Let’s discuss “Short-Cycling”

[yeldogt]

Back in 2019, GJ member @Mabalzach wrote in a thread that manufacturers “spec“ (I assume he meant “design for”) 2 to 5 cycles per hour. How do you and others here feel about that?

Cycles depend on delta .... IE set temp vs outside temp. At 50 degrees outside the cycle is going to be more vs what is going on a 10.

This is why one needs to look at how the load calculation is factored. The calculation uses both a max high and max low outside temp in the area --- target temp you are trying to achieve.

All too often systems are simply matched to what was in the house ... that's what most home owners are doing when they get "new" equipment --- they are replacing. There are also various "rules of thumb" -- one being 1 ton of cooling for 500sf of space. If you are an HVAC company going into a house and you ask the homeowner if the current system worked OK .... you just replace with the same.

With spray foam and decent construction I have no issues going over 1000sf per ton of cooling ... my new house is past 4k with 3 tons of cooling and the system has no problems in the upper 90's at 2pm. Most companies said 5T -- one said 7T. Heat pumps can pose another problem .... often in a given area the heat load and cooling loads don't match. Obviously in a cold climate one must provide enough heat and using electric strips to match up with cooling makes for costly running.

In South Carolina it's perfectly reasonable to size a system so as to use strips (electric resistance) a few days a year .... you would not want to size so it can't keep up for a large part of the winter. Sadly -- the vast majority of people never get a load and don't know where it all shakes out.

 

[jar944]

Cycles depend on delta .... IE set temp vs outside temp. At 50 degrees outside the cycle is going to be more vs what is going on a 10.

This is why one needs to look at how the load calculation is factored. The calculation uses both a max high and max low outside temp in the area --- target temp you are trying to achieve.

All too often systems are simply matched to what was in the house ... that's what most home owners are doing when they get "new" equipment --- they are replacing. There are also various "rules of thumb" -- one being 1 ton of cooling for 500sf of space. If you are an HVAC company going into a house and you ask the homeowner if the current system worked OK .... you just replace with the same.

With spray foam and decent construction I have no issues going over 1000sf per ton of cooling ... my new house is past 4k with 3 tons of cooling and the system has no problems in the upper 90's at 2pm. Most companies said 5T -- one said 7T. Heat pumps can pose another problem .... often in a given area the heat load and cooling loads don't match. Obviously in a cold climate one must provide enough heat and using electric strips to match up with cooling makes for costly running.

In South Carolina it's perfectly reasonable to size a system so as to use strips (electric resistance) a few days a year .... you would not want to size so it can't keep up for a large part of the winter. Sadly -- the vast majority of people never get a load and don't know where it all shakes out.

Whoever specified the system system in my house was either drunk or incompetent. The upstairs heat pump went out 6 years ago and I've not bothered to do anything about it. The lower unit does just fine heating and cooling the entire (5700sqft) house. Yes it didn't keep up when temperatures hit 102, but on a normal 95 degree day it's fine. Considering its the smaller tonnage ac unit (lower is propane heat) the whole system was grossly oversized.

On the plus side humidity is 20% or so in the summer now.

 

[yeldogt]

Whoever specified the system system in my house was either drunk or incompetent. The upstairs heat pump went out 6 years ago and I've not bothered to do anything about it. The lower unit does just fine heating and cooling the entire (5700sqft) house. Yes it didn't keep up when temperatures hit 102, but on a normal 95 degree day it's fine. Considering its the smaller tonnage ac unit (lower is propane heat) the whole system was grossly oversized.

On the plus side humidity is 20% or so in the summer now.

Twin systems just double the problem. A second floor is designed independent of the first -- the first independent of the second. They are not independent .. it's all working together. So you end up with a second floor system as if it is all out there alone .... knowing it's up in a hot/cold attic the HVAC company goes even bigger.

One hears all kinds of reasons for second systems --- redundency is a common one. Zoning is always discounted .... people saying variable speed units are not a good fit because of the blowers. I always zoned my houses and zoning is now even easier due to the VS equipment. Now there are reasons that a 2nd system may be better (very large house over 5T) but zoning works very well.

Most don't want to do the match and figure it out ..... it's easier to stick a 2T in the attic and a 3T in the basement when the whole place only needs the 3T of cooling