System not keeping up

[tykester]

I was involve with the HVAC design of a custom home. It is a zoned system, upstairs and downstair. Well two years after the homeowner moved in, we got a call that the cooling was not working. The homeowner did not call until two years after he move in because he does not use the upstairs. It was only noticed when he had company staying upstairs. There are other simular homes that we did and they are not having any problems. The difference is a skylight, but even when I figured it to be the worst case it only add roughly 2000 BTu/hr to the load. The total load is roughly 20,000 BTU/HR. We installed a 2.5 ton condenser and a 3-ton furnace. We have had problems with the manufacturer, so we changed the condense to another manufacturer, but the same size. We also changed out the furnace to a 5-ton to get more airflow. But no matter what we do, it seems like we can only drop the upstairs an intial 4 degrees and then it will just idle there. It stays roughly 10 degrees lower than the outside temperature. If the outside temperature goes up so does the indoor temperature. We have tried everything. It seems to me that maybe we are missing insulation in the house? Any ideas as to what is going on? Has anyone run into something like this before? Please help.

 

[Waramanga]

I would first check the airflows, ie get the system balanced, and check it against design. Then get a good tech in to trouble shoot system, ie amps, pressures etc, T on T off etc. How does the system perform on heating??

 

[ChasBean1]

A 10°F drop and how much dehumidification at the coil? 2.5 tons in a residence should be good for at least 2,500 square feet of house. If it's 90° and humid out and you get 80° and relatively dry, that's not so bad. At night, the 90 might typically go as low as 75, which allows comfortable sleep. If they want it to be 72°F and 50% mid-day, expectations may be the problem. They should maybe go out to the beach. There are many more problems typically with oversized DX systems than undersized systems...

 

[IRstuff]

2.5 tons would a bit light for a 2500 sq.ft. house. We had a 2200 sq. ft. house with a 3 ton A/C and it ran quite a bit in the summer. Your typical A/C installer would try put a 4-ton A/C on a 2200 sq.ft. house.

What's the breakdown on the 20,000 BTU/hr? That doesn't seem to jive with a house big enough to only live downstairs.

How is the zoning accomplished? It sounds like there's only one condenser, so is the zone control done by dampers?

TTFN

FAQ731-376

 

[tykester]

The house is roughly 1600 square foot. The downstairs has no problem cooling. It is a dampered system. I have not heard of an issue from the homeowner as far as heating. This system is in Reno, Nevada (very dry) so there is no latent load. We are a design/build company and we have had our techs out there. Pressures, temperatures, superheat, subcooling, and amp draws are all where they should be with the old condenser and the new condenser. We do not have an airflow hood, so we check airflow with static pressures. The original furnace (3-ton) was pushing roughly 1200 cfm. Since we changed to a 5-ton the furnace pushing 1900 cfm and it is still not cooling the upstairs down. We are going to drop the fan speed to better match the 2.5-ton condenser.

 

[imok2]

After taking a heat load and determining how much CFM to each room did you balance the system or are you just throwing darts? Do you know for a fact that the insulation was installed as specified and installed correctly. Take infrared readings and see if you are absorbing a lot of heat on the second floor as opposed to the first. An infrared heat gun can give you a thorough understanding of where your home is losing heat in winter, or gaining it in summer

 

[Compositepro]

In my experience if you provide all of the cooling air to the top floor you will not get too warm on the first floor. Cool air has a very strong tendency to sink. Do you have ceiling registers in the second floor? Having a return air register in the second floor ceiling can help tremendously by removing the hot air from where it collects. This is costly and unusual, however. In winter you want to take return air from the lower floor. A ceiling fan can help.

Building HVAC systems are some of the worst engineered systems I see. Getting perfect heating in in winter and cooling in summer is a very complicated control problem and cannot be done with one simple thermostat. You get what you pay for.

 

[vmarho]

Just a thought,
Is there unfinished attic space above 2nd floor?
Is there attic ventilation, such as gable or soffit vents and roof ridge vents to let heat out?

 

[BM4BRANES]

Compositepro offers very sage advise, 2nd floor returns are crucial for the system you described and the associated complaints.Regardless of the dificulty involved, do nothing prior to correcting the return air location (you will only be treating the symptoms)

 

[IRstuff]

I'm not following something. Are you able to see air being moved into the top floor? A strip of tissue paper would tell you lots about the flow and velocity. Sounds like you need to find out where all the air is going. Maybe the ducting isn't conveying all the air to where it's supposed to go.

All my ducting is in the attic, and it's got maybe R15, and it's HOT up there. Nonetheless, there's plenty of cold air getting into the 2nd floor, although we had to twiddle the registers a bit.

A simple thing might be to simply shut all the registers on the first floor and see what happens.

TTFN

FAQ731-376

 

[BronYrAur]

What is the diffuser layout upstairs? I assume you have floor supply registers. Do you have any returns upstairs? if not, the air may be short-cycling back down the stairs without proper distribution in the rooms. I assume these are bedrooms upstairs. If they don't have returns, you need either transfer grilles or very undercut doors to get the air out. Finally, if you have only 1 point of thermostat control, it's no doubt downstairs. Once it's satisfied, you lose cooling.