Residential HVAC unit 4

[MotorCity]

Hello HVAC friends.....structural guy here hoping you can point me in the right direction.

The problem is that when the AC unit runs (not sure what size, need to check) very little of the cold air makes it up to the second story.

The air that does come out upstairs is cold, but the flow is minimal. So, I'm thinking that because the air is cold, the issue is not with the AC unit but is probably due to an undersized fan in the furnace which blows the cold air through out the house. If that's the case, I thought of installing in-line duct fans (which are not cheap) or installing a more powerful fan in the furnace.

Am I on the right track, what would you do?

 

[lukaiENG]

you could also have some leaky ductwork or or maybe some vents open off the system to other areas? in my house we have a furnace for the 2nd level which also has some grilles in the basement. I have to close these down in the summer to make sure the cooling goes where it is needed.

 

[IRstuff]

If the AHU can handle the furnace's air low ; it should be able to handle the A/C's air flow, since it's typically the same air flow, i.e., the A/C's evaporator coil is inserted into the AHU's airflow from the furnace. A good check would be to turn on the furnace and see if there's adequate flow

What temperature is the A/C air at the furnace? It should be in the high 50s to 60s °F to be operationally sufficient.

I second the leak possibility; when we moved into our most recent house, the ducting was completely disengaged and the AHU merrily pumped its air load into the attic.

Certainly, when the A/C is on, you ought to divert more of the A/C load to the upstairs, since it's hotter to start with upstairs, and hot air rises from the lower story.

Another possibility is that the evaporator coil has frozen over, blocking the air flow; this typically occurs if the condensation drain is blocked.

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[GT-EGR]

Along with the previous suggestions, it’s possible that your fan and ductwork is fine, and you just have a much easier duct path through the first floor grills than the second floor. A simple solution may be to close dampers on the first floor, which will drive more air up to the second floor.

This especially happens if your thermostat is on the first floor, and then it calls for cooling, and since all your air is coming out on the first floor it cools everything down quickly, and stops cooling even though the second floor is still hot. So if you shift more airflow to the second floor by closing dampers on the first floor, you may eventually just find the sweet spot.

Or that ductwork is just too small or too leaky. Then you’d need to fix the ductwork or try the in-line fan.

 

[Compositepro]

You cannot keep the grill dampers in the same position for heating and cooling and expect good results in both cases. You need to make adjustments from winter to summer.

 

[MotorCity]

Thanks for the suggestions. I did close the dampers in rooms that are rarely occupied (laundry room, guest bedroom, etc) in an attempt to increase air flow to the other rooms. But I think I do need to go one step further and experiment by closing off a few more dampers on the first floor since the thermostat is on the first floor.

The leaky ductwork is also a strong possibility. The house was built in 1965 so there was probably little, if any, thought given to energy efficiency and conservation. None of the duct joints are taped or sealed.

I have seen companies that provide duct sealing where they essentially pressurize the ducts with this airborne "sealant" that finds its way into all of the leakage points.

Installing a larger fan in the furnace may be the cheapest/easiest option but then I run the risk of dislodging segments of ductwork.

 

[IRstuff]

I suggest that you verify heater air flow first. If that's behaving correctly, then your evaporator coil is freezing over.

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[itsmoked]

Besides the good suggestions above you could check flow in an upstairs bedroom then open the bedroom window and recheck. It could be you have a lot of back-pressure between the upstairs and downstairs.

Also, could there be a return upstairs that has a clogged filter?

You've checked filters right?

You should probably video your ducts. Could be giant dust clogs perhaps a dead body, of sacks of money?

If you can get it working by subtle adjustment of registers mark them with a sharpie as cooling or summer.

Most systems have fans that have several speed taps. Lots of systems I work with have different speeds for heating and cooling. Cooling always run faster. Slow can cause fast evaporator icing and subsequent choked flow. Possibly the speed-up command it not happening? Icing is always a possibility.

If not icing and the speed commands coming out of the thermostat are good and the ducts actually have been proved to be clear then see if you can speed up the blower with faster taps.

My furnace has about 7 speeds and I run it on middle one for 'fast'.

Definitely check on icing. If the coils run cool enough you may need defrost. My cheesy window AC needs defrosts or it ices up.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

If the problem were icing the A/C would run all the time, and even the lower floors would not get adequate cooling.

