How to size a heat pump water heater for large range of demand
Questions are at the end.
Background – environment, infrastructure and loads
We are building an addition on our 60-year old home, making this a good time to replace our 27 year-old water heaters (two 50-gallon propane). We’re re-doing our electric panel, re-routing propane lines, and adding a generator for rolling power outages during wildfire season and downed power lines during avalanche season.
Year-round, the home is used 30% of the time. Seasonally, the home is used about 50% of the time during the winter (temps of -5F to 35F), and 20% of the time during the remainder of the year (temps up to 88F, but mostly 60s-70s). When we are there, it’s often for a 1-2 week continuous stretch depending on work and our family schedules. We are often joined by extended family and friends throughout each stay.
We’re at 6600-ft elevation, on the cusp of IECC climate zone 7B (9500 HDD some winters) and 6B (8800 HDD most winters). Incoming water temp is 47F during winter and 55F otherwise.
Our 60 SQF equipment room in the 600 SQF basement (insulated, unconditioned) is maxed out for space containing the propane HVAC and two 50-gallon hot water tanks. Fortunately, we have a large and tall protected area under our driveway (spans the hillside to the garage) where we can put heat pumps and a back-up generator adjacent to the home. We have every form of rodent here, but we can design outdoor protection around that.
After the remodel, the house will be 4 full baths (4 shower heads, 1 tub), plus the usual kitchen (1 sink, dishwasher) and laundry (1 old top-loading washer). The headcount is sometimes 3-4, often 6-10, and we’ve crammed in up to 12-14 on a holiday (kids, grandkids, friends – we have bunk rooms).
We have 1PH 120/240V (200A service) and a 1000 gallon propane tank. No natural gas available. PV is not an option due to snow load (350 lb/sqf on the roof). We suck down a scary amount of expensive propane and want to change that by moving from propane HVAC and water heaters to electric heat pumps if it makes financial sense.
SIDE NOTE: when comparing propane vs electric, using our actual billing rates, and de-rating for elevation and accounting for appliance efficiency, the $ per equivalent BTU is actually the same in our situation. The only way to move the needle is to find more efficient appliances – hence the heat pump.
We’ve gotten away with 100 gallons of hot water by taking breaks between showers. The propane water heaters reheat relatively fast (45 minutes). But with a cold climate heat pump water heater (HPWH), the breaks between hot showers might be too long (4-6 hours in winter with outdoor temps in the 20s, teens, and lower?), especially if we have up to 10 showers to be taken.
SIDE NOTE: 10-14 showers is not typical. Our preference, as well as our crew of family and friends, is to be able to hang together into the wee hours. This is often on holidays when short-term rentals and hotels are fully booked and most expensive. So although we could reduce our shower load by having some people stay elsewhere nearby, if we can eliminate the shower limitation, that would be ideal.
All of our showers have isothermal controls. We did this to provide a stable temperature experience at all the showers as different showers go on and off, and as the tank water temperature goes down with use. Although costly, this was a game-changer for us (although I think shower duration has increased due to the comfort of water temperature stability!).
Our friend/family lifestyle involves outdoor mountain activities on the weekend and catch-as-catch-can during the week. This usually translates into many showers in the late afternoon (up to 10 on the weekend, or 10 gpm for 4 simultaneous showers), along with dirty sport clothes in the washer.
Questions
See above for load details, environmental temperatures and other constraints.
1. What size HPWH to support 4 simultaneous showers taken, for a total of up to 12 showers (call it 120 minutes of 100F shower time to be generous)?
1a. Maybe design for 8 showers taken and let people draw straws for who might get the cold showers? (80 minutes of 100F shower time)
1b. Or maybe we know how many minutes we can deliver 100F showers before running out, and let everyone figure out the shower schedule with a multi-hour gap for the HPWH to reheat.
