ASHP water heaters

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Ashful

Minister of Fire
Mar 7, 2012
20,058
Philadelphia
I guess it's been a few years since my hunt for a good tankless ASHP water heater was stalled by lack of good options. Anything new on the market?

I've got a Amtrol boilermate tank tied off my oil-fired boiler, which has all the storage and continuous regen capability I need. In fact, it's a great setup, with one big problem: heating all water via the boiler makes my basement too warm in summer. My primary electronics lab is adjacent to the boiler room, as is the kids rec room, and they both get too warm in summer.

I'm looking for a solution to pre-heat the water from 50F to 120F before it goes into the boilermate tank. I have no interest in actually trying to run a household off ASHPWH alone, too many stories here of luke-warm water, limited regen capability, the thing having to go into resistive mode to keep up. Not worth it, when I have the boiler with effectively unlimited capacity. The boiler will probably still be used to heat the water to final temperature, at least in winter, we may play with running cooler final temperature in summer.

Our hot water usage is very high, and the system is presently plumbed all 3/4" copper, yet I still get cold shocked in the shower if too many additional appliances are turned on at the same time (we have four showers, two dishwashers, a perpetually-running clothes washer, etc.). It's not a matter of heater capacity, but line/flow-rate capacity, so I have no interest in anything with less than 3/4" in/out.

Options?

Yes, I did look at a Rheem tank unit, but between the low ceiling height in my boiler room, and just the sheer floor space they consume, I don't see it as a very attractive option.
 
Not to my knowledge. There was a unit called the geyser. It was meant to retrofit for a standard tanked resistive unit.

The flow rate would be really high so the capacity would need to really really high. That really goes against the highest efficiency at low output for heatpumps.
 
Not to my knowledge. There was a unit called the geyser. It was meant to retrofit for a standard tanked resistive unit.

The flow rate would be really high so the capacity would need to really really high. That really goes against the highest efficiency at low output for heatpumps.
Yeah, same I found years ago. Unfortunately, setting up a loop to allow recirculation on a lower-flow unit plays against using the same tank for low-temperature HP and higher-temperature boiler.
 
Finding out why you go through so much hot water would be the key to lowering it.

Say you find out it's the kids taking 30 minute showers.. Pre rinsing helps with the dish washer. I'd chase the kids around the yard with either the garden hose or a super soaker. This might be less effective with the wife.
 
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Yeah, same I found years ago. Unfortunately, setting up a loop to allow recirculation on a lower-flow unit plays against using the same tank for low-temperature HP and higher-temperature boiler.
I thought About swapping to a DC element in my tank and going solar (without an inverter). You really need a tank though.

Electric tankless and solar. That’s my only solution.
 
Finding out why you go through so much hot water would be the key to lowering it.
Oh, I know exactly why we use so much. But fighting that isn't worth the interruption to domestic tranquility. We're on a well, and really only using ~1 gallon of oil per day for DHW, so the latent heat in the basement is really the only penalty I'm suffering here.

I thought About swapping to a DC element in my tank and going solar (without an inverter). You really need a tank though.

Electric tankless and solar. That’s my only solution.
I have a tank! Sorry if it wasn't clear, but that's what an Amtrol Boilermate is, a tank with a built-in water/water heat exchanger. There's a loop off the boiler with a dedicated circulator motor, which heats the tank. The trouble is, that loop runs WAY too hot to put it thru any heat pump that I know of.

Solar isn't an option. Ugly roof panels, and our southern exposure is shaded by trees, which I have no interest in changing. Also, this isn't about cost, although I always love shaving cost, it's really about air-conditioning my basement!

Other than the lost floor space and potential ceiling height issues, it appears the best solution is a classic tank-mounted ASHP upstream of the Boilermate tank, with bypass valving that allows me to completely bypass either unit for service or failure management. In winter mode, have the ASHP feed the Boilermate, so we can get acceptable hot water temperature and volume. In summer mode, just shut down and bypass the boiler rig altogether.

While there's a big open space in the middle of the boiler room (18' x 18'), nearly every bit of useable perimeter space is already filled with other utility equipment. Since HPWH's tend to vibrate hard plumbing, the recommended hookup is PEX or other soft connections. That does make putting this in front of another piece of equipment, and just keeping it mobile for easy disconnect/relocation a bit more feasible. So, maybe it's the way I should be looking to go.
 
