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Discussion Starter · #1 ·
I live in the relatively moderate Pacific Northwest. I'm going to replace my old electric 5500W 50gal water heater and I'm trying to decide to stay conventional or go with a hybrid heat pump type.

My concern is the water heater heat pump's heat extraction from the living space during the winter months. Our manufactured house has marginal heating capacity and I don't want to add more capacity.

Has anyone ever added a winter-time manually engaged duct system to bring in outside air (and also exhaust the cooler exhaust air through a separate duct) for the water heater heat pump?

Are there any other options I can consider to use a hybrid but minimize the winter-time cooling? (Other than just purely use the resistive heating during the winter.)
 

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I see a couple of issues with your idea:
  • The fan on the water heater is sized for zero pressure loss from ducting, so by adding ducting you’ll cut down on the airflow through the unit.
  • The heat pump on the water heater is sized to pull heat from “room temperature” air. It’s doubtful that it would be able to extract the required amount of heat from outdoor air that can be significantly colder.
(I’m not against heat pumps. Our HWT is heated by one, but it’s a component of our whole house hydronic heating system)
 

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Discussion Starter · #3 ·
@Chris616

I hear you…

Some systems are designed for a minimal auxiliary ducting system for just this situation. My ducts (8") would be less than 4 feet long (intake and exhaust, each) with a single elbow and wall cap - hopefully not creating too much resistance.

As for the temp, the general specs are 40-90degF but of course the higher the better for the purposes of heating the water.

Maybe I'm not thinking about this the right way. Maybe I should - take advantage of what can be taken advantage of. If my environmental and physical limitations suggest that I can only use the heat pump portion cost-effectively for only 7-8 months of the year then why not get that advantage? Use the resistive heating for the other 4-5 months of the year - I'd still be on par with a conventional water heater.
 

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It would be useful if you have a home electrical energy monitor so that you know how much you spend to heat hot water every year. Then you could calculate the savings in that electricity from using the heat pump for 7-8 months of the year and see if that pays for the additional capital cost of the heat pump water heater over its expected lifetime.
 

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Discussion Starter · #6 ·
Does 7 months of heat pump water heating usage per year pay for the increased capital cost?

Water heating resistive energy usage - 900 kWh monthly usage * 0.25 typical electricity share for heating water = 225 kWh/month
Electricity is about $0.10/kWh

Conventional
$1000​
UEF .94​
Warranty 12 years​
Water heating cost yearly total 225kWh * 12months * $0.10/kWh = $270/year​
Savings over 10 years $0​

Hybrid
$2000​
UEF 2.8​
Warranty 10 years​
Water heating cost yearly total​
(225 kWh * 5 months * $0.10/kWh)​
+ ((225 kWh/2.8) * 7 months * $0.10/kWh) = $57/year​
Cooling cost avoidance in summer 3 months​
heat pump cooling is 50% of a window AC (12A) 1440 W * 50% = 720W​
which runs for 8 hours per day for 3 months = 518 kWh​
518 kWh * $0.10/kWh = $52 (avoided/saved)​
Savings over 10 years (($270-$57)+($52)) * 10 years = $2650​

$2650 Savings over the life of the unit pays for the $1000 increased capital cost
 

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Should this calculation not look like this?

Water heating cost yearly total
(225 kWh * 5 months * $0.10/kWh)
+ ((225 kWh/2.8) * 7 months * $0.10/kWh) = $169/year

Using a heat pump hot water heater will remove heat from the house, displacing some of the load of the air conditioners, but I don’t understand your calculation. Let’s assume that the water heater and air conditioner both remove the same amount of heat from the air in the house per kW of input energy. Doesn’t your calculation show that using (225 kWh/2.8) = 80 kWh of energy monthly in the water heater would remove the same amount of heat as using (720W * 8 hrs * 30 days) = 173 kWh of energy in the air conditioner?
 

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Your "Cooling Costs Avoided" doesn't quite look right.
I am not quite following the logic there, but I estimate my own hot water energy usage as 113 kWhr/month equivalent in the summer (10 cu meters natural gas/month).
So, if you are putting 518 kWhr into the water over 3 months, that is a reasonable number. Higher than my own estimate, but reasonable.
But you do not seem to be including the electricity costs of running the water heater.

The other thing is that future savings need to be depreciated. If your investments are otherwise returning, say, 8%, than $52 saved 10 years from now is worth half that amount today.

My gut feeling is that a heat pump water heater is a good idea in Arizona, but not so much in the Pacific Northwest.

Read that warranty on the heat pump water heater carefully. Your first payable service call could wipe out all the energy savings of the previous 10 years. A resistive electric water heater is a pretty simple device.
 

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More needed and heretofore unknown and hard to obtain. information.

How much electricity (kilowatt hours) the water heater heat pump consumes to use 30 F outside air to heat so much water to 120 F compared with how much electricity the space heating heat pump uses to heat air from 30 F to 70 F and the water heater heat pump uses that 70 degree air to heat the water to 120 F. I am guessing that a heat pump that needs to heat something like 120 F will be much less efficient starting with air under 30 F or may not work at all compared with a heat pump that only needs to reach a target temperature of 75 F.

