Where are you? Shouldn't deviate from proper design temp.

 

In your experience, does an accurate application of Manual J provide a good match (is it conservative or otherwise) or how do you select a size for all of the cases that fall between two sizes..

Manual J cannot compensate for bad or old insulation in walls, also it don't compensate for leaks in your home, windows,door seals,etc. this is why it does take a little "Human" experience. If the company has been in your city for a long time and has done many installations, this always help build the complete system, and will factor in on it's future performance. I personally would never,ever,use it exclusivity, without injecting my own personal experience of the city, especially in Houston Texas, you will have some unhappy customers trust me, been there done that.

 

Well, I'm glad I'm not the only one who thinks like this.

I was just afraid to say it in public.

 

Consider All the Options...

...
I am spending good money and would like to get the most efficient system (see note on saving electricity, above), but I am concerned that when we most need it, it won't be able to cool our house adequately.

I have three questions for the collective experts. In your experience, does an accurate application of Manual J provide a good match (is it conservative or otherwise) or how do you select a size for all of the cases that fall between two sizes. And two, if it really is a 2-ton load and I were to install a 2.5-unit, how much efficiency would I be wasting? Finally, I have a 2.5-ton unit currently running on a simple thermostat. Can I use the next couple of days of approximately 90-degree heat to do a quick empirical study of cycle time to see if 2- or 2.5- ton would be better? (BTW, I do get the sense that our current 2.5-ton unit cycles too often when in use.)

You would have to test the performance of your 2.5-Ton system to see if it is delivering its rated BTUH at said conditions.

Cycling too often can be caused due to other things besides an over-capacity system; room t-stat location cool source 'on-cycle' & heat source 'off cycle' - on & on, cooling anticipator & on/off too small a differential, - check it out!

There are many reasons why it may not even be delivering 2-Ton operating performance. Therefore, unless you do an accurate test, its run time doesn't tell you anything useful.

The right kind of manual J will show you the multiplier's for optional retro work that might get it well within a properly performing 2-Ton's capacity.

If you want to reduce both heating & cooling that is the option to play with.

Normally, there are a number of retro options that would make the 2-Ton work. An air infiltration blower door test, & a manual D (Air-Flow!) are critically important. :det: - Darrell

 

Cycling too often can be caused due to other things besides an over-capacity system; room t-stat location cool source 'on-cycle' & heat source 'off cycle' - on & on, cooling anticipator & on/off too small a differential.

Cycling too often can also be caused sometimes by setting the stat at to high a setting, 78-79-80 depending on the outdoor conditions, and air filtration. What is the humidity at? that will tell you a lot.

 

Hi,

I am getting quotes for a furnace and A/C change out to take advantage of the tax credits. Three contractors have done calculations to determine the A/C sizing. In each case, the indoor temp used was 76 and the outside temp was 89. We try not to use our A/C a lot, to save on electric costs, but when we use it, we want it to cool the house. Typically, we would not turn on the A/C unless the daily high were 83 to 85 or above. Therefore, the majority of the days we would want the A/C to be working are likely near or above the ASHRAE (97.5%?) design temp of 89.

First of all, I sized the cooling needs at 75 degrees and at 50% RH indoors always.
With this I can go up to 115% max of the sensible heat.
But of course, you have to get close with the proper inputs, your local companies should know what type of house/s they are in your city.

The contractor's calculations seem to show somewhere between 2- and 2.5-ton, generally around 26K for latent and sensible combined. Two of the three contractors are recommending 2-ton and one 2.5-ton (his number was 27.5K).

I am spending good money and would like to get the most efficient system (see note on saving electricity, above), but I am concerned that when we most need it, it won't be able to cool our house adequately.

Perhaps you should ask them how long have they been doing Load-Calcs?
I have many competitors that are beginning to do load calcs now, why, I don't know. It takes some experience (a few projects) to get close and understand how to properly input the correct info.

I have three questions for the collective experts. In your experience, does an accurate application of Manual J provide a good match (is it conservative or otherwise) or how do you select a size for all of the cases that fall between two sizes. And two, if it really is a 2-ton load and I were to install a 2.5-unit, how much efficiency would I be wasting? Finally, I have a 2.5-ton unit currently running on a simple thermostat. Can I use the next couple of days of approximately 90-degree heat to do a quick empirical study of cycle time to see if 2- or 2.5- ton would be better? (BTW, I do get the sense that our current 2.5-ton unit cycles too often when in use.)

Thanks for your response.

These questions have been already answered above!

