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Help with planning my upgrades! =)

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jordew82
jordew82 Member Posts: 3
in Gas Heating
Howdy folks! New homeowner here from Long Island NY! The boiler, water heater, and AC in my place are all 20 years old so I know they're 1) not energy efficient and 2) approaching their useful life-span. I'm trying to plan out their replacement/upgrade.

Some details: 2,800 sq.ft. 2-story house w/ basement. Currently has: 5-ton SEER 13 AC unit and single-speed AHU w/ single-zone ductwork in unconditioned attic, approx. 80% AFUE gas-fired boiler for 2-zone hot water baseboard heat (1 zone per floor), and a 50 gallon gas-fired HWH of unknown efficiency. The house also has 7.5KW solar panel system.

So... going to make my attic semi-conditioned to protect a new variable-speed AHU and ductwork, and install an air-source heat pump (since I generate a lot of electricity) to handle HVAC for three of the four seasons (basically anytime the outside temp is above 32). Undecided about whether to get two units (one for each floor) or try 2-zone ductwork, or just replace in kind (single-zone, whole house). Feedback welcome!

I'm trying to decide on the boiler/water heater with Winter in mind. My thought is to install a non-condensing boiler and a heat-pump hot water heater with an optional hot water exchanger. Spring, Summer, and Fall - the boiler is off and the HWH runs in heat-pump mode on solar power (bonus cooling and dehumidification). Winter - boiler is on and the HWH works in "indirect" mode. Since I have hot water baseboards throughout (designed for 180F supply) and will be adding an indirect water heater, I'm 99% sure a condensing boiler would not help me because I feel the return will very likely be above 135F.

What supply temp is typically needed for an indirectly-fired water heater?
Is a condensing boiler still more energy efficient than a non-condensing one, even when it's not in condensing mode?

Thanks all!

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 23,286
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    Just for reference, you 7.5 KW is equivalent to 25,000 BTUh. When the sun is shining...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    STEVEusaPA
  • jordew82
    jordew82 Member Posts: 3
    edited September 2020
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    Hi Jamie, thanks for replying! Possibly I expressed something incorrectly about the solar, or I may have my math wrong about the heat pump? Here's what I've calculated so far:

    I'm going for a high-efficiency heat pump (20 SEER), and my understanding is SEER is calculated as BTU/Watts. Based on that, a 5-ton machine (60,000 BTUh) would need 3KWh to operate at maximum capacity, which is less than half of the 7.5KWh solar production (on a screaming sunny day, I agree, when I'm very likely to need maximum cooling). I generate about 55KWh total on a sunny summer day and about 30KWh on a cloudy one.

    Admittedly, that same heat pump has a HSPF of only 10, but that "season" rating apparently includes a lot of electric heat usage during defrost and sub-freezing cycle testing, so the actual heating performance of the unit in Spring and Fall is more efficient. Since I won't need all 5 tons of heating in the temperate months, it should still mostly fit within the diminished solar production at those times.

    Of course if I'm way off base, I can switch to AC only and then a condensing boiler makes sense easily (to cover Spring and Fall). What do you think? =)
  • Jamie Hall
    Jamie Hall Member Posts: 23,286
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    No, I don't think you are off base, really -- I was just sort of trying to temper you enthusiasm for "lots of solar" providing your heating and cooling... your solar will provide some. It won't come close to providing all of it, day in and day out.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • jordew82
    jordew82 Member Posts: 3
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    Terrific! Yes, I agree, the solar power doesn't cover everything. July was especially bad as we had a run of hot, humid, cloudy days (and I was over-circulating my pool, my mistake).

    Thankfully my utility provides net metering, so any excess generation is accumulated in a "bank" I can draw from - this really helps even things out daily and monthly.
  • psb75
    psb75 Member Posts: 833
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    If you chose a modulating, condensing gas boiler it probably won't condense for much of your heating season with baseboard convectors. But...it will certainly modulate--a definite benefit for fuel savings, esp. in the shoulder seasons of spring and fall.
  • Ironman
    Ironman Member Posts: 7,376
    edited September 2020
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    jordew82 said:

    Hi Jamie, thanks for replying! Possibly I expressed something incorrectly about the solar, or I may have my math wrong about the heat pump? Here's what I've calculated so far:

    I'm going for a high-efficiency heat pump (20 SEER), and my understanding is SEER is calculated as BTU/Watts. Based on that, a 5-ton machine (60,000 BTUh) would need 3KWh to operate at maximum capacity, which is less than half of the 7.5KWh solar production (on a screaming sunny day, I agree, when I'm very likely to need maximum cooling). I generate about 55KWh total on a sunny summer day and about 30KWh on a cloudy one.

    Admittedly, that same heat pump has a HSPF of only 10, but that "season" rating apparently includes a lot of electric heat usage during defrost and sub-freezing cycle testing, so the actual heating performance of the unit in Spring and Fall is more efficient. Since I won't need all 5 tons of heating in the temperate months, it should still mostly fit within the diminished solar production at those times.

    Of course if I'm way off base, I can switch to AC only and then a condensing boiler makes sense easily (to cover Spring and Fall). What do you think? =)

    You're being a little too generous with your numbers: a 5 ton, 20 SEER heat pump uses 5kw at maximum capacity, not 3kw. That comes directly from a manufacturer's performance data sheet.

    As far as solar contribution goes: a well sized array will usually contribute about 1/3rd of your heating demands.

    Have you done an accurate Manual J heat loss/gain calc? That's the first step. From that, system sizing and estimated energy usage can be calculated.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • hot_rod
    hot_rod Member Posts: 22,142
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    The very best first step and place to spend $$nis tightening up the home and getting the load as low as possible. this pays back summer and winter.
    Next I would size or add heat emitters to get the SWT as low as possible. That opens up more options like A2WHP that could heat, cool, and provide some DHW.

    https://www.caleffi.com/sites/default/files/file/idronics_27_na.pdf

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • bleeder
    bleeder Member Posts: 30
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    in english what is heat emitters open swt to low? a2whp?
    thx
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 4,002
    edited September 2020
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    Adding or sizing up heat emitters allows you to lower the SWT (supply water temperature) which gives you many benefits:

    1) Let's your boiler condense, reducing fuel costs.
    2) If you have a A2WHP (air-to-water heat pump), it will supply 140°F water which may be enough to heat the house. If not, the boiler will bump it up.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab

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