Oh, I know exactly why we use so much. But fighting that isn't worth the interruption to domestic tranquility. We're on a well, and really only using ~1 gallon of oil per day for DHW, so the latent heat in the basement is really the only penalty I'm suffering here.


I have a tank! Sorry if it wasn't clear, but that's what an Amtrol Boilermate is, a tank with a built-in water/water heat exchanger. There's a loop off the boiler with a dedicated circulator motor, which heats the tank. The trouble is, that loop runs WAY too hot to put it thru any heat pump that I know of.

Solar isn't an option. Ugly roof panels, and our southern exposure is shaded by trees, which I have no interest in changing. Also, this isn't about cost, although I always love shaving cost, it's really about air-conditioning my basement!

Other than the lost floor space and potential ceiling height issues, it appears the best solution is a classic tank-mounted ASHP upstream of the Boilermate tank, with bypass valving that allows me to completely bypass either unit for service or failure management. In winter mode, have the ASHP feed the Boilermate, so we can get acceptable hot water temperature and volume. In summer mode, just shut down and bypass the boiler rig altogether.

While there's a big open space in the middle of the boiler room (18' x 18'), nearly every bit of useable perimeter space is already filled with other utility equipment. Since HPWH's tend to vibrate hard plumbing, the recommended hookup is PEX or other soft connections. That does make putting this in front of another piece of equipment, and just keeping it mobile for easy disconnect/relocation a bit more feasible. So, maybe it's the way I should be looking to go.
Thing is if you are drawing down that quantity of water quickly you really need an 80 gallon unit. Means it takes more space. More cost. Three years now all I have had to do do to mine was the occasional hard reset 1-2 times a year and clean the filter. Probably will have anode replaced and coils cleaned in two years. Given your low electricity cost and high usage it probably makes cents. But $x000 buys a lot of oil. Unit will cost $3k+ and then add install cost. The free dehumidification and cooling (great in the summer) is noticeable in my 1000 sq ft basement.
 
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Thing is if you are drawing down that quantity of water quickly you really need an 80 gallon unit. Means it takes more space. More cost. Three years now all I have had to do do to mine was the occasional hard reset 1-2 times a year and clean the filter. Probably will have anode replaced and coils cleaned in two years. Given your low electricity cost and high usage it probably makes cents. But $x000 buys a lot of oil. Unit will cost $3k+ and then add install cost. The free dehumidification and cooling (great in the summer) is noticeable in my 1000 sq ft basement.
I haven't really put much thought into this math, but a 20 minute shower on a 2 gpm shower head with a 70/30 (winter) mix of hot/cold is going to use 28 gallons of hot water. Two of those back to back... 60 gallons. With our laundry running almost continuously, and a random dishwasher cycle, your number may be a reasonable winter target.

But remember that I'll have the boiler running all winter, and that has its own capacity with infinite regen capability. So running out of hot water is a total non-issue, all that varies is how much pre-heating we're actually getting from the ASHP. Then again, in winter, I don't really mind the waste heat coming off the boiler.

In summer, figure more like 30/70 mix for hot/cold, and thus more like 15 gallons of hot water used in a 20 minute shower. This is why I was thinking we might get away with shutting down the boiler in summer. Maybe the cooler water coming off the ASHP will cause us to shift the hot/cold mix from 30/70 to 50/50, but even that might be okay, with anticipated laundry + dishwasher usage.

Heck, even if I leave the boiler turned on, I'd guess it won't be firing nearly as long after each shower, just doing storage and a little temperature top-up. This could be lowered even further if I lowered the boiler temperature for summer.

Funny aside, I remember now that each of our houses and rental properties growing up had a "summer/winter" switch, that we'd flip around May and October. I'm guessing my father must've had two thermostats on each boiler, and the switch that eventually became my job to flip twice per year must've been toggling between the two thermostats.
 
20 minute shower? That must be a teenage daughter calculation. :cool: I'd be twiddling my thumbs to take a 10-minute shower. Let the water go cold to hint that they need to take shorter showers.
 
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Could run 2 ASHP units if storage volume is an issue, either in series or parallel.

I should calculate our daily energy usage for hot water, we put in central AC 2 years ago, I wonder if an ASHP water heater would put a reasonable dent in our cooling needs?
 
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Could run 2 ASHP units if storage volume is an issue, either in series or parallel.

I should calculate our daily energy usage for hot water, we put in central AC 2 years ago, I wonder if an ASHP water heater would put a reasonable dent in our cooling needs?
It should be accounted for in a manual J in some fashion. But I don’t know how the manual J accounts for usage. I did the math at one point. It really affects me in the winter. I figure it’s between 1/3-1/2 ton of cooling. Running 4-6 hours a day for us. What is saves me in dehumidification costs for a basement is worth something.
 