 

Thanks for all of your responses so far. In particular, it is clear that there is no good way to use my existing 2.5-ton system to determine if it is oversized. Since that appears to be the consensus of responders, I will not try and address any questions related to that.

Otherwise, in the order the comments were received:


seatonheating: Where are you? Shouldn't deviate from proper design temp.

It is my understanding that 89-degrees is the correct ASHRAE design temp for Minneapolis. What do you mean "Shouldn't deviate from proper design temp.?

Mr Bill: Manual J cannot compensate for bad or old insulation in walls, also it don't compensate for leaks in your home, windows,door seals,etc. this is why it does take a little "Human" experience. If the company has been in your city for a long time and has done many installations, this always help build the complete system, and will factor in on it's future performance. I personally would never,ever,use it exclusivity, without injecting my own personal experience of the city, especially in Houston Texas, you will have some unhappy customers trust me, been there done that.

My house is not typical of surrounding homes. It was built extra tight, with carefully detailed insulation and vapor barrier sealed at all wall penetrations. It has an HRV, which changes the air every 3-4 hours. This was explained to each of the contractors.

udarrell: Normally, there are a number of retro options that would make the 2-Ton work. An air infiltration blower door test, & a manual D (Air-Flow!) are critically important.

Are you saying that I need a blower door test to determine if I need a 2 or a 2.5?

arc8: Perhaps you should ask them how long have they been doing Load-Calcs?
I have many competitors that are beginning to do load calcs now, why, I don't know. It takes some experience (a few projects) to get close and understand how to properly input the correct info.

I can only assume, at this point, since I have nothing else to go by, that each of the contractors is equally competent at using Manual J. (Their results are quite similar.)

Given all of your response, I guess I should try to simplify my question. If we assume that my house will actually require 26K to cool, what other factors besides the load analysis would you use to choose between 2 and 2.5? What other questions should ask?

 

Given all of your response, I guess I should try to simplify my question. If we assume that my house will actually require 26K to cool, what other factors besides the load analysis would you use to choose between 2 and 2.5? What other questions should ask?

1. People!!

2. HRV: since they are not very effective at removing moisture (RH) in the summer months; they should highly consider the latent portion of the Heat Gain very carefully.

3. Cooking

4. Max of 50% RH control in your house at 75degrees.


BTW: My cooling degree day design is at 90 degree (further North than you).

 

Thanks, Arc8,

I think we are getting closer In response to your comments:


1. People!!

It is my understanding, that our family of four people has been included in the MJ calc at 300 each for a total of 1200 BTU/Hr.

2. HRV: since they are not very effective at removing moisture (RH) in the summer months; they should highly consider the latent portion of the Heat Gain very carefully.

Because it is an HRV and not an ERV, it is almost always off in the summer with it only being used when it is switched on high in the bathrooms during showering.

3. Cooking

This is also included in the MJ at 1200 BTU/Hr IIRC.

4. Max of 50% RH control in your house at 75degrees.

It is my understanding that my MJ was run at 76-degrees and 55% RH, which is extremely similar to your design standards.

BTW: My cooling degree day design is at 90 degree (further North than you).

Apparently, ASHRAE says 89 for my location. Another responder did not recommend changing the ASHRAE design parameter.

So, would you install a 2 or a 2.5-ton unit?

 

76-degrees and 55% RH, which is extremely similar to your design standards.

Apparently, ASHRAE says 89 for my location. Another responder did not recommend changing the ASHRAE design parameter.

So, would you install a 2 or a 2.5-ton unit?

Sometimes there's a big difference between a 50 and 55% RH.
I do believe in your area you have more water vapor than us, more humidity, a higher latent probably not much more, but, it still can be significant between two sizes of A/C's??

I would need these numbers to help you out.
You say your Total Heat Gain is 26,000!

What's your sensible heat gain and what's your total heat lost?
Sometimes the heat loss is significant enough to offset the CFM (airflow) from the cooling CFM (I'm just curious)!

 

My house is not typical of surrounding homes. It was built extra tight, with carefully detailed insulation and vapor barrier sealed at all wall penetrations. It has an HRV, which changes the air every 3-4 hours. This was explained to each of the contractors.

udarrell: Normally, there are a number of retro options that would make the 2-Ton work. An air infiltration blower door test, & a manual D (Air-Flow!) are critically important.

Are you saying that I need a blower door test to determine if I need a 2 or a 2.5? My house is not typical of surrounding homes. It was built extra tight, with carefully detailed insulation and vapor barrier sealed at all wall penetrations. It has an HRV, which changes the air every 3-4 hours. This was explained to each of the contractors.

udarrell: Normally, there are a number of retro options that would make the 2-Ton work. An air infiltration blower door test, & a manual D (Air-Flow!) are critically important.

