Should I replace my Aquatherm hydronic heat system with a heat pump?
Hello GBA community,
My Aquatherm air-handler (22 yrs old) is paired with an axillary line on my gas fueled water heater (10 years old) to provide heat in my home. For the summer time, we have an outdoor air conditioner (12 years old). The outdoor air conditioner is leaking freon and I’m not sure how much longer we’ll be able to recharge it.
As I make plans for replacement in the future, a lot of HVAC companies are suggesting we go with a 2.5 ton Heat Pump for our 1400 square foot home. (One estimate was about $7000 for a 16 SEER / 9 HSPF heat pump that would use our hydronic heat as a back up system. In comparison, I was given $6000 estimate to replace our existing Aquatherm air handler with a 14 SEER air conditioner.) I’ve considered a mini-split set up, but unfortunately, the cost estimates are too high for our two-story, five-room home. ($10,000 to $15,000)
The challenge is, I’m having a hard time learning about the efficiency of my Aquatherm / Water Heater, so I don’t know how to compare it to the HSPF rating of a heat pump.
(FYI, we live in Richmond, VA, so we have a few days out of the year that drop to the 20s, but it’s very seldom. Our average gas / water bill is $150 to $190, and our electric bill can range from $50 to $110. Calculating future energy bills is an important part of our decision.)
If you have any advice from in respect to suggested energy savings / cost savings, I would be incredibly grateful!
Best,
Blake
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Replies
Blake,
Replacing your existing system with a new air-source heat pump is reasonable (assuming that the installer knows how to perform a Manual J load calculation and knows how to size the equipment).
The efficiency of your existing heating system depends on the efficiency of your water heater. A 10-year-old ordinary gas-fired water heater might have an energy factor (EF) as low as 0.59 or 0.60 (although its EF could conceivably be much higher, if an unusual model was chosen).
Thanks Martin.
Is there an easy way to compare the HSPF of a heat pump to the EF of a water heater . . .??
Overall, I'm concerned with utility costs. If I go with a heat pump, I assume my electric bill will kick up during the winter (staying at that peak of $110); however, my gas bill will go down considerably since my hydronic system is no longer my primary source of heat, correct?
Best,
Blake
If you go the heat pump route be sure the installer does the Manual J. 2.5 ton sounds on the high side for your 1400 sf home.
Thanks James!
Okay, I tried to do some additional homework.
It looks like a dual fuel system will be a better buy in the long haul (1. the responses on this forum are supportive, 2. who knows how long gas will continue to be cheap: http://www.zdnet.com/article/the-questionable-economics-of-shale-gas/. and 3. Although a water heater with an auxiliary line is more expensive ~ $2000 ~ it seems like having a Heat Pump with electric as my back up is a bad idea unless I have one that can perform is some extremely low temperatures: https://www.greenbuildingadvisor.com/blogs/dept/building-science/using-hot-water-heat-air-hydronic-furnace).
The next question is how do I find the economic balance point with a hybrid Heat Pump / Hydronic heating system? A lot of calculators out there don't seem to be able to compare the cost hydronic heating to running a heat pump. I assume this is hard to answer given that a hydronic heat source shares the water with other appliances. Regardless, any tips in this area would be welcomed. Thanks in advance for your help!
The EF of a hot water heater is not a legitimate statement of it's AFUE when used as a combi-heater using a hydro-air coil. The steady-state efficiency (about 80% for non-condensing gas fired water heaters) would be closer, independent of what it's EF test number is. EF testing presumes a much lower duty-cycle on the burner, with much longer standby periods, which add up to significantly larger standby losses as a fraction of the total fuel input. When used as a space heater the EF would only be close to the space heating efficiency if you used almost no fuel for space heating, and the air handler only runs a few 10s of hours per year.
Since you have a heating history on this place, you can calculate a firm upper bound and optmal sizing for the replacement equipment based on the wintertime fuel use of the existing system. This only works if you aren't using significant auxiliary heat, and if it is not a super-insulated house with designed-in solar tempering. But the math is very straightforward- you are using the existing heating system to measure the heat load, within the accuracy of the steady state combustion efficiency (again, about 80% for most non-condensing hot water heaters.) If you have not been using a wood stove or pellet stove or other auxilliary heating on a regular basis, gather some wintertime gas bills and apply the methodology outlined in this bit o' bloggery:
https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new
Note Richmond's 99% outside design temp is +18F. Even though it gets colder than that at time, use +18F as the outdoor design temperature for calculating the load, and only very judiciously apply an up-sizing factor, no more than 1.4x (the ASHRAE recommendation) for new equipment. With a 1 or 2 stage heat pump you're better off with a very low oversizing factor or even sizing it for only 90-95% of the calculated load, with heat strips to cover the difference for the few hundred hours per year the heat pump alone can't deliver the full amount of heat.
Use the extended temperature heating tables or an HSPF test submittal sheet (which conveniently has a capacity number at +17F) and NOT the rated cooling BTUs for selecting the sizing for heating.
The HSPF test is only a crude approximation of the seasonal average efficiency, and is highly affected by oversizing factors. The bigger the oversize factor, the more on/off cycling the equipment experiences over the season, which cuts fairly significantly into as-used efficiency. To operate efficiently it needs to run a low number of fairly long cycles. If sized correctly it is somewhat valid for heating zone IV (which includes Richmond VA. See: http://www.fsec.ucf.edu/en/publications/html/FSEC-PF-413-04/images/Figure5_lg.gif ), based on the number of hours per year there is a heat load.
