New construction in Metro Detroit.

8 years ago

Hello,

I am building a new home in Metro Detroit. It is 2900 square feet with a two-story great room. The house is being built with SIPs exterior walls, superior Xi basement, low U windows with high solar heat gain on the southern facing windows. I modeled the home in the government's BeOpt software and calculated the following

Heating capacity 35.3 kBtu/hr

Cooling capacity 19.6 kBtu/hr

Does this seem reasonable? Natural Gas is available at the property. With these loads what sized furnace and AC should I be considering to be properly size. What options would also be beneficial two stage/variable? . I would assume 90% AFUE and at least 13 Seer.

Thanks

Fred

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Comments (5)

Fred M
Original Author
8 years ago
Heard back from a contractor after looking at my plans.....he was stating I would need around 120,000 BTU furnace and a 4 or 5 ton AC unit. I told him all of the building technologies and building envelop, but didn't share the load calcs. Does this seem way over sized??
mike_home
8 years ago
What the contractor is proposing is very over sized. Any new home build must have a load calculation. Ask to see the load calculation results. If he refuses then you can assume he has not done a calculation. You need to find someone else.

What software did you use to do your load calculations? What indoor and outdoor design temperatures did you use? The numbers you posted seem low even for a highly insulated 2900 sq. foot house in Detroit. Something is not right.

Detroit gets very cold in the winter so I suggest getting a 95%+ natural gas furnace. A 13 SEER AC is the minimum allowed by federal law. Stay away from the builder's grade equipment which is usually rated only 13 SEER. If you plan to live in this house a long time then invent in 2-stage equipment.
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ionized_gw
8 years ago
Higher quality, variable speed cooling equipment can probably do a lot to control humidity in low cooling demand times. You seem to be able to grasp the fact that with a super envelope on a home, investing in super efficient heating and cooling appliances is not always cost effective.

When you get to the point of operating your new HVAC equipment, keep in mind that variable speed/capacity equipment, sometimes operates at a lower efficiency than at partial speed. Unless the controls can know the difference between a set-back and a normal high demand, set backs can result in higher operating costs than not using set backs.
Fred M
Original Author
8 years ago
Thanks for some feedback.

I used the government's software BeOpt. I used the flint airport weather file and indoor Temps of 71 for heating and 75 for cooling. I too was a little concerned the numbers being on the low side, but some knowledgeable folks over on greenbuildingtalk seem to agree with them. I am not looking to push the envelope in terms of sizing the equipment, but I don't want a grossly oversized system either. Nor do I feel like I need a Cadillac of systems. I am more concerned with the tightness of the house and having adequate air change over.

Ionized.... I can most definitely appreciate what you said.

Thanks

Fred
ionized_gw
8 years ago
This study concerns mini splits, but, in a general sense should be of use to you concerning operation of high efficiency, variable speed, equipment in efficient, well-built structures. I believe that good advice about the use of setbacks is difficult to achieve as it depends on the rate of heat transfer from the building and the operational characteristics of the equipment. If equipment manufacturers would have a 'setback recovery" mode that allowed reasonably fast recovery at an efficient output, the discussion would be over. This is true for heat pumps and boilers as I have looked into it, and I am reasonably sure that furnaces are similar.

Long-Term Monitoring of

Mini-Split Ductless Heat

Pumps in the Northeast

K. Ueno and H. Loomis

Building Science Corporation

November 2014