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The free dehumidification and cooling (great in the summer) is noticeable in my 1000 sq ft basement.
Sane....don't even use the dehumidifier anymore.
In winter the hpwh is using "waste heat" from the Kuuma wood furnace that is 15' away...so I look at it as wood fired hot water ::-)
It would be kinda cold down there in winter otherwise
 
20 minute shower? That must be a teenage daughter calculation. :cool: I'd be twiddling my thumbs to take a 10-minute shower. Let the water go cold to hint that they need to take shorter showers.
Ditto. I'm usually in and out in about 3 minutes, but I take at least 2 showers most days, occasionally 3 or even 4, if I'm running back and forth between yard work, business, kids activities, etc. My teenage son is the primary long-shower offender, and while we can all imagine what he's doing in there, he was just as bad at age 6. He daydreams and sings in the shower, just seems to like standing under the hot water. And my wife, what can I say... don't most of them take long showers?

Could run 2 ASHP units if storage volume is an issue, either in series or parallel.
Why? They make some pretty large single-unit ASHP's.

In my case, storage volume is secondary, of less importance. I'll be running out of that ASHP into a boiler, at least in winter where we want to have our water hotter than the ASHP can deliver. The boiler has basically infinite capacity, I think it can regen faster than we can use hot water. In summer, I would play with shutting down the boiler, and living only on the ASHP if possible. But I assume hot water usage is lower in summer, maybe offset a bit by the drop in the hot water temperature on ASHP only, but maybe that's when I switch the ASHP out of Eco mode and crank up it's temperature a few degrees to compensate.

What people need to appreciate is that the amount of hot water you use in a shower will be less, if the water temperature is hotter, as you'll adjust your mix ratio to compensate.

I should calculate our daily energy usage for hot water, we put in central AC 2 years ago, I wonder if an ASHP water heater would put a reasonable dent in our cooling needs?
What's your present method of water heating? If you're on a boiler like me, then yes, I suspect it will make a noticeable dent in your cooling needs. The boiler uses very little fuel to heat water, so there's not much savings in fuel directly attributable to heating of water, but the waste heat it adds to the house must be removed by your air conditioning if you're running AC.
 
Could run 2 ASHP units if storage volume is an issue, either in series or parallel.

I should calculate our daily energy usage for hot water, we put in central AC 2 years ago, I wonder if an ASHP water heater would put a reasonable dent in our cooling needs?
I have two hot water tanks in series. The first one is plumbed into my geo desuperheater, and OWB plate exchanger. The second is a HPHW, if the water coming in isn't 125F then it will run to get the water up to temperature. You could do something similar with your current setup.

Something to be aware of, a HPHW does make a decent amount of ambient noise. I know some of the newer ones are a quieter, but mine is definitely not quiet by any stretch of the means. My bedroom is right above it, and it took me awhile ( no TV / ambient noise sleeper ) to get use to the noise.

Speaking of cost. My Marathon electric hot water heater was costing me roughly $50 a month to run when the ex-wife ( she loved long / hot showers too ) and the kid were here full time. With the HPHW it dropped the cost to roughly $5 a month, and I didn't have to run a dehumidifier in the summer. Back when I installed the HPHW the ROI was 18 - 20 months roughly. That's when a HPHW cost 1k, I think they are about double that now for a 50 gallon unit. *sigh*
 
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What's your present method of water heating? If you're on a boiler like me, then yes, I suspect it will make a noticeable dent in your cooling needs. The boiler uses very little fuel to heat water, so there's not much savings in fuel directly attributable to heating of water, but the waste heat it adds to the house must be removed by your air conditioning if you're running AC.

On demand natural gas fired. There is no heat loss from the water heater itself only minimal from the hot water lines.

Only reason would have been to avoid buying an AC unit. After further review though an ASHP water heater wouldn't pull enough heat from the house to be effective for that use case. And I can heat hot water with gas for less than half the energy cost an ASHP would.
 
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Yeah. The amount of cooling provided is relative to the amount of work the ASHP is doing. So, if DHW usage is low, cooling provided by this unit will also be very low. In my case, the comments about keeping my basement cooler were as much (or more) about preventing the boiler from running, as any cooling actually provided by the ASHP.
 