The units of HSPF are BTUs per watt-hour. Electricity is normally billed in 1000s of watt-hours, "kilowatt-hours" (kwh), so the BTU per kwh would be HSPF x 1000, eg: Say the HSPF is 8.3, the heat pump should deliver 8.3 x 1000 = 8300 BTU per kwh of power used. If you know your power rates you can calculate the cost of a million BTU (MMBTU) of heat using that equipement (assuming proper sizing, etc.) Using the above example, it takes (1,000,000 / 8300= )120 kwh/MMBTU. If your all-in retail electricity costs 11.5 cents/kwh (the current average for VA: https://www.eia.gov/electricity/monthly/epm_table_grapher.cfm?t=epmt_5_6_a ), your heat would cost $0.115 x 120=) $13.80/MMBTU
You can do a similar calculation on the exising gas-burner using 80% efficiency and the all-in retail price per therm. Say you used 110 therms of gas on the last billing, the bill was $136. That's $136/110= $1.24 /therm, source-fuel heat. One therm=100,000 BTU. You used 110 therms in source-fuel BTUs, but in an 80% water heater 20% went up the flue, so only (0.8 x 100,000=) 80,000 BTU went into the hot water, so your net cost per therm would be ($1.24 / 0.80 = ) $1.55/therm of heat deliver to the house. One MMBTU is 10 therms so the cost per MMBTU would be (10 x $1.55=) $15.50/MMBTU.
If you can wait until summer for the replacement equipment, you could measure the duty-cycle of the existing AC system to estimate the true cooling load, or you can run a Manual-J, or take a WAG based on a rule of thumb. Typical small-to-mid size house homes would have a 1% cooling load to floor area ratio of about a ton per 1000' (though HVAC hacks use a ton per 500 or ton per 750, intentionally oversizing it, to the detriment of the homeowner). Odds are pretty good that a 1.5 ton heat pump could handle both the heating and cooling loads of your 1400' house, but if the ducts and air handler are in the attic above the insulation it could easily add a ton to the peak loads.
If the ducts are all INSIDE the insulation and pressure boundary of the house, a 2.5 tonner is almost certainly sub-optimally oversized, unless you have one of the crummiest houses in Richmond (in which case spending the money on insulation & air sealing is probably a more worthwhile investment than newer-better HVAC equipment.)
@D DORSETT. Dang . . . This is really helpful. Thanks for taking time to break this down to a novice like myself. I'll be spending some time doing some calculations in the weeks to come. In respect to a back up heat pump system, I'm hesitant to go with heat strips because it seems like I'd be putting all of my "eggs" in one basket (i.e., electric), instead of having a dual fuel system (electric / gas hydronic) that allows me to make system balance point adjustments based upon energy costs. That said, I'm open to pushback on that. (As in, is maintaining and installing a dual fuel system worth it when you factor in the cost of a water heater that needs an auxiliary line, etc.?) Regardless, thanks again for all your time. I'm incredibly grateful.
Blake,
Q. "Is there an easy way to compare the HSPF of a heat pump to the EF of a water heater?"
A. You can follow Dana's suggestion, or you can use a space heating fuel cost comparison calculator. A decent one is available on this web site: Energy Cost Calculator. On that page, click the blue words "Energy Cost Calculator" to get an Excel spreadsheet. You can enter your fuel costs into that spreadsheet, along with the efficiency of your equipment (for example, 80% for a propane water heater, or 300% for an air-source heat pump -- or whatever efficiencies best fit your situation), and compare the annual cost to heat your home using a variety of fuels.
@MARTINHOLLADAY Great, thank you!
When using Martin's recommended spreadsheet widget, converting HSPF to a percent efficiency would be:
100 x HSPF / 3.412= % efficiency
eg:
An HSPF of 9 would be:
100 x 9 / 3.412= 264% efficiency
An HSPF of 8.3 would be:
100 x 8.3 / 3.412= 243% efficiency
@DANA DORSETT Awesome, thanks!
Dear, Dana, and Martin,
About three years ago, you provided helpful insights as I tried to determine the most efficient HVAC unit for our 1400 square foot home. (Long story short, I found a trustworthy mechanic who does this work on the side, and who properly filled our system with freon. The outdoor air conditioner has been running like a champ and hasn't "leaked" since. Go figure.)
If you have time, I'd love to get your perspective on some additional analyses that I've done. (See attached.) For starters, I've finally found a local HVAC company that knows mini-splits in and out. They're also very flexible and willing to do whatever makes sense, in contrast to being dogmatic about what the "best" option is (i.e., that sense of trust that's hard to quantify). Secondly, I'm much more confident in some of my base calculations, which inform my subsequent conclusions. System cost aside, do the attached numbers seem accurate?
(1) On the heating side, we're currently using about 13.5 MMBTUs per year. However, when doing a crude heat load calculation paired with a 1.4x sizing factor, it looks like we only need a 1-ton unit. Do my numbers seem right?
(2) On the cooling side, I used some equations referenced by PSU's College of Earth and Mineral Sciences (https://www.e-education.psu.edu/egee102/node/2106, example 4) to compare the cost of our current system to new ones using different SEER ratings. When doing so, I kept the "time of usage" constant in this equation to make my comparisons (similar to keeping MMBTUs constant when looking at heating systems). Am I using the proper logic here?
Finally, I'm curious about your thoughts of running a ducted mini-split air-handler combo (12,000 BTU/1-ton) to cover my upstairs (700 square feet, 3 rooms/1 full bath), and a non-ducted mini-split (9000 BTU/.75 ton) to cover my downstairs (700 square feet/2 large rooms). It seems like we might realize some cost savings by utliziing a multi-zone set-up; however, it would be slightly oversized in comparison to just using one single-zone ducted mini-split air-handler for the entire home (18,000 BTU / 1.5 ton). What would you say is better in this scenario? A two zone system, or a system that's more accurately sized?
Thanks in advance for your time and insight.
Best,
Blake
Richmond, VA
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