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AC short cycling on/off
northernsoul
Member Posts: 134
Hello
Moved into an old house that has the wall programmable thermostat about 3 feet away from a floor supply vent (floor vent is about 16x16). When AC turns on to maintain the set temp only runs for about 5 minutes then off for 10-15. The main floor is cool but upstairs is hotter as it’s not runnng for long.
i am not sure if the systèm isn’t balanced or the thermostat is too close to cold air supply so it’s shutting off fast.
not sure if I need to get a tech into check the system and compressor etc, or if I buy a smart thermostat and run it off a remote sensor in a spot that isn’t near a vent or have an electrician move the current one to a new spot entirely.
i am not sure if the systèm isn’t balanced or the thermostat is too close to cold air supply so it’s shutting off fast.
not sure if I need to get a tech into check the system and compressor etc, or if I buy a smart thermostat and run it off a remote sensor in a spot that isn’t near a vent or have an electrician move the current one to a new spot entirely.
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probably poorly designed. could probably be balanced better. open the vents up stairs and close the vents most of the way downstairs, especially the one by the thermostat. the load up stairs and downstairs are very different, it will never be great with both on one zone.
could probably set the cycle time on the thermostat longer too.0 -
mattmia2 said:probably poorly designed. could probably be balanced better. open the vents up stairs and close the vents most of the way downstairs, especially the one by the thermostat. the load up stairs and downstairs are very different, it will never be great with both on one zone. could probably set the cycle time on the thermostat longer too.0
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are you heating and cooling with your hvac system? and are you sure that the 16 x 16 is a supply. as supplies go that is big, more like a return size. and if it is a supply i would expect more short cycling than what you are experiencing as it would create wild swings with the thermostat readings. it wouldn't take more than a minute to satisfy the thermostat being only 3' away.0
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pedmec said:are you heating and cooling with your hvac system? and are you sure that the 16 x 16 is a supply. as supplies go that is big, more like a return size. and if it is a supply i would expect more short cycling than what you are experiencing as it would create wild swings with the thermostat readings. it wouldn't take more than a minute to satisfy the thermostat being only 3' away.
Yes sometimes it runs very short.Is there a fix? Other than moving the thermostat or using a remote temp sensor?0 -
You can move the thermostat and it might help a little but its not going to solve the underlying problem. You have a a physics issue. You see when you running a combined system like that your seasonal demands change. meaning you need more air supplying the upstairs in the summer than you do in the winter. warm air is always rising regardless of the seasons. you need to zone the duct system. easier said than done. or have separate systems. one upstairs and one downstairs. Or you could install boiler with baseboard loops for heat and just use the hvac system for cooling only. And balance it for cooling. You have options. It just cost money0
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Is the ductwork a converted gravity system?0
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mattmia2 said:Is the ductwork a converted gravity system?
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northernsoul said:Is there a fix? Other than moving the thermostat or using a remote temp sensor?
Even if everything else is perfect, it's still always going to be warmer on the second floor if the thermostat is on the first floor.
Is there adequate return air on the second floor?
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HVACNUT said:northernsoul said:Is there a fix? Other than moving the thermostat or using a remote temp sensor?
Even if everything else is perfect, it's still always going to be warmer on the second floor if the thermostat is on the first floor.
Is there adequate return air on the second floor?
there is no return air register on second floor. There are only baseboard registers in the 4 bedrooms that are fed from 2 duct risers in the wall (registers on either side )
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Add a fan blowing down the stairs as well. That'll help move the upstairs air down towards the return on the first floor. Way cheaper than adding a return air duct upstairs. If there are any balancing dampers, make sure the ones serving the first floor are partially closed and the ones serving the upstairs are completely open—you want to force the cool air to go up where it doesn't want to go. (You'll need to reverse the dampers in the heating season.) Don't close them off completely, you still have to maintain a minimum amount of air through the system or you'll run into issues.
Another thing I've done is add those little booster fans in the runs going upstairs, wired to come on with the blower.
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ratio said:Add a fan blowing down the stairs as well. That'll help move the upstairs air down towards the return on the first floor. Way cheaper than adding a return air duct upstairs. If there are any balancing dampers, make sure the ones serving the first floor are partially closed and the ones serving the upstairs are completely open—you want to force the cool air to go up where it doesn't want to go. (You'll need to reverse the dampers in the heating season.) Don't close them off completely, you still have to maintain a minimum amount of air through the system or you'll run into issues.