Are you saying that I need a blower door test to determine if I need a 2 or a 2.5?

The information you gave above negates my last post, because you may not be able to reduce the heat-gain by very much.

I'd rather not advise you from here on condenser sizing.

However, you could go with a 2.5-Ton & lower end blower motor airflow that would get a colder evaporator coil & somewhat longer run times.

I would advise using a room t-stat that would help you somewhat with adjusting for longer runtimes.

Depending on heatload conditions that may get you somewhat wider room temp variations toward colder temps & lower humidity.

If your home is as tight as you say, the 'off times' should rather long, not short...

I'd let the indoor blower cycle with the condensing unit. - Darrell

 

I don't know how much you want to spend for a unit. It sounds to me like this would be a good application for a two stage condensing unit. Let the first stage do the cooling when the load is lower. When the load increases you have the second stage to pick up the slack. I believe you would also need a variable drive blower for the two stage unit.

These are just my thoughts. The high efficiency two stage units were just getting popular, and more affordable, when I went strictly to commercial/industrial HVAC/R service. Maybe the residential guys and gals here could recommend a unit that would fit your application.

 

I wish I understood some of these posts better. This is the first time I have ever heard it suggested that Manual J was even supposed to "compensate for bad insulation in walls". Does anyone really want to say that? Like any model if you input wrong data then you can expect a wrong result.

Has ACCA Manual S become unnecessary? I am a homeowner and not qualified to tell another what the answer is. But I have purchased the Manual S book and read the way the AC is supposed to be de-rated for the design day conditions, and then selected so that it matches both latent and sensible loads separately. In other words I have always been told that if your house has 24,000 BTUH combined sensible and latent loads, you usually don't want your AC to have 24,000 capacity rating.

Hope this helps -- Pstu

 

There is no such thing as bad insulation or old insulation, its either there or its not. There are "quality" levels like everything else. the only "old" insulation is attic blown-in that might settle over time.

For manual J-the "human factor" is determining the infiltration rate and properly defining the thermal envelope.

 

http://www.wunderground.com/NORMS/D...tCode=KMSP&StateCode=MN&SafeCityName=Minneapolis&Units=none&IATA=MSP&normals=on

http://www.wunderground.com/history...ort/KMIC/2009/7/1/MonthlyHistory.html?req_city=NA&req_state=NA&req_statename=NA

I wonder if someone from the state or near it, would take pity on this Texan and explain to me whether it is even reasonable to expect an AC to do much humidity removal if the homeowner only turns it on when outdoor temperatures will exceed 83F.

Humidity removal from what I understand, is supposed to be one of the key reasons to size an AC according to Manual J. I have heard arguments about energy savings, but have also seen a study which failed to establish significant savings by "right sizing" a previously larger AC. I believe the true energy savings is 6% or less which gets lost in the noise -- in other words too small to measure. If the OP cares to email me I will go to the effort to locate that research paper and pass it along. Email address is in my profile.

Best of luck -- Pstu

 

Sorry. I know you are an experienced and honorable man, I just had trouble interpreting the meaning of what you wrote.

Best wishes -- Pstu

 

Manual J is accurate if the input is accurate,insulation in walls doesn't "get old" in a way that changes the R value.

No extra "input" or "guessing" is needed or wise.

 

Manual J & S

The reason for the increase use of these manuals is to qualify for the rebates that are offered in some states. I am in NJ and to get the rebate from the state run program they want to see a Man J and S calc within 15% variance. I am a builder and do most of my own HVAC work that only applies to A/C. I use boilers for heat. Not many hvac contractors (in my area)know how to do these calculations, they sub it out to an engineer. They size the units from experience. Sometime they hit it dead on other times.... well oversizing = musty problems. I may consider purchasing the manuals from ACCA.ORG.

 

The reason for the increase use of these manuals is to qualify for the rebates that are offered in some states. I am in NJ and to get the rebate from the state run program they want to see a Man J and S calc within 15% variance. I am a builder and do most of my own HVAC work that only applies to A/C. I use boilers for heat. Not many hvac contractors (in my area)know how to do these calculations, they sub it out to an engineer. They size the units from experience. Sometime they hit it dead on other times.... well oversizing = musty problems. I may consider purchasing the manuals from ACCA.ORG.

Every contractor in the country should have and use ACCA manuals.