Good article on HVAC design for high-performance homes

[Paul12644]

http://www.acrightsize.com/files/ele...n_the_room.pdf

[Carolinarider]

Ok, so I am reading this article and come across this statement

"Many HVAC contractors intentionally size multistage equipment based on the first stage capacity and consider the high stage as reserve capacity for extreme weather conditions or a big party. By taking this approach, contractors unknowingly cheat homeowners out of the efficiency they paid
for. Even when sized properly, payback for multistage usually exceeds the life of the equipment."

So if I buy a new multistage heatpump with a seperate variable speed air handler to try and take advantage of the multistage system, It's not worth the extra cost. However, if will provide a better level of comfort. Is that a correct assumption?

[pstu]

Ok, so I am reading this article and come across this statement

"Many HVAC contractors intentionally size multistage equipment based on the first stage capacity and consider the high stage as reserve capacity for extreme weather conditions or a big party. By taking this approach, contractors unknowingly cheat homeowners out of the efficiency they paid
for. Even when sized properly, payback for multistage usually exceeds the life of the equipment."

So if I buy a new multistage heatpump with a seperate variable speed air handler to try and take advantage of the multistage system, It's not worth the extra cost. However, if will provide a better level of comfort. Is that a correct assumption?

That is pretty close to what I have always heard as a homeowner. I have a 2-stage system and am glad I do, however have not analyzed it in a pure dollars and cents way. But come to think of it, if I wanted to save the absolute maximum amount I would not have air conditioning at all.

I have a lot of respect for good equipment in both single and two stage versions. In all probability the dollar optimum comes with the single stage.

Hope this helps -- Pstu

[BaldLoonie]

2 stage equipment is for comfort, not efficiency. So there is virtually no (if any) payback on it. I love a 2 stage gas furnace since we rarely need high except the coldest days. But still not convinced how ooodles $$ more for a 2 stage outdoor unit wouldn't make my home more comfy. It's very comfortable in the summer with what I have. Of course many 2 stage outdoor units have higher SEER (though not always EER or HSPF) than their single stage counterparts so there would be some possible payback there. Some say longer run times on low help with moisture removal but on some models, latent capacity on low is lacking and on most 2 stage outdoor units, low isn't that low anyway, effectively around 80%.

[Carolinarider]

That's interesting, and thansk for the feedback.

My new house will need a HVAC system. I started out looking at geo-thermal, after monitoring this site for a few months I figured a two stage system with variable speed blower would be best. Now, I am not sure that's the best bet.

So, my plan is select two or three contractors, tell them what I want (conditions not equipment) have them give me some advice on improving the thermal envelope of the house and then see what they suggest.

One thing I do know now is installation first, equipment second. That and open communication between me and the contractor. When in doubt, ask.

[beenthere]

Ok, so I am reading this article and come across this statement

"Many HVAC contractors intentionally size multistage equipment based on the first stage capacity and consider the high stage as reserve capacity for extreme weather conditions or a big party. By taking this approach, contractors unknowingly cheat homeowners out of the efficiency they paid
for. Even when sized properly, payback for multistage usually exceeds the life of the equipment."

So if I buy a new multistage heatpump with a seperate variable speed air handler to try and take advantage of the multistage system, It's not worth the extra cost. However, if will provide a better level of comfort. Is that a correct assumption?

2 stage can provide savings. IF it allows you to set the thermostat set point temp a degree or more higher then you would with a single stage unit.
On new construction. You don't know if its giving you that ability or not.


With 2 stage equipment. If you compare a single stage 16 SEER system.
To a 2 stage 16 SEER system for operating cost alone. You won't get a worth while savings.

However. The comfort of the 2 stage can more then pay for it though (properly sized unit). by keeping the humidity under better control. if you live in a humid area.

Hate to think that saving money on operating cost is worth sweating your butt off.

[teddy bear]

Few articles really explain the issue. A/cs cold coil condense moisture from the air. After 30 mins, a couple lbs. of moisture builds up on the cooling coil. now water moves from the coil to the drain until the cooling cycle ends. During the of cycle the couple of lbs. of moisture on the coil/pan re-evaporates back into the ducts home. During high cooling loads, a properly setup a/c system removes enough moisture to remove the moisture from the occupants and infiltration/ventilation. During low/no cooling loads, the a/c removes very little moisture from the home. The results are high indoor moisture levels when adequate fresh air and occupants are present during low cooling loads. This why the a/c trade opposes fresh air ventilation. Supplemental dehumidification is a must for real comfort and indoor air quality. These are the sad facts of life. Regards TB

[geonovice]

Good Question! I also read the article, & am now wondering if I should stop focusing on the Carrier Infinity (or equivelant) system. I could opt for a less sophisiticated, lower cost system & spend the money saved on even more insulation & duct improvements??? I'm also wondering if the $49 home owners load calc. available online has the up to date (8th edition manual "J" + the 2 revisions) calcuations mentioned in the article, or is it just another load calc that isn't very accurate? I've received two estimates on an Infinity zoned system but the more I read about Infinity, zoning, & system sizing the more confused I get.

[beenthere]

HVAC Calc is not based on Manual J ver 8.

Improvements should be made to your home first. Even if 2 stage equipment saved you 15% over singled staged equipment.

Since improvements to insulation and infiltration rates. Will save far more by lowering the size of equipment you need.

[geonovice]

HVAC Calc is not based on Manual J ver 8.

Improvements should be made to your home first. Even if 2 stage equipment saved you 15% over singled staged equipment.

Since improvements to insulation and infiltration rates. Will save far more by lowering the size of equipment you need.

I didn't mean to indicate that I won't make improvements first, I've made quite a few already. In my preperation for choosing a contractor I would like to have good back up data so I know if I am getting sold an oversized system. I don't want the contractor that I select to throws in an upward size fudge factor on his manual J. The contractors I have talked to so far seem to be reluctant to do a manual J. I know they will do one to get the job but I betting they will still over size the system in my house. That's why I was asking about the HVAC load calc. is this a good calc for the HO? Manual J seems to be such alot of work for such an inexact calcuation.

[pstu]

It is my experience that Manual J7 was pretty good in its day, and Hvac-Calc is based on that. No doubt J8 is better, but it seems weird to claim J7 is junk now that someone has made a more complex model. I am a homeowner and try not to let "the perfect be the enemy of the good". Both are just models.

Hope this helps -- Pstu

[beenthere]

Yes HVAC Calc is a good load calc program.

But, the info you enter needs to be accurate. The same as a version 8 program.

[pstu]

http://www.acrightsize.com/files/ele...n_the_room.pdf

Thank you for the article, but it does seem to me to overstate its points. One way of making a paper entertaining is to be smug and glib. But that method compromises its wisdom and accuracy.

Best wishes -- Pstu


P.S. The paper says author Butler has a M.S. degree in Engineering and a B.S. degree in Business and Finance. Does anybody else think that combo is curious? Still, I would very much like to see him participate in a panel discussion with other experts in the field.