Another thing I've done is add those little booster fans in the runs going upstairs, wired to come on with the blower.
ratio said:Add a fan blowing down the stairs as well. That'll help move the upstairs air down towards the return on the first floor. Way cheaper than adding a return air duct upstairs. If there are any balancing dampers, make sure the ones serving the first floor are partially closed and the ones serving the upstairs are completely open—you want to force the cool air to go up where it doesn't want to go. (You'll need to reverse the dampers in the heating season.) Don't close them off completely, you still have to maintain a minimum amount of air through the system or you'll run into issues.Another thing I've done is add those little booster fans in the runs going upstairs, wired to come on with the blower.
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You're going to need a good HVAC tech anyways, or eventually. Wouldn't be a bad idea to have them come out, go over your system, do a AC pm, clean condenser coil, check evaporator coil (clean if necessary) & check/balance the airflow.
Before it gets hot.There was an error rendering this rich post.
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ratio said:Add a fan blowing down the stairs as well. That'll help move the upstairs air down towards the return on the first floor. Way cheaper than adding a return air duct upstairs. If there are any balancing dampers, make sure the ones serving the first floor are partially closed and the ones serving the upstairs are completely open—you want to force the cool air to go up where it doesn't want to go. (You'll need to reverse the dampers in the heating season.) Don't close them off completely, you still have to maintain a minimum amount of air through the system or you'll run into issues.
Another thing I've done is add those little booster fans in the runs going upstairs, wired to come on with the blower.
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If you turn the tstat down as low as possible does it still short cycle?
And does the inside blower cycle on and off with the AC unit?
It is not impossible that the outside unit is cycling on either low or high pressure switches......but inside blower would remain on while doing this.
HVAC pro is a good idea.
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JUGHNE said:If you turn the tstat down as low as possible does it still short cycle? And does the inside blower cycle on and off with the AC unit? It is not impossible that the outside unit is cycling on either low or high pressure switches......but inside blower would remain on while doing this. HVAC pro is a good idea.When the thermostat calls for cooling to maintain set temp (you can hear the click) it will run for a few minutes then I hear the thermostat click again and blower shuts down shortly after. So I’m not sure if it’s short cycling due to an issue with unit or the oversized supply register (12x12) nearby the thermostat cools it down fast and hits set temp fast.0
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It might be? The ducts upstairs in bedrooms are on interior walls at baseboard level that share one riser from basement between adjoining two rooms if that makes sense.northernsoul said:mattmia2 said:Is the ductwork a converted gravity system?
Are the ducts huge and probably covered in asbestos paper? Ducts sized for gravity circulation were much larger than modern forced air ducts. If the system was designed for gravity warm air heating it probably won't perform well with ac.
Balancing with the register dampers or the duct dampers, either one is ok, whichever it has, the effect is the same.
I would look very carefully for a company that can sort this out, very few will put the effort in to it or have the needed skills and you will pay them for essentially nothing.0 -
mattmia2 said:
Are the ducts huge and probably covered in asbestos paper? Ducts sized for gravity circulation were much larger than modern forced air ducts. If the system was designed for gravity warm air heating it probably won't perform well with ac. Balancing with the register dampers or the duct dampers, either one is ok, whichever it has, the effect is the same. I would look very carefully for a company that can sort this out, very few will put the effort in to it or have the needed skills and you will pay them for essentially nothing.mattmia2 said:Is the ductwork a converted gravity system?The basement ductwork for main floor was all repiped it seems about 10 years ago, but they kept some of the old 12x12 registers and added two more coventional ones.0 -
That probably started out as a gravity system with a coal furnace.
Not having returns upstairs you will need to have the doors open or undercut to get any air flow. The air has to be able to get out to come in. Even with that having the returns downstairs will make the natural tendency of the cold air to fall down the stairs worse.0 -
@mattmia2
You’re probably correct.
my first priority is to get a tech here to inspect the system for any issues, and clean the coils - although the prior owner covered it in the fall/winter - I can see some buildup of dirt on the outside of the fins.
I also want to see about moving the thermostat . Having it close to a big supply register seem problematic.
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A gravity system is very low velocity and higher temp differential. ac is high velocity and low temp differential. it takes someone that really knows what they are doing to convert a gravity system to work well with ac. since you say there are no returns upstairs my guess is that it is functioning exactly as it has been constructed.0
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