2. Does a large propane tankless make more sense than HPWH due to dynamic usage and equipment room space limitations? Propane tankless is marginally “green”, but definitely easy when it comes to delivering 1 shower or 4 simultaneous showers for 10 or even 14 people. Might help with standing losses if I want to feel “green” about it.
3. Does a hybrid system with a HPWH (80 minutes of 100F shower time) and one small point-of-use (2.5 gpm shower) electric tankless heater make sense for when we run out of HPWH 100F shower time? People can then take turns on the one shower that has the tankless back-up while the HPWH is reheating.
4. Just stick with propane hot water heater tanks because our environment and loads are too variable.
There’s something appealing about option (3) with HPWH + tankless. The tankless can be small and possibly benefit from the remaining “warm” water in the HPWH (rather than the 47F cold water inlet). Not sure how to manage the transition from HPWH to tankless at the one shower. Or maybe just 3 showers are on HPWH, and the 4th shower is on dedicated tankless? This could give good flexibility as well as redundancy if the HPWH fails.
I’m generally a fan of hybrid solutions of all kinds, as one-size-fits-all tends to have harsher compromises and consequences.
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Replies
My biggest recommendation would be low-flow showerheads. I'm a fan of High Sierra, specifically the 1.25 gpm head. It's a great shower, you don't notice the difference, and you use half as much hot water. It's like doubling the size of your tank.
Thank you for the excellent suggestion and recommendation.
I'll give the High Sierra a try ASAP and get feedback from family members. That would be a big and almost "free" gain (compared to the cost of bigger appliances).
The other way to effectively double the size of your tank is to add drainwater heat recovery, e.g. the "powerpipe" brand. It's a great fit for your application because it works best when there are many simultaneous and/or sequential showers.
"SIDE NOTE: when comparing propane vs electric, using our actual billing rates, and de-rating for elevation and accounting for appliance efficiency, the $ per equivalent BTU is actually the same in our situation. The only way to move the needle is to find more efficient appliances – hence the heat pump."
Can you talk more about this? Specifically, when you accounted for appliance efficiency, what number did you use for electric and propane?
Great question. Not sure if this calculation is the right way to think about this, but here it is (it works out to about 3% difference between propane and electric resistance, with propane being 3% cheaper). I consider 3% a negligible difference, especially because propane rises and falls month to month (I used December 2023 costs directly from our utility bills).
Propane calculation:
Propane at sea level is 91,452 BTU/gal
We pay $3.756 per gallon (with taxes and fees, 12/2023)
$/BTU = $0.000041 @ sea level
Propane then derated 26% for 6600 ft elevation (4% per 1000 ft above sea level)
$/BTU = $0.000056 @ 6600 ft
Electricity calculation:
Electricity is 3412.1 BTU/kWh
We pay $0.214 per kWh (with taxes and fees, 12/2023)
$/BTU = $0.000063 @ any elevation
Next take propane vs electric resistance efficiencies into account:
Propane Water Heater (1997) = 82% efficiency
Propane Water Heater (2024) = 90% efficiency
Electric Water Heater (2024) = 95% efficiency
$/BTU = $0.000068 @ 6600 ft with 82% efficiency (propane 1997)
$/BTU = $0.000062 @ 6600 ft with 90% efficiency (propane 2024)
$/BTU = $0.000060 with 95% efficiency (electric resistance 2024)
Using the 2024 efficiencies, the cost difference between electricity vs propane is 3.3%, with propane being less.
Resistance heat is the same price as propane, not heat pump.
So at least that answers your hot water needs. With a place that is not used often, get a couple of larger resistance tanks. This will cost you less than any HPWH solution and be cheaper than propane due to the reduced standby losses. If you are really tight on space a large propane tankless unit is the next best thing, keep in mind it can only do about 3 showers at the same time without dropping the output temperature.
Anything HPWH, especially a larger one, is not worth the cost unless occupied full time.