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I am in the market to replace my 23+ year old LP water heater. current units are in the $1000-2000 range LP i have been interested in the on demand units but i am concerned about the heat ex-changers plugging up, again LP fired just because the electrical from grid cost is rising so fast. Currently I am using around 250 gallons of LP/ year 90% is for the water heater( has a standing pilot) other units on lp are standby genny ( runs once a week for apx 30min,) and conventional forced air furnace ( which almost never runs due to wood stove supplying 98% of heat). heat pump water heater or combination are priced even higher but those i have seen are all electric- do not want to go that route - there would be a fair cost to redo the electric panel to accommodate same. comments requested.
 
I am in the market to replace my 23+ year old LP water heater. current units are in the $1000-2000 range LP i have been interested in the on demand units but i am concerned about the heat ex-changers plugging up, again LP fired just because the electrical from grid cost is rising so fast. Currently I am using around 250 gallons of LP/ year 90% is for the water heater( has a standing pilot) other units on lp are standby genny ( runs once a week for apx 30min,) and conventional forced air furnace ( which almost never runs due to wood stove supplying 98% of heat). heat pump water heater or combination are priced even higher but those i have seen are all electric- do not want to go that route - there would be a fair cost to redo the electric panel to accommodate same. comments requested.

What's your water quality like? I have a gas fired on demand unit that is now 6 years old. (Navien 199k btu). Not a single issue with it yet, I've just flushed it twice with Citric acid, although it's due to be done again.

Our water is hard, but doesn't produce a lot of scale, our city water comes from the nearby river which is mountain runoff, but doesn't build up much for deposits. I wouldn't run such a heater on well water that produces a lot of scale.
 
well water , fair amount of iron in it . lot of lime stone here so that likely would be an issue. about once a year i have to run vinegar through coffee unit. haven't had to much build up in faucet screens but then i am only one here. do have softener that helps some. likely need a more intense system as to iron
 
I installed a shut off valve on the kids shower and ours, the kids water pressure is turned down so it doesn't hurt as much with the longer showers and they never noticed. Mine/wife shower is turned up a little bit more but still way down from regular pressure. The valve blends in well if you have chrome fixtures.

I did try turning off just the hot on my son a few times when he stayed in too long. Unfortunately his mixing valve has a balancing spool, which aims to keep inlet pressure on hot and cold balanced, such that you don't get scalded when someone flushes a toilet. The result of this is that when I cut off only the hot, in an effort to make him hurry up and get out, the balancing spool in his mixing valve also shuts off the cold, and the shower runs dry.
 
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well it got him out, yes ? My middle son was the same way .He would run the tank out completely, and you know who got the cold shower!
 
20 minute shower? That must be a teenage daughter calculation. :cool: I'd be twiddling my thumbs to take a 10-minute shower. Let the water go cold to hint that they need to take shorter showers.
My sister-in-law lived with us for a while and liked to take long showers.
On several occasions, I would throttle back the supply valve on the water heater to encourage her unplanned shower exit.
I have to admit it made me snicker when I heard her shut the shower off.
 
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I scanned most of this and only have one idea. We used to have an insulation blanket on our old water heater. Technically, mine is a hot water storage tank like the OP, with water actually getting heated in my hard working oil fired whole house boiler.

So my old hot water heater had an insulation blanket on it. When we replaced the oil fired boiler and the hot water storage tank May 2022, we put off replacing the insulation blanket on the hot water storage tank. In the winter months, BTUs in the envelope are BTUs in the envelope. However, I have noticed my garage space is notably warmer this year than in years past.

So if there is not one already, a good quality insulation blanket on the hot water storage tank might be worth maybe 3-5 degrees F cooler rooms in the summer months.

I got nothing for on demand inline water heaters. When I get one, I want a lockout timer on it.
 
Interesting. By comparison to the room, my hot water tank does not feel warm, so I wouldn't think it's radiating any appreciable heat into the room. Even in winter, the room seems to be as nearly warm as the sides of the tank.

I likely do have a lot of heat loss off the fitting and piping at the top of the tank, which could be reduced with pipe insulation on ALL of my hot water piping. I've never bothered, probably since I am always in a cold weather mindset when considering that room, but this might help shave a few degrees in summer. Of course, if I move forward with the ASHP, that waste heat won't go as much to... waste.

This is honestly probably project #4 on the present list, there are three much larger projects in the works ahead of it, so I have time to consider. Ironically, one of the projects currently underway actually includes an ASHP water heater, but it's mounted outdoors.
 
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