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Wrightsoft heat loss question (Getting ready for gas in CT)

WalnutFarmer Member Posts: 42
I posted this at the end of my original long thread, but thought it might merit starting a new one to get some quick feedback. Sorry for the duplication. (Original thread)

For those of you who don't recall, in planning for my conversion to gas (from oil) I ran my own heat loss analysis a few weeks ago. My contractor now did his using commercial software and they don't agree,

The contractor walked the house with me and I provided construction details, so he had the same (or similar) R values and measurements that I used for my Excel based heat loss calcs. He used Wrightsoft to do his calcs (Manual J 8th Edition).

My original calculations came out to around 55k BTU/Hr. Wrightsoft came up with 76k. (See table)

He sent me the Wrightsoft report late last night. One thing caught my eye that might explain the discrepancy. 41.5% (31.7k btu) of the heat loss is through "ducts". Remember this is a hydronic system. I do have central AC, but I'd be shocked if the AC ducts are passively losing that much heat (or even 10% of it) during heating season. The AC systems are entirely separate from the heating system, except for one zone that has the fan coil (zone 3) and its own compressor. Hard to believe the ducts for a 400 sf "zone" could lose 41% of the house's heat!

When I look zone by zone, it appears that a standard 41.5% for duct load is applied to all zones.

Are any of you familiar enough with Wrightsoft to know if there is a setting he should have turned on/off to eliminate the impact of ducts? Or is this a normal outcome?

Thank you again. Obviously a 41% swing in load is a big decision factor!


  • Tinman
    Tinman Member Posts: 2,804
    I have not read your other thread but unless you're using fan coils with uninsulated duct, that's not accurate.

    If you don't have any duct, there can't be duct losses.

    And, of you did have duct losses that account for 41.5% of your load, something is seriously wrong with your ducts.

    Steve Minnich
  • WalnutFarmer
    WalnutFarmer Member Posts: 42
    Thanks Stephen. I guess my fundamental question is this:

    For a house heated with hydronic baseboards does a separate central AC duct system contribute materially to heat loss, given that trunk and some lines are in the attic (which is foamed, so partially "conditioned" space).

    Apparently, Wrightsoft is insisting that the cooling ducts need to be considered as a source of heat loss.

    I'm trying desperately not to fall into the trap of oversizing my boiler, but don't want to undersize either!

    Thanks for any additional feedback.
  • Brewbeer
    Brewbeer Member Posts: 616
    @WalnutFarmer where are the ducts in question? Are they in the attic or in outside wall, or are they located within the conditioned envelope of the building?
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread: http://forum.heatinghelp.com/discussion/154385
    System Photo: https://us.v-cdn.net/5021738/uploads/FileUpload/79/451e1f19a1e5b345e0951fbe1ff6ca.jpg
  • WalnutFarmer
    WalnutFarmer Member Posts: 42
    edited October 2016
    @Brewbeer I have two AC airhandlers in the attic. They each have short trunks plus supply ducts in the attic that feed into registers on the ceiling of second floor rooms. Additional supply ducts drop from attic through interior second floor closets to first floor ceiling registers.

    The attic has 4-5 inches of icynene (?) foam under the roof deck, so the attic temps are very moderate in summer and winter.

    All the ducts and trunks are insulated. Not sure of r-value
  • Tinman
    Tinman Member Posts: 2,804
    The way I see it, the only way that your AC ducts can be included as part of the loss is through warm air rising up into the duct via return air grills or supply registers. Close them or seal them and they are no longer part of the equation.
    Steve Minnich
  • njtommy
    njtommy Member Posts: 1,105
    edited October 2016
    Following the other thread and reading what you found for heat loss with the numbers you came up with and the contractors numbers. You still be the same boiler size of the KHN085.

    The only thing that would be in question is how much of the year would you be running at condensing return temps. Along with dialing in your out door reset curve. Honestly even doing the math on the ODR curve may change a bit. Efficiency Is great and we always try to push for the highest possible, but if you are not comfortable in the house efficiency means nothing.