Don't forget. When figuring the cost per kkilowatt hour, combine all of the itemized charges that are billed by the kwh. Things like stranded cost, distribution cost, carbon emission cost, etc. as well as energy cost. Or you can take the total bill in dollars, subtract the fixed charges like the minimum charge, the monthly charge, the paper billing charge, and things like that and divide what's left by the number of kwh to get the cost per kwh
 
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Discussion Starter · #10 ·
...$169/year

...Doesn’t your calculation show...80 kWh...= 173 kWh...?
Thank you for the math error "catch". It really changes the trajectory upon which the decision should be made.

I understand your point about the equivalency of the water heating and AC cooling. I'm going to chalk it up to "assumptions" and suggest that it leads to a more conservative approach to the decision making (meaning - don't "over" assume and bias the calculation towards buying the hybrid unit; in other words, avoid a form of confirmation bias).

I'll post a new calculation summary soon.
 

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Discussion Starter · #11 ·
Your "Cooling Costs Avoided" doesn't quite look right.

....depreciated...

My gut feeling is that a heat pump water heater is a good idea in Arizona, but not so much in the Pacific Northwest.

...resistive electric water heater is a pretty simple device.
Thank you for imparting some rigor.
I'll post a new summary calculation soon.
Agreed that other financial dimensions should be considered.
Agreed about reliability and maintenance cost for simple vs. complex machines.
In sum - I think I'm changing my mind on buying a hybrid unit and am just going to stick with a conventional unit.
 

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Discussion Starter · #12 ·
More needed and heretofore unknown and hard to obtain. information.
...
Don't forget....
Agreed that info is hard to find or create and to get slightly better assumptions than the "zone" view of the USA (for a particular region, like the PNW) is really hard. I'm trying to ignore the hype and make reasonable but conservative assumptions. The power company wants everybody to convert because even small energy savings are then multiplied by thousand of customers and the power company doesn't have to account for increased maintenance cost and inconvenience due to a more complex machine.

I had accounted for the "burdened" electricity cost in my energy cost - a "low-ish" electricity cost is one of the few benefits of living in my area.
 

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Discussion Starter · #13 ·
A new summary:

Does 7 months of heat pump usage per year pay for the increased capital cost?

  • Water heating resistive energy usage - 900 kWh monthly usage * 0.25 typical electricity share for heating water = 225 kWh/month
  • Electricity is about $0.10/kWh
  • Conventional water heater
    • $1000
    • UEF .94
    • Warranty 12 years
    • Water heating cost yearly total 225kWh * 12months * $0.10/kWh = $270/year
    • Savings over 10 years $0
  • Hybrid water heater
    • $2000
    • UEF 2.8
    • Warranty 10 years
    • Water heating cost yearly total
      • (225 kWh * 5 months * $0.10/kWh) + ((225 kWh/2.8) * 7 months * $0.10/kWh) = $169/year
    • Cooling cost avoidance in summer 3 months
      • heat pump cooling (according to some sources) is 50% as effective as my window AC
      • My window AC uses 12A ⇒ 1440 W which runs for 8 hours per day for 3 months = 1037 kWh
      • 1037 kWh * 50% avoided/year * $0.10/kWh = $52 avoided/year
    • Savings over 10 years (($270-$169)+($52)) * 10 years = $1530
$1530 Savings over the life of the unit pays for the $1000 increased capital cost

But, are these slim savings ($530) enough to justify a more complex unit that will likely have higher repair cost (and the attendant inconveniences for repair)?
 

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Discussion Starter · #15 ·
In my poorly insulated manufactured house plus stick-built addition (total ~1650ft2), I have:
  • 3000W of electric wall heaters
  • Propane fireplace insert tstat-controlled variable output 19kBTU-40kBTU 70% AFUE
(Propane is much more expensive here.)
 

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I have never had a hybrid, I have a separate unit hp for mine.

Can you just turn off the HP part on the unit during heating season?
Then turn on the HP part during non heating season?
Of course that means you will run on elec elements for hot water 5 months of the year, but then for the other 7 months you use the HP and add cool air to the inside of the home and slightly reduce the ac bill and dehumidify somewhat as well.

It will take longer for breakeven though.
 

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In my poorly insulated manufactured house plus stick-built addition (total ~1650ft2), I have:
  • 3000W of electric wall heaters
  • Propane fireplace insert tstat-controlled variable output 19kBTU-40kBTU 70% AFUE
(Propane is much more expensive here.)
A reversible heat pump that can be a heater or air conditioner is much more efficient than the electric heaters. The best electric heater can get is a heating seasonal performance factor (HSPF) 3.41 (https://bit.ly/3deoJji ), whereas heat pumps get at least double that and there are models that can go to over 4x that, this unit can get you up to 15.0 HSPF (Infinity Residential Ductless System Heat Pump with Basepan Heater - 38MPRA | Carrier - Home Comfort). It can heat in weather ranging from -22F to 86F (at 100% efficiency) and cool in weather from -22F to 122F. YOu could use a heat pump to take heat from inside to heat the water, and take heat from outside to heat inside, and use less power than using electric heat. If you get temps below -22F you can use the electric heat as a backup.

Those split mini are a DIY dream.. super easy. Just have an electrician do the breaker box and the rest you can do.
 
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