For space heat, a properly sized heat pump will produce between 2 to 3 units of heat for each unit of electricity, so in this case it would be 1/2 to 1/3 the cost of propane to heat. It is a no brainer for overall operating costs. Make sure it is sized correctly for both load and climate conditions.
If larger power outages are an issue, some pilot operated propane backup heat is a good idea at least in the basement to keep things from freezing. A through the wall vented propane heater would work quite well.
I'm assuming there are fireplaces upstairs, so those will work as backup heat when occupied.
Akos, thank you for pointing out my oversight.
I should be calculating energy cost using UEF so that flue and pilot losses are included for the gas heater.
Comparing propane (UEF = 0.69) and electric resistance (UEF = 0.92), it turns out that electric resistance will cost 40% less than propane for the same amount of hot water. This is using my actual billing costs for electricity and propane.
Good point regarding your comment, "Anything HPWH, especially a larger one, is not worth the cost unless occupied full time" - it would take a lot of showers to break even.
We certainly have a heavy shower load when there, but then we're only there 30% of the year. So basically if we're averaging 8 showers a day when there, that's really like 2-3 showers a day on average over the course of the year.
Let's say your incoming water is at 47F, you installed the low-flow showerhead so your shower is at 1.25 GPM at say 107F. So that's 60F of temperature rise, at 1.25 GPM that's 37500 BTU/hr or 11kW. At 240V that's 46A. That's a big chunk of your 200A service, depending on what else is installed you may not be able to install that.
Generally for on-demand a fuel-burner is more feasible than electric.
Agreed on both counts - a 11kW tankless would be a big chunk of the 200A service, and a fuel-burner is better for on-demand.
Although we're trying to reduce our propane use, a small propane tankless on just one shower might be a good trade-off if we are all electric on the HPWH.
We don't have a good location for a small propane tankless near the shower. As much as I'd like to put the tankless unit closer to the shower (easier with electric), it might have to go down in the equipment room. Not the end of the world.
I'd put the tankless inline and after the HPWH. That way you can use any shower. If the thermostat is set slightly below the HPWH it should only run when the HPWH is struggling.
What you don't want to do is set the thermostat on the HPWH higher. That destroys its efficiency.
I disagree. Set it higher for those days when you have hoards of people showering, and then set it back down for normal situations. That still better than relying on the electric backup.
1. As DC said, low flow showerheads would help.
2. Drain water heat recovery is perfect for this situation if you can access the pipes.
3. I think the easiest equipment option would be 80G HPWH with a large propane tank heater too. Combustion tanks all look the same but there’s a ton of variation amongst them actually: you can get a propane tank heater with a burner that easily matches that of an on demand unit.
Since you have room for two tanks, I see no benefit to tankless. They are mediocre products and should be avoided.
Ordinary propane tank heaters have terrible efficiency and terrible standby loss. And if you are plumbing it in series with the HPWH, you'll get its standby loss even when you are using the HPWH and not burning any propane.
Right, don’t buy a bad propane heater. They make versions as efficient as tankless.
Can you tell me more about the HPWH / propane option you suggested? This has potential - it fits in our space, and it doesn't involve a propane tankless.
Are you thinking 80G HPWH for 3 showers, and propane tank on the 4th shower? Or would the propane tank be in line, with the 80G HPWH feeding the propane tank?
As for the drain water heat recovery (DWHR), we can easily access the new shower (yet to be built), but the three existing showers are a challenge. The distance between the hot water heater and the showers range from 20 ft to 80 ft. The new shower is the furthest (80 ft, maybe a little less). For the existing showers (20-40 ft from the water heater), we might be able to remove a section of existing pipe about midway between the water heater and the shower.
For further distances from the water heater, would you feed the DWHR output to just the shower, or would you still tee the DWHR output to both the HPWH and the shower?
Highest efficiency is when the DWHR pipe feeds both but practicality matters too. Your layout will determine what options you have better than I can.
I think propane tank in-line. The HPWH can be set higher.
