I have a 30yr old Tappan Gas Furnace. I replaced the blower motor and wheel. Furnace now keeps running and running to just make a few degrees difference in the house. Air ducts now feel too hot and I can't feel much air coming out of the ducts. Is this a failed fan/heat limit switch?

If you have a clamp on ampmeter, I would check the amperage on the fan motor and see what it's amping at while its running.. look at the name plate of the motor and it should match closely and under its rated amperage. If it is amping OK.. I'm sure you have a restriction in your ducts somewhere.. hope this helps..

I bet the motor is running backwards or the blower wheel is on backwards. Happens all the time.

hvac1000:
I wouldn't be surprised if the rear wheels on my car were running backwards, too. I'll see if the supplier shipped me a CW instead of a CCW blower wheel.

Hvac 1000:
You were right about something being backwards. I inspected the curvatures on the original blower wheel to the new. They were opposite. The supplier sent me a CW wheel instead of the CCW wheel I ordered. The airflow, of course, is now normal. I have to see if its still running hot. I would think the increased airflow would cool the air somewhat.

Let me know what you think.

I think the correct wheel will fix the problem.

I reversed the rotation on the electric motor to coincide with the wrong wheel, and its working much better. The ducts feel hot as they get closer to the furnace, but they feel normal further away.

That also will work. I mentioned that in my other post. It is somewhat normal for the duct to be hotter near the furnace.

I've rebalanced the ducts. The net result so far is I've lost some blowing power with the new blower wheel. It has 12.5% less blade surface area than the original. If I need more blower power I may need to replace the new electric motor (1075rpm) with a high speed motor (1625rpm).

I replaced the blower motor and wheel. Furnace now keeps running and running to just make a few degrees difference in the house



By increasing motor speed you will also increase noise. Good luck with your project.

You are correct. However the new motor and wheel are running very quietly, so I don't think it will bother us. You should have heard the noise from the original blower and motor. It sounded like a freight train.

Good luck.

hvac1000:
I've ordered the 1625rpm electric motor that will give me a 50% boost in air velocity. I don't trust the air flow for the January deep freeze. It gets down to 10 below zero around here.

Thanks for your help.

Blower motor speed has nothing to do with heat in general. There is only so much heat available for a given size furnace. All you are going to do is blow the same heated air around and crerate drafts,overtax your duct system, and increase your electric bill. It will be like living in a wind tunnel but I do not live there so enjoy.

You really should check the heat rise across the heat exchanger. Compare this with the range listed on the name plate. This should give you a pretty good indication of whether the blower you installed is pumping out too much or too little air.

Hi MarkwithAK
Thanks for the suggestion. I will do that. I know the blower wheel is pumping out less air than the original. We can tell at the air vents. It's much less velocity than before with the original, but also hotter. Also, the new wheel has 12.5% less blade surface area than the old.

When I changed the rotation on the motor to coincide with the CW blower wheel design, the ducts were cooler. They're still hot, although not as hot as before.

I know from experience with the old blower wheel and motor, what the temperature of the air ducts should be: warm to the feel. My guess 110-110 degrees.

Question: What should the temperature of the air be coming out of the air duct at the further point from the furnace?

hvac1000:
You are right that blower speed has nothing to do with the temperature of the furnace. However it has everything do with the velocity of air and cfm produced. The difference will be in the time it takes to warm the house in dead of winter. i.e. more air, less time. This will determine how often the furnace has to cycle, and my guess is that the higher 1625 rpm will pay for itself in reduced utility bills.

The difference will be in the time it takes to warm the house in dead of winter. i.e. more air, less time.

You can have all the air you want but since the furnace is LIMITED to the amount of BTU'S it can produce all you are doing is blowing around the same amount of BTU'S and that will not heat the home any faster in fact it can have the reverse result.

The faster air moves across the skin the more moisture it removes. Less moisture in the air has a determental effect on the human body in regards to sensation. The sensation part is where the person will feel chilled and very uncomfortable.

Did you do a calculation on what this extra CFM will do to all the air leakage values in the home? All homes leak air but higher velocity will increase that problem.

Engineers found out long ago that the best speed for heating is usually the lowest speed available while staying within the design heat curve of the furnace (IE temperature rise across the heat exchanger as mentioned by another poster). This provides the maximum output of heat for a given BTU imput. By incerasing blower speed beyond design temp it will actually heat less or have less actual output. The heat exchanger will never be allowed to stay at full operating temperature for any length of time causing more run time and allowing more spent fuel up the flue no matter what the efficiency of the furnace is. You see the negative effects the speed of the fan can have on a furnaces proper operation?

You started posting because you could not figure out why your new motor and fan cage would not mover enough air. You had to be told about proper rotation of the motor and cage. Then all of a sudden you are a fan expert.

I suggest you read up on fan laws to start with. This way at least you will have a little knowledge on the subject.

As you can see in the below link it takes approximately 50+ percent more power (energy) to increase the fan speed 20 percent. Since you are talking a 50 percent increase in air flow with your system your energy bill will go up quite a bit on electrical usage. But I might as well tell you now that the higher speed motor that you are getting will never spin the fan up to that speed. Here again it is a violation of the fan laws since you are not installing a 5 HP motor. In fact you might stall the motor so to spaek and burn it out sooner than you think because placing to small a HP motor under extreme stress will cause a quick failure. Then you can go out and buy another one. I just tell it the way it is and since it is your home and not mine it does not make a diference what you do. Our job here is to try and give good advise and if the posters ignore it that is fine with us. There are many good helpers on this board and many are more knowledgeable that myself under various circumstances. We try hard to give the best information possible. Enjoy the fan law link and become educated.

http://www.nyb.com/Catalog/Letters/EL-02.pdf

hvac1000:
You are right about BTU's. You are also right about best speed for heating is the slowest. The faster the air moves across the heat exchanger, the less time it has to heat. Thus slower air velocity produces a higher temperature at the air vents. This is basic common sense.

What I'm trying to accomplish is the air is too hot at the air vents, and I wish to cool it by moving it faster to compensate for the less efficient blower wheel. The wheel is rated to 1750 rpm's, so a 1625 rpm motor should be OK. The proof will be in the performance upon installation.

I'm not trying to get something beyond performance specs. I'm simply trying to get the air movement to where it was before I installed the new blower and motor.

I don't claim to be an HVAC expert, I'm just a do-it-your-selfer, and I've tapped into your experience to help me and you did quite well. You helped me go back and look again at the wrong blower that was shipped to me. I thank you for that.

I didn't have to be told about the proper rotation of the blower. I ordered a CCW and was sent a CW. I trusted the supplier and he made a mistake. The shipping document didn't have the rotation. I had to inspect the curvature of the blower blades and compare it to the original which was CCW. Don't get twisted in your shorts over fan theory. This is common sense.

It is not theory it is Fan Laws. They cannot be changed. Good luck.