HVAC advice for new construction

8 years ago

I am seeking advice for hvac in new construction. Previously I posted about geothermal options, but are no longer considering this (http://ths.gardenweb.com/discussions/2937563/geothermal-in-new-construction)

Some specifics:

1 story, walkout basement, 6500 total finished space.

ICF for the lower level only.

insulation below slab

10' lower level ceilings. main level ceiling heights vary, some deep cathedrals.

large great room and kitchen windows on the main level, south facing. Andersen Eagle aluminum clad.

no west facing windows, few north facing.

detached garage

region 4

We have access to natural gas.

The house is long (~114 ') and narrow.

Most of the time there will be 2 of us, and will do most of the living on the main level. Lower level has extra bedrooms/family room that will be used mostly for when grown children visit.

One HVAC professional did a thorough load calculation and suggested a 2.5 or 3-ton unit for the lower level, and a 4-ton unit for the upper level.

A second HVAC pro did not do a load calculation yet, but is planning to. He is suggesting we would want 2 5-ton units. He justifies this by saying that since we would use variable speed units, they would work in the low speed and act as thought they are 3 ton units, UNTIL we have a greater need (when the house is full of guests), and therefore we won't suffer the complications associated with oversizing the HVAC. He stresses that he wants to be sure that we are comfortable and wants to be sure that we do not undersize our system. He mentioned a concern that with an undersized unit, the air will not circulate appropriately to keep us comfortable. I don't know or understand enough about HVAC to know if this makes any sense or not.

Additionally, professional #2 is strongly suggesting a heat pump with gas back-up instead of a gas furnace. Is there a clear reason for one option over the other?

Thanks for any help you can offer.

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Comments (7)

ionized_gw
8 years ago
In this day, with low gas prices, it is likely to cost more to heat with a heat pump than with a natural gas furnace. You have to run some numbers to be sure. That is on a pure BTU/$ fuel basis. Furnaces cost less to buy and maintain than heat pumps.

Variable load calls for variable speed. Just two of you in the house will present a lot different load than a house full of family or a party. A variable speed compressor will run at high efficiency at low load/speed. I am an amateur.
mike_home
8 years ago
Pro no. 1 is doing the right thing by first doing a load calculation assuming it is correct. Have him explain it to you and see if it makes sense.

Pro no. 2 has done no calculation but has already decided that you need 10 tons of cooling. That is a red flag for me.

The only reason to use a heat pump instead of a gas furnace is operating costs. Ask him to show you how he calculates the costs of each.

My recommendation would be variable speed gas furnaces and 2-stage AC condensers.
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Really depends on the construction and that is where the manual J comes in. You have some real heat gains with a 2 story & lots of glass. It would be beneficial to have a local do a heatload for you. A lot less money than having the wrong size unit and unhappy wife for years to come. With that big sqft split you may consider two systems too. If you don't want to go that route it is usually 400 sqft per ton here in s fla. See if there is a local Johnstone Supply in the area, if you have one that does not lock out non-tradesman you should find what you need. Besides Goodman A/C is made in Tx. Good luck Been more than a few years since I did installs but a cheap ductulator (slide rule for duct sizing)will give you the duct size. Use .08 for the multiplier on the ductulator for quiet ducts. Return air grilles are 144 sqin per ton less they get noisy.
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Yes, Ionized, you are right.....it seems likely that there is a need for 4 zones due to the overall length of the house (-110'). Will use the main level mostly, with master suite area taking up 1/3 of the level, great room/ kitchen taking up the remaining 2/3 space. Lower level has 2 bedrooms, bath and storage under the master suite level, with a family room, bar, theater and another bedroom under the great room/kitchen. Open staircase centralized, going from great room to family room. Bedrooms here are for when the grown kids come to visit....a couple times a year. However, I am sure that we will use the bar and theater more often. While the load for the lower level is smaller than the main level, it still seems like we would want separate zones on each level to address the needs of the far reaching rooms. If anyone wants to throw out any other thoughts, I would greatly appreciate them!
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weedmeister
8 years ago
Having a hybrid system (HP and Gas) has to do with your cost of electricity versus gas. Without that you cannot make a good decision.

Basement loads are usually a bit less than non-basement areas, requireing less cooling. This is what #1 is telling you. Since the home isn't built, he has to get his information off the drawings. Knowing that the ceilings are high and cathedral is places makes a difference since many programs assume 8' ceilings.

It could be that the downstairs only needs 3 tons but that actually sounds a bit large. It would need to be justified to me. And multistage on a level that isn't occupied that much would be a cost I would put elsewhere. But that's just me.
betttyloo
Original Author
8 years ago
Well, I met with a 3rd HVAC pro yesterday.....and I am now more confused. The manual J calculation that he performed is vastly different from pro #1. Pro #3 suggests a 2-ton unit for basement, and 2 3-ton plus a 4-ton unit for the main level (12 tons total, compare to 7 tons total from pro #1) The load values vary so much between the 2 manual J reports, yet both professionals used the same software. How is the consumer supposed to sort through this information? Pro #3 accounts for infiltration values and duct loads that were much higher than Pro #1. Suggestions on where I go from here? Is there a simple way for me to understand the Manual J report, so that I will know which pro may have calculated incorrectly?
weedmeister
8 years ago
things that affect the results would be design temperatures (outside highs in summer, lows in winter, and indoor desired temps), outer wall design (brick, stucco, wood, alum siding, insulation type and amount, window sizes, etc), sun loads and shading, and the high ceilings you have.
betttyloo
Original Author
8 years ago
Am I correct in assuming that if the design temperature is fiddled with, either with outside or desired inside temps, this can affect sizing? Because it seems to me that this, indeed, could be an area where fluctuations can result in varied outcomes.
ionized_gw
8 years ago
Yes, but to get a 40% difference someone would have to be using the WRONG temps.

HVAC advice for new construction

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