The SanCO2 units are expensive, but may be a viable option. The storage tank is separated from the heat-pump, allowing storage size to be tuned to the application. It also heats to higher temps (145-150F), increasing its first hour delivery rating. While most folks think of installing the SanCO2 heat pump unit outdoors, installation within the building envelope is allowed. This is what I did at our house where it replaced a Stiebel Eltron Accelera 300 that had a manufacturing defect.
The Gen4 model claims a COP of 5 at 67F ambient (about what my unconditioned mechanical area/deep crawl space hovers at) for a 90F rise. At 47F incoming water temps, the COP would drop a little, but probably not below 4. Paired with an 83Gal tank, you get a 115Gal first hour rating (125Gal on the Gen5 which is coming out now). Recovery rate is ~20Gal/h of 150F water. They also sell a 119Gal tank and multiple tanks can be ganged if you need even more storage.
Most HPWH on the market are "Hybrid" water heaters with resistance elements - typically 30A matching traditional electric tanks. These come on during periods of high demand. If you installed 2 that would be 60A worth of "instant hot water", equivalent to 1.8 gallons/min at a 55F rise. Shouldn't that be fine?
I went back and read the original posting, and this bit jumped out at me:
"But with a cold climate heat pump water heater (HPWH), the breaks between hot showers might be too long (4-6 hours in winter with outdoor temps in the 20s, teens, and lower?), especially if we have up to 10 showers to be taken."
There is no such thing as a "cold climate heat pump water heater." A heat pump water heater is pumping heat out of the house, it doesn't care how cold it is outside.
The heat that it takes out of the house has to be replaced. Since electricity is your low-cost fuel, the decision to be made is whether, once you factor in the cost of the heat that needs to be replaced, a HPWH is cheaper to run than a resistance one.
If you're burning propane for heat, the heat for the hot water comes from that propane, and since propane is so much more expensive than electricity that ends up costing you more money than just running a resistance hot water tank.
If you're getting heat from heat pump, the answer is determined by whether the combined COP is over 1.0. If the heat pump has a COP of COP1, and the HPWH has a COP of COP2, to move one BTU of heat from outside the house to inside the house takes 1/COP1 units of electricity, and to move that BTU into the water heater tank from inside the house takes 1/COP2, a total of (1/COP1+1/COP2). So the combined COP is one over that, or 1/(1/COP1+1/COP2).
HPWHs typically have a COP around 3.5, so your heat pump has to have a COP over about 1.4 in order for a HPWH to make sense. My recommendation for heating (and cooling) the house would be something like a Mitsubishi M Series, which is rated for a COP of 1.5 at -13F and 1.9 at 5F, which should be fine for you.
The combination of a heat pump and HPWH with say 65 gallon capacity will serve you well with 4-5 people, but what about when you have 14 people? You need to be able to bring more capacity online. I see two ways. One is to have a couple more big resistance electric water heaters. Like maybe two 80 gallon units. It's hard to say if even that is enough. The other way would be to bite the bullet on the cost of propane and have an on-demand propane heater.
In either case I'd recommend having them in series and downstream of the HPWH. The HPWH always gets first crack at heating the water, if it has the capacity the backups do nothing.
You should look at first hour hot water delivery for the rheem and Ao smith hpwh and figure it out from there. As stated above both of these units can be run as resistance (and are in high demand situations) and so the only benefit to a resistance only is the initial cost. This is, in most places offset by rebates. My hpwh was actually cheaper than a resistance water heater due to electric company rebate.
If you need more, as dc points out the on demand unit would be ideal for the situations where the hpwh isn’t enough, as it will run forever. I’m not sure how that would work in concert with a preheat from hpwh but I’m sure there is a way to make it work, maybe even simply inline. I know they kick on with water flow, but they also measure temp so may not kick on until drops enough, though this might be a little finicky. I would talk to the on demand manufacturer prior to install.