AC will not shut off sometimes.

[krueger86]

I have a Colman furnace with central air I was told when we bought our trailer that we had a 3ton AC unit installed. I am not sure where to get the numbers for the A-coil and the Condenser. If someone can point me in the right direction that would be great!

My problem has been going on for awhile. My AC just was not keeping up year after year and this year it got my attention with a high electric bill, an electric bill that we had while we spent 2 weeks on vacation. I first noticed that the AC was not shutting off and this has happened on and off over the past couple of months. I started by changing the thermostat and the problem is still there. Well, we got a new thermostat out of the deal. Digital and programmable. Next I did what I should have been doing every year but have not done and that was to clean the A-coil of the dust that has accumulated. That helped some but the unit would still not keep up on average weather days. Next I read that I needed to clean the condenser. So, I did this and it has made a world of difference. I knew that the unit was just not exchanging the heat efficiently. The unit will now keep the set temp on average temp days and is even cycling on and off. Yet it still will not shut off from time to time, just happened yesterday.

When the system was new it would cool without problems. We also had a shade tree that helped keep the house cooler in the summer. Ever since we lost the shade tree the system struggles to keep up and it will not keep up when the temps are in the 90’s. It does seem that we need more volume from the blower. I think the blower is a 3 speed blower. But, it doesn’t seem that it runs at different RPM’s. A little help with a bigger replacement blower would be appreciated.

Thanks,
Anthony

Furnace model # 7966A856

AC control box 7900-6021

Blower Assembly 7900-6031

[Robert]

Hi Anthony,

Model numbers are on side of Condenser on data sticker if still legible and on data sticker on front plate of a-coil.


Blower motor size cannot be increased, mobile home ducts are smaller and limited to air volume and speed.

Plus size of system dictates amount of airflow which it has now.


Shade on condenser is almost a must in the temps we have been getting nationwide.


Hot days, they will run alot.


Check inside controls area of condenser outside for a stuck contactor when it won't shut off.



Thanks,
Robert

[krueger86]

Hi Robert! Thanks for the reply. Can you tell me the HP of the 7900-6031 and what speeds it runs at? I have not been able to get sufficient airflow from the ducts in the middle of the main run since we bought the trailer. At the ends it blows really well. Restricting the flow at the end ducts does not help much in the middle rooms. That was just one reason why I wanted to consider a larger blower. As far as the contactor is there just one common part number so I can order and just replace it? Or do I need to get numbers off of the old one to make sure I order the correct one?

[krueger86]

I also wanted to explain. The system is not shutting off at the set temp on the thermostat. I set it for 75 and when it does finally get to a point where it will cool below set point I notice the the thermostat says 72 and the system is still cooling. I have to turn the swith at the furnace on heat or turn off power to get it to stop. This is an intermitted problem right now. I did replace the thermostat with a newer digital one. I don't know the model number right off hand. But it was doing the same thing with the older programmable thermostat. That is why I am thinking it is something else. I am an industrial electrician. So, if there is some voltage measurement that I can make to check to see if it is the thermostat or another relay I can do that. Thanks.

[Robert]

Hi,

What is the condensing unit model number ?

How many wires actually connect to the t-stat ?



Thanks,
Robert

[krueger86]

Robert,

I can not read the model number off the label. The t-stat is two wire. I will try to post some pics. Thanks.

[krueger86]

Here are photos of the furnace and condensor.

http://s197.photobucket.com/albums/aa28 ... ce%20pics/

I checked the t-stat today. Checked continuity and the t-stat checks out OK. Just wondering if it could be the sequencer relay?

Thanks,
Anthony

[Robert]

Hi Anthony,

Your blower motor is 1/3hp, 1100rpm. It has 3 speeds: High/black=a/c and Fan On Cont., Medium/Blue not used, Low/Red=Heating.

The sequencer is not in the cooling circuit.

IF you have a mode switch on your wall t-stat, ALWAYS keep it set to HEAT and AUTO. Switches at furnace control box set to Heat for Heating season and Cool for cooling season and Auto always.


There is a thermostat reversing relay inside the control box that controls the a/c in a 2 wire system.


Thanks,
Robert

[Robert]

Hi Anthony,

Looked at the pics, both run capacitors and the contactor need to be replaced. The contactor contacts are pitted and that could also be reason for a/c running continuously.


I would make sure t-stat is set to HEAT if applicable and replace the run caps, contactor and t-stat reversing relay.

Also be sure the blower motor black wire is hooked up for cooling speed.

That is about all you can do as for as DIY without calling an hvac tech to do some testing of system.

Also, remove all floor registers, start at one end and lay flashlight down in duct, go to next register and hold mirror down in duct and look back at light.

Keep moving light one register at a time to inspect entire duct trunk line. Also inspect where register boot connects to trunk and be sure nothing has separated.

Will see if I can determine condenser model and give some part numbers and ordering info if interested.


Thanks,
Robert

[Robert]

Hi Anthony,

Is there any legible numbers on the run caps. Need mfd/uf ratings if legible.


Also, open a/c control box and take some pics of components so I can which reversing relay it has.


Thanks,
Robert

[krueger86]

Hi Robert,

Thanks for looking at the pictures. I am interested in ordering the correct parts. I will pull the run caps as soon as I can and see if there are any legible numbers. I will also take pics of the control box and I think I know which one is the reversing relay. I will post when I have them updated.

Thanks,
Anthony

[krueger86]

Robert,

Another question. Can I replace the blower assembly with a 1/2 HP blower to get more volume? Just looking at my options.

Thanks,
Anthony

[Robert]

Hi Anthony,

I'll check and see about the 1/2 hp motor.




Thanks,
Robert

[krueger86]

Robert,

If it is an option I would replace with the 1/2 HP blower assembly that has a bigger squirrel cage. I will measure because I think the replacement would be part number 7900-6051/C Blower Assembly and it is 16" high. Bottom of housing measures 11.5 wide" x 15.5" deep (lip included in measurement). I was thinking this might be a direct replacement.

Thanks,
Anthony

[Robert]

Hi Anthony,

1/3hp that you now have is the largest you can go for a 3 ton unit. The air volume is decided by the system, not desires of the person. Each system has a set amount of airflow per ton of the unit.


Be sure there is not any blockages or gaps or openings in the duct. IF all is ok or made ok, then if there is still any problems, that is unfortunately associated with mobile home ducts being smaller and the airflow is effected by it.

The only true cure, is normal conventional home ducts and conventional home units and blower assemblies. Basically a total overhaul of the systems and total replacement.

Usually not a monetary option. Use ceiling fans throughout to also keep air flowing better.



Thanks,
Robert