Bids vs HVAC-CALC results

[rwilkers]

Building a new home, it is 2000 sqf on the first floor, and has a partial loft area with a 8/12 pitch cathedral ceiling. Usuable floor space total for first and second is approximately 2800 sqf, however all the floor space, usable/unusable is within the conditioned area.

Walls are Polysteel 4000 series ICF, the roof is 2x12 on 16"centers with 1+ inch of closed cell foam topped with a full stuffing of BIB. Gables upstairs are 2x8 with the same treatment. Windows are double pane, low-e and moderately sized.

House will have a 90 cfm ERV system, and separate unidirectional exhausts for bathrooms, and kitchens. Slab has 8 zones of radiant tubing, my plans were to heat radiant on the first floor from the geothermal and solar collectors, use geothermal forced air heat pump/backup heat strip on the second floor (if needed). For cooling, the geothermal would supply all via forced air to upper and lower floors, each room or floor with dedicated adequately sized return air supply.

I had two bids for a closed loop vertical geothermal, one bid called for 4 ton on the bottom and 2 ton on the top. The second called for 5 on the bottom and 3 on the top. I frankly did not believe that this house could need 6 to 7 ton of AC, especially when I was trying my best to design energy efficiency into it.

For that reason I purchased a home owner copy of HVAC-CALC and ran the numbers. Being liberal in my assumptions I came up with the following...

Whole House

Sensible Heat Gain 19,547
Latent Heat Gain 4,018
Total Heat Gain 23,565
Total Heal Loss 25,071

These numbers are pretty different from my bids, and I don't understand why they want to install two units and not a variable speed zoned single unit. Any ideas/help/suggestions here?

[Zeiss Nut]

I'm just a noob in hvac industry. so i'm waiting for other to put in their 2 cents.

But does the HVAC-Calc number suggest the whole house only need about 2 or 2.5 tons at the max ?

[jdh]

I believe your numbers are a lot closer than the geothermal heating professionals. Most of the ICF homes are down in the 13 to 10 Btu/ft/sqft range in heat loss, for my area.

Of course, Location of your project can change those numbers. Did anyone show any form of calculations for those guesses?

[rwilkers]

The upstairs heat gain was 17,213 btu's for cooling
The upstairs heat loss was 20,433 btu's for heating
Nominal tonage on a 2-ton is 24,000 btu's

The downstairs heat gain was 42,879 btu's for cooling
The downstairs heat loss was 44,377 btu's for heating
Nominal tonage on a 4-ton is 48,000 btu's

[garya505]

My first try at HVAC-Calc gave me numbers that seemed way too low. At the advice of people here and elsewhere, I changed all of my windows to unshaded and no blinds (do we really know we'll close the blinds?). That reslulted in the biggest change in the results. I think I made some other changes but I can't recall what they were - mostly I went conservative to make sure wasn't underestimating the loads. I also made the sure the square footage calculated by HVAC-Calc was correct according to my other calculations. My final HVAC-Calc loads were very close to those calculated by two contractors who both used Wrightsoft.

[AirCareTech]

I think you should trust Contractor A. I just did some quick "field calculations" and I came up with the house needing almost 6 tons. There are all kinds of things that go into figuring the load. Things like amount of people that you could have at the house, not the the ones living there but guests as well, or the type of refrigerator/cooking equipment or even the lighting fixtures being installed. All these things will add heat to the dwelling and need to be accounted for. You should ask the guy how he got his figures. Good luck.

[rwilkers]

Sorry, should have given more info, When I ran the program I made the following assumptions..

Put in 3 people in each zone, actually only two will live in the house.
Put in 2000 Watt "Misc" in each zone for extras (program suggested 1200
All windows unshaded inside and outside except front door and patio door which actually are under porches.
Allowed 40cfm of force air infiltration to make up for the worst case loss in the ERV I am installing.

And I double checked the sqf that the program is using, it is correct.



I went back to Contractor B whom did supply his design criteria.. They assumed unconditioned attic space, assigned R19 stick built for the walls, assumed R30 for the roof. Assumed medium level of infiltration, in other words pretty much ran it for a standard contractor grade new spec home.


Since the house is up now, walls are in place, and insulation in.. I have asked both to do a site visit and actually look at what is there, then rebid..

[tedkidd]

How did this turn out?

[rwilkers]

Seems the best proof is the empirical. During construction I put in a 7k and 5K window upstairs. During the worst days of summer the interior temperature upstairs or downstairs never exceeded 77 degrees. Seems like that was good rule of thumb that my heat calc were on the money.

I had four contractors total bid the project. Prices ranged from 21k for a single 4 ton unit (no zoning) to 36k for dual 3 ton units. Based on the new numbers I went back to the lowest dual unit bidder and had them reprice based on two 2 ton units with ECM (nobody makes a 1 ton unit, and the ECM will allow them to run at lower capacities).

My price ended up being 25K turnkey. Seems like WAY too much money based on some of the other numbers I have seen on this forum but since all the bidders were in that general area I did not seem to have any real choice.. BTW, a air source 16 seer was bid as an alternate. That price ranged from 12 to 16 thousand.

[tedkidd]

better rmv pricing.

so, 3 ton air or ground source with zoning would likely be plenty for cooling... and heating?

[rwilkers]

Mainly I was concerned about cooling, I have radiant floor heating with a solar primary so I only needed forced air for the upstairs, but yes, three ton would be fine for both heating and cooling.