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energy use: forced air vs hydronic (continued)

duerstad Member Posts: 3
I appreciate all the opinions offered on this issue yesterday and think the better way I can respond is to offer some of the information requested along with a table of more information from BEopt results representing the wider range of heating options it offers. This wide range still demonstrates much more energy consumed and heat delivered by hydronic setups than forced air. This difference is specifically what I'm trying to understand--or estimate whether it is realistic or not. Many or most of these possibilities I wouldn't even consider, but they may help evaluate overall this large, odd difference suggested between forced air and hydronic systems.

The house as modeled has 2" of XPS under and around the 4" slab. BEopt says that adding more really changes very little.
All ducts are specified as inside conditioned space.
No fan or pump loads are included in the numbers here. You can pretty well estimate those differences between the various designs if that helps you. They aren't big enough to change the overall hydronic vs forced air energy use difference that concerns me. BEopt offers those numbers; they can be added in if you like, but I don't think they contribute to my question, i. e., why this big forced-air/hydronic difference.
I understand "comfort" is an issue, but I think this difference in energy use should be evaluated first. Then comfort can be included. (It's not that I think it is unimportant.)
There are no temperature setbacks in any of these designs.
Ventilation is set exactly the same in all designs.
My understanding is that BEopt assumes that air in the building is "well-mixed," which should help to control the issue of stratification.
The reason that radiant and low-temperature aren't getting included is that BEopt's sponsor (NREL) is spending its time transitioning to more flexible software, which does offer these possibilities, but so far is much more difficult to use. I've tried to use it (with great difficulty!) and its results are consistent with what I'm showing here.

Btus delivered Btus consumed Gas unit AFUE, outlet temp, etc.
141.4 193.4 Boiler 3 72%, 180, forced draft
141.4 174 Boiler 5 80%, 180, forced draft
141.4 163.8 Boiler 7 85%, 180, forced draft
125.1 144.3 Boiler 8 95%, 180, condensing, modulating, OTA reset
125 140.8 Boiler 9 96%, 150, condensing, modulating

76.4 127.3 Furnace 3 60%, 120 temp supply, flue open
76.4 100.5 Furnace 7 76%, 120 temp supply, flue open
74.8 83.1 Furnace 10 90%, 120 temp supply, flue not open
74.8 76.3 Furnace 14 98%, 120 temp supply, flue not open

You can see why I'm concerned enough to try to get some kind of "second opinion" on this. If Btus to heat the place were roughly similar for forced-air and hydronic, I wouldn't be concerned very much. When the one says it will take roughly 75 million Btus and the other says between 141 and 125 million Btus, I think this deserves considerable thought before making further plans. In other words, I'm basically starting with HVAC, which I understand is good though uncommon.

Thanks very much! At least most of you have more experience with this than I do. BEopt/Energyplus/OpenStudio, on the other hand, should not be casually dismissed when it presents us with this.


  • Brewbeer
    Brewbeer Member Posts: 616
    Perhaps there are errors in the programs assumptions. For example, a furnace delivering a 120F air supply temp is not 98% efficient.
    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
  • pecmsg
    pecmsg Member Posts: 4,389
    Manual "J" heat load / loss calculations are going to tell you that the building will LOOSE X amount of BTU's per hour. How you replace them is up to you, Gas hot water or forced air are going to be basically =.
    A100,000 96% forced air or 96%WH boiler will deliver the same output 96,000BTU's at there design conditions!
  • Jamie Hall
    Jamie Hall Member Posts: 22,162
    A difference of 2 to 1 in the BTUh requirements between different heat transfer media -- which is what you are talking about -- leads me to be extremely suspicious of the entire software package in question. Something is seriously wrong with it at that level -- and who knows what else is wrong, just not quite as obvious.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Gordy
    Gordy Member Posts: 9,537
    edited March 2019
    I think the team that wrote the program does not have a full understanding of hydronics. Some how they are trying to base it on forced air btu delivery. Which is quite different than btu delivery in hydronics, or steam. Average water temperature of any hydronic emitter verses discharge temperature at the Forced air diffuser. Big difference when you have a whole floor in a room as the emitter verses say 3-4 diffusers for FA.

    Not having available for comparison a low temp radiant emitter says much about the program. Quite easy to heat the space with less than 120 degree water. Tight envelopes can be less than 100 degrees.

    In general water has the ability to carry more btus than air. A 1 1/2 copper pipe can deliver 225k. Compare that to a forced air duct size to deliver same 225k.

    Btu loss in the delivery to the emitters with in the conditioned space will be less from small diameter pipe than larger diameter ducts. Either way it’s a wash since that loss is to the conditioned space.

    Boiler 9 has a 16 mbtu deficit for delivery of 150 swt with 96% efficiency.

    Furnace 14 has a 1.5 mbtu deficit for delivery with 120 air temp with 98% efficiency.

    Between the two it is assuming the boiler 10 will use 64.5 mbtu more than furnace 14???

    Not buying it. Someone’s pushing a FA agenda.....