AC short cycling on/off

northernsoul

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.

mattmia2

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.

northernsoul

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.

Thanks.  The vent near the tstat is an old metal grille style one with no damper to close.  I could place something over it to block most of it if that will slow the cooling at the tsat to run longer. 

pedmec

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.

northernsoul

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 both gas furnace and AC central air. Oddly enough it is a supply vent.  I had to go down and double check myself on where is was leading from and it’s the supply side of the furnace.  I don’t know If an installer at some point just used an old vent hole from a prior system. It’s 100 plus year old house.  I’m sure they’ve done some odd stuff.  The return vent is same size just in the next room.

Yes sometimes it runs very short. 

Is there a fix? Other than moving the thermostat or using a remote temp sensor? 

pedmec

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 money

mattmia2

Is the ductwork a converted gravity system?

northernsoul

mattmia2 said:

Is the ductwork a converted gravity system?

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.  

HVACNUT

northernsoul said:

Is there a fix? Other than moving the thermostat or using a remote temp sensor? 

If everything else is copacetic, then I would relocate the thermostat to the second floor, or like you said, get a remote 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?

northernsoul

HVACNUT said:

northernsoul said:

Is there a fix? Other than moving the thermostat or using a remote temp sensor? 

If everything else is copacetic, then I would relocate the thermostat to the second floor, or like you said, get a remote 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?

So put a remote sensor on the second floor wall and run a smart thermostat from that ?

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 )  

ratio

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.

northernsoul

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.

Thanks - by damper do you mean the butterfly screws/levers you sometimes find in the basement ductwork or the register dampers ? I’ll see if we have any damper knobs in the basement ductwork.

STEVEusaPA

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.

northernsoul

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.

So no damper screws/baffles in the basement duct pipes so my only recourse would be to partially close some of the 5 registers on the main floor but that might cause issues ?

JUGHNE

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.

northernsoul

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.

No it wil run until it hits set lower  temp.   From what I know they cycle on-off the same.

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. 

mattmia2

northernsoul said:

mattmia2 said:

Is the ductwork a converted gravity system?

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.  



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.

northernsoul

mattmia2 said:

mattmia2 said:

Is the ductwork a converted gravity system?

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.  

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.

The 2 vertical ducts in the walls to second floor are rectangle and about 8x3 and 6x6.   Looks like the vertical section on one at least that I can see has paper on outside - inside is metal. 

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.   

mattmia2

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.

northernsoul

@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. 

mattmia2

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.