by wkearney99 at 10:23 AM on January 15, 2011 [1 favorite]

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