 

[IRstuff]

Do you even know whether it's a single unit A/C? On a larger house, there might be a separate A/C for each floor.

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[willard3]

Bad ductwork is also a distinct possibility, ie, damper closed, elbow leaking and etc. You can probably change the fan hp- to overcome this, but noise will increase. Get a velometer and measure discharge grills

 

[MotorCity]

To answer a few of the questions:

Its a single AC unit for the entire house

I checked all of the dampers in the duct work and they are all open. Air flow to each room is controlled by a grill/register in each room

Duct work was cleaned and video'd about 2 yrs ago. Video did not reveal any clogs or ductwork issues.

The same lack of airflow exists whether the heat or AC is on. Usually its not an issue in the winter when the heat is on (I'm guessing) because the heat from downstairs eventually rises upstairs to supplement the heat coming from the 2nd floor registers.

The only filter is at the fan in the furnace. There are no filters on individual return/supply vents.

I am not sure if the fan is single or variable speed. How/where would I check this? Based on the consistent noise level, it seems to run at the same speed for cooling and heating.

 

[LittleInch]

If all the dampers are open then you might just be pumping all the cold air into the lower floors with not much left for the upstairs.

Try closing all the lower floor grills or dampers (what is a register??) and see if you can generate some flow. Try the trick about opening some windows a little bit - Do you have a return air flow system or does it all just "leak" out of the room?? Then if you start to get air upstairs slowly open the lower dampers / grills and see if you can balance the system.

This does sound like someone has just decided they wanted more flow and opened it all up and not realized that some sort of balancing is required where you have variable length ducts. You often get the same in hot water heating circuits where the top floor radiators only get luke warm because someone opened up the valve son the ground floor radiators.

The hot air rising issue will probably help in the winter to send more hot air upwards. Cold air will do the reverse.

The data on the motor / fan should tell you if its fixed speed or not and give you some idea of the rated flow it can produce.

Or you could just have a fan that is too small / knackered!

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Compositepro]

Opening a window should not be necessary, or effective, unless the A/C takes all its air from outside. Just make sure there is a return path to the A/C inlet/return, which is likely on the first floor. Open doors will do it, but you probably want your A/C to work with the doors closed as well.

Do start with all the grills closed on first floor. If this does not provide adequate flow to upper floor you need new ducting or blower. Then start opening grills on first floor, in sunny rooms away from thermostat. Just keep in mind that a single zone A/C for the whole house will never be perfect. Leaving the blower on all the time will help to mix air between warm and cool rooms (fan set to on rather than auto).

Another problem you may have is that the grills on the second floor are in the floor and the cold air you do get never reaches the ceiling, where most of the heat load is. In this case a ceiling fan or floor fan is needed.

An attic exhaust fan on a thermostat can help save money and increase comfort.

 

[IRstuff]

OK, so the lack of heating flow pretty much eliminates the A/C, per se. The choices are then:
> possible imbalance between upstairs and downstairs
> blockage of all the upstairs ducts
> leakage or broken branch duct to the upstairs ductworks


Do you know where and how the ducts split out to each floor and each room?

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[berkshire]

Little inch,
A register is an outlet grille with a built in damper / flow regulator .
B.E.

You are judged not by what you know, but by what you can do.

 

[LittleInch]

Well I kind of gathered what is was but honestly had never heard of it before. The lack of a/c built in systems in the UK probably has something to do with it!

We await the results of the test with them closed on the lower floors.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[itsmoked]

Comp I don't believe any of us were advocating opening windows 'as a solution' only as a data point.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

I realize that but I am saying that it is a "pointless" data point. The idea is to give the air flow some place to go. It needs to get back to the fan inlet. That is located inside the house, not outside. So open the door, not the window.

 

[MotorCity]

So last night I closed all of the 1st floor grills and ran the AC. Not surprisingly, the second floor grills nearest to the fan had a lot (almost too much) air flow and the grills at the end of the duct run had minimal air flow. I did this with all interior doors open and windows closed if that makes a difference.

I am leaning toward an undersized fan as the culprit. I looked at the fan last night and did not see any controls or labels that indicated it had variable speeds.

Label indicated its a 1/2 hp fan motor