calculations

[John q home own]

Last two ac units I have had installed short cycled. This time I want to do my home work prior to selecting a contractor.

Geographics = Thornton Colorado
House faces North East
Double pain windows
built 2004
two story
2200 sq ft.

I have not moved in yet so I do not know how bad the duct leakage is yet.

I would like to get an Idea of what size compressor and coils i will need

[nike123]

Last two ac units I have had installed short cycled.

Please, explain what you mean by short cycled.

[mchild]

Please, explain what you mean by short cycled.


With the typical U.S. system having a single capacity compressor the system is effectively over sized for all temps until the design conditions the system was matched to have been reached. Here in the U.S. the term a/c means cooling only. Short cycling with a single capacity system causes the indoor humidity to be high, electric usage to be high, and temp swings normally more than is comfortable.

John q needs to have a contractor perform a Manual J calculation on a room by room basis that will completely analysis and determine the heat gain of the home in order to size the cooling system to the actual load. A common problem here in the U.S. are contractors not willing to take the time to do this calculation and guessing at the size needed and most often the systems end up over sized. Thus, the common problem of short cycling and poor indoor comfort.

The other common problem we have is undersized ducting with significant leaks (hackers slapping systems together without care for quality). Ducts that leak that are installed in attics and other areas outside of the conditioned space of the home then cause infiltration of air into the home and making matters only worse.

John q should start by calling HVAC contractors in his area and ask them if they will size the system using Manual J, design and or evaluate/remediable the duct system using Manual D and match the system to the load using Manual S. If they say they don't need to do all that because they have numerous years of experince he should keep dialing as they are only guessing. Two homes with oversized systems is enough guessing.

The J calc is the foundation for all the rest. With that done then it is known (assuming they did a full room by room J calc) how much air flow must be delived to each room so that the duct system can be addressed with Manual D, and finally matching the system capacities to the actual load by using Manual S.

Good luck John q as finding competent contractors is difficult, but it can be done.

[nike123]

With the typical U.S. system having a single capacity compressor the system is effectively over sized for all temps until the design conditions the system was matched to have been reached. Here in the U.S. the term a/c means cooling only. Short cycling with a single capacity system causes the indoor humidity to be high, electric usage to be high, and temp swings normally more than is comfortable.

John q needs to have a contractor perform a Manual J calculation on a room by room basis that will completely analysis and determine the heat gain of the home in order to size the cooling system to the actual load. A common problem here in the U.S. are contractors not willing to take the time to do this calculation and guessing at the size needed and most often the systems end up over sized. Thus, the common problem of short cycling and poor indoor comfort.

The other common problem we have is undersized ducting with significant leaks (hackers slapping systems together without care for quality). Ducts that leak that are installed in attics and other areas outside of the conditioned space of the home then cause infiltration of air into the home and making matters only worse.

John q should start by calling HVAC contractors in his area and ask them if they will size the system using Manual J, design and or evaluate/remediable the duct system using Manual D and match the system to the load using Manual S. If they say they don't need to do all that because they have numerous years of experince he should keep dialing as they are only guessing. Two homes with oversized systems is enough guessing.

The J calc is the foundation for all the rest. With that done then it is known (assuming they did a full room by room J calc) how much air flow must be delived to each room so that the duct system can be addressed with Manual D, and finally matching the system capacities to the actual load by using Manual S.

Good luck John q as finding competent contractors is difficult, but it can be done.


Then I would suggest him to take consulting firm to make all calculations and supervision of instalation work.
He then could choose between contractors and their equipment according to their portfolio after contacting happy customers from their portfolio.
That is probably little more expensive then "key in hand" package from one contractor, but at the end, result is value for money.

Also, as heating/cooling load varies thru the year, he should consider some equipment with capacity control.
I would not recommend use of roof top (or split-type central AHU) unit and ducting for cooling/heating. Instead, I will recommend ducting only for ventilation purpose, with heat recovery and indoor fan-coils, for every room individualy.
It could be Chiller/heat pump based or VRV (VRF) system.

It is time arrived, even for USA, to reject that concept of central AHU and cheap energy and to adopt Japan leading, new, energy efficient technology.

I know that is more expensive than that current mainstream, but at the equipment exploitation life span, it is cheaper in total.

[mchild]

Then I would suggest him to take consulting firm to make all calculations and supervision of instalation work.
He then could choose between contractors and their equipment according to their portfolio after contacting happy customers from their portfolio.
That is probably little more expensive then "key in hand" package from one contractor, but at the end, result is value for money.

Also, as heating/cooling load varies thru the year, he should consider some equipment with capacity control.
I would not recommend use of roof top (or split-type central AHU) unit and ducting for cooling/heating. Instead, I will recommend ducting only for ventilation purpose, with heat recovery and indoor fan-coils, for every room individualy.
It could be Chiller/heat pump based or VRV (VRF) system.

It is time arrived, even for USA, to reject that concept of central AHU and cheap energy and to adopt Japan leading, new, energy efficient technology.

I know that is more expensive than that current mainstream, but at the equipment exploitation life span, it is cheaper in total.


nike123,

Yes, your thought of having a design firm and an installing firm would be the optimal method as the design firm is not trying to sell product and the installing firm only has to worry about a quality installation which would be specified by the design firm. All work would have to meet design firm requirements. But, you are also correct in that it would increase cost substantially.

There are installing firms who take their efforts seriously and will do all the correct calculations to make sure the system is correct for that home. John q just has to be diligent about determining the criteria used by any company to do the work.

Again, I agree with you that time has come to change the way our homes are conditioned. I am one of a very few who have gone with a VRV-S system for my home. It does seem like the acceptance of these systems is starting to gain traction. Higher energy cost will determine how fast the change occurs.