Properly Sized System?

[GJWSATX78209]

How do I know if my system is properly sized? In other words, what defines a system that is working properly according to ACCA?

I hired one of the most reputable companies in San Antonio, TX to install entirely new ductwork and a new split unit forced air system in my 1950 ranch style 1,760 SF house. They designed the system according to all of the ACCA Schedules. However, I have a feeling the unit is slightly too small.

Today the high was 93 and we had the thermostat set at 71. It started off the day at 71 and maintained that temperature at noon when I checked it. However, by 5 pm it was 74. It has been peaking at 8pm, so I would assume it will be up to 76 by then.

I'm meeting with the company tomorrow to discuss this and I want to make sure I'm well educated about this topic. A properly sized system for San Antonio should be able to maintain a temperature of 71 inside when it is only 93 degrees outside, right? What indoor temperature should a properly sized system be able to maintain when it is 100 degrees outside?

Thanks in advance for any help with this.

-George

[beenthere]

Depends on what temp it was designed/sized to maintain.

[jimj]

75* is normally ID design temp for manual J. Did your contractor discuss any special needs you wanted for your system?

[GJWSATX78209]

They did specifically ask us what temperature we like to keep our thermostat set to and we told them 71. However, since I haven't yet seen the manual J calculations, I'm not certain they didn't just plug in 75 since that is standard.

If they did use 75, then what does that mean? Does it mean that the system should maitain 75 degrees when the outside temperature equals the design temperature of 100 degrees (which is what I believe it is for San Antonio, TX).

[Patty Joe]

I need your help sorry for jumping in this way.

[udarrell]

They did specifically ask us what temperature we like to keep our thermostat set to and we told them 71. However, since I haven't yet seen the manual J calculations, I'm not certain they didn't just plug in 75 since that is standard.

If they did use 75, then what does that mean? Does it mean that the system should maitain 75 degrees when the outside temperature equals the design temperature of 100 degrees (which is what I believe it is for San Antonio, TX).

San Antonio TX summer design used to be listed at 97-F dry bulb, 73-F wet bulb or around only 31% Relative Humidity.

If there is air movement 75-F will usually keep you comfortable.
However, they should have designed for 70-F at listed design conditions.
What indoor humidity did you want at 71-F?
What percent indoor humidity did the prior unit usually hold, - at 71-F?

[jimj]

They did specifically ask us what temperature we like to keep our thermostat set to and we told them 71. However, since I haven't yet seen the manual J calculations, I'm not certain they didn't just plug in 75 since that is standard.

If they did use 75, then what does that mean? Does it mean that the system should maitain 75 degrees when the outside temperature equals the design temperature of 100 degrees (which is what I believe it is for San Antonio, TX).

That is correct,however there is a fudge factor built in where you should be able to do better than that. Before you jump to conclusions, there maybe an equipment issue that's causing your issue. Start there.

[GJWSATX78209]

I just want to make sure I am being reasonable when I have the discussion with them. They have been great to work with and they have ensured me that they will replace the system if it ends up that they undersized it. However, what I didn't know before was what defined an undersized system.

At this point it is my understanding that they have exhausted all of the testing options and everything is checking out. We actually left town all weekend and gave them the keys to the house so they could periodically check on it and perform any tests necessary. When we came back in town the thermostat was set to 75 and it was still 75 at 2 pm. However, by 8pm it was 77. The high for the day was 92

I guess the next step is to ask to see the manual j calculation to see what design temperatures they used. Assuming there are no system issues and assuming they used 75 as the design temp then that would mean the system is undersized, right?

[jimj]

Udarrell says your design temp is 97*DB and 73* WB ( I think you can depend on his info). Its appears that in less than those conditions you cannot maintain design indoor temps, so I would say if everything is working correctly there has been a miscalculation in manual J.

[udarrell]

Here is what I would do - were I in your position.

Providing the duct system is sealed, airflow is on target & everything else checks out, I would look at all the ways possible to bring the heat-gain calc down to where it would meet your 71-F target... manual J can also show you the HTM multipliers to reach your 71-F goal.

The WHOLE-HOUSE ARENA, plus duct system Manual D airflow, is where your increased savings will optimally accrue from...

Changing to a larger unit will NOT save on your operating costs; other options will.

Look first at the envelope areas of your home to reduce air infiltration to a minimum.

Look at insulation & ways to reduce solar gain, & more.

This WHOLE-HOUSE ARENA is where H-VAC Techs will be fully involved with - very soon. - Darrell

[jimj]

Here is what I would do - were I in your position.

Providing the duct system is sealed, airflow is on target & everything else checks out, I would look at all the ways possible to bring the heat-gain calc down to where it would meet your 71-F target... manual J can also show you the HTM multipliers to reach your 71-F goal.

The WHOLE-HOUSE ARENA, plus duct system Manual D airflow, is where your increased savings will optimally accrue from...

Changing to a larger unit will NOT save on your operating costs; other options will.

Look first at the envelope areas of your home to reduce air infiltration to a minimum.

Look at insulation & ways to reduce solar gain, & more.

This WHOLE-HOUSE ARENA is where H-VAC Techs will be fully involved with - very soon. - Darrell

Agree, 1000%

[Ed Janowiak]

Are you drawing all the blinds on all the windows that the sun shines through?

I've seen doing that simple thing reduce the gain on many homes.

[pstu]

Among other things, I would recommend you get radiant barrier in your attic if you don't already have that (assuming the usual TX ventilated attic). And just a few solar screens on West facing windows might make a real difference. Air leaks in the duct system are a real energy waster, you need to have that sealed the best possible.

I am a homeowner just west of Houston, so we have an idea of what you are experiencing. My own AC benefitted from having a new technician look it over, he found a problem I'm pretty sure the old tech was overlooking -- the result was the AC cools and dehumidifies better than ever before. It is possible there is a Manual J problem, but IMO much more likely that some component of your AC is not working as planned. Let us know what you find out.

Hope this helps -- Pstu

[GJWSATX78209]

I appreciate everyone's input and I'll let you guys know how things turn out.

If you guys are telling me to add more insulation, radiant barrier, etc then in essence you are telling me there was a manual j error, right? (My understanding is that insulation, radiant heat, air infiltration, etc are all already factored into those calculations.)

The problem with that solution is that it puts the burden on me, the homeowner. Since we purchased a system specifically designed for our house, then it should work properly without any other modifications to our house. The burden to get it working properly should fall on the HVAC company, not me. (We were very clear about the amount of insulation contained in the walls and attic during the design process so there should be no excuses there.)

Of course I would like as efficient a system as possible to keep my electricity bills low, but not at the expense of my comfort. Also, I don't want to have to add any insulation, radiant barrier, solar shades, etc in order to have a more efficient system. I just want a system that works properly for our house... not a perfect house... our house.

Thanks again for all of the suggestions.

[beenthere]

Contractor may not have allowed for capacity derate for your design conditions.

[jimj]

What you have said is correct, however it may be cheaper for your contractor to have your insulation upgraded than for system replacement. In this case you both win.

[GJWSATX78209]

...very good point, jimj. I'll mention that as a potential solution. Thanks.

[genduct]

It isn't just the air temp that your unit is removing heat from, it's also all the "stuff" the walls, furniture, everything in the space that is holding heat, So the fact that your unit is not so oversized that it will quickly reduce the inside temp means that your contractor DID THE RIGHT THING!!! There have been several people who have already pointed out to you that 75 degrees is what the unit was probably the inside design temp used.

[pstu]

I feel it important to say this might or might not be a Manual J error. Only if you can establish the AC is putting out its full rated capacity of cooling BTUH, or if you can identify the Manual J error, can you really believe it exists. Believe it or not, contractors sometimes make mistakes in installing.

That said, in another thread a poster raised the question of too much reliance on Manual J as the solution to all cooling problems. He said it might become a scapegoat for other problems, and that is possible. What has been observed in the majority of installations over the years, is that in the absence of Manual J there is a distinct tendency to have AC grossly over sized (and to recommend bigger as a solution to any cooling problem). Over sizing is a flaw and I see that can be its own scapegoat, with people blaming the too-large AC on mostly theoretical grounds when other problems are actually responsible.

At the time I suggested radiant barrier, I thought your main priority was to make the problem go away. I almost wrote suggesting a window unit, calling it the "poor man's two stage"<g> which would add capacity without compromising the humidity removal of the existing unit. Agreed, that would not be a pretty solution to your problem.

Regards -- Pstu

[jimj]

I feel it important to say this might or might not be a Manual J error. Only if you can establish the AC is putting out its full rated capacity of cooling BTUH, or if you can identify the Manual J error, can you really believe it exists. Believe it or not, contractors sometimes make mistakes in installing.

That said, in another thread a poster raised the question of too much reliance on Manual J as the solution to all cooling problems. He said it might become a scapegoat for other problems, and that is possible. What has been observed in the majority of installations over the years, is that in the absence of Manual J there is a distinct tendency to have AC grossly over sized (and to recommend bigger as a solution to any cooling problem). Over sizing is a flaw and I can see that can be its own scapegoat, with people blaming the too-large AC on mostly theoretical grounds when other problems are actually responsible.

At the time I suggested radiant barrier, I thought your main priority was to make the problem go away.

Regards -- Pstu

I agree, if manual J is done correctly and the system is working correctly there will be no issues.