Dual 40 gallon versus single 80 gallon water heater

PC7060

Met with a friend who is updating her house and needs to replace a 80 gallon electric water heater. What’s the preference of the group of using two 40 versus a single 80 given sufficient space & power exists?

Seems like two 40 gallon WH would provide faster recovery time due the relatively higher KW/gallon rating and provide redundancy in case of failure. Another consideration is the lower cost of 40 vs 80 gallon units ($500 vs $2500).

Intplm.

If piped correctly and you have the room doing two 40's is fine. However, I'm not so sure of the recovery rate(s).

psb75

Get one 80 gal. heat pump domestic water heater. There are often good incentives offered—federal, state and local.

Intplm.

https://forum.heatinghelp.com/discussion/comment/1824481#Comment_1824481

A good idea. I have installed quite a few and they do offer many advantages. @PC7060 Consider one as well as a conventional WH.

PC7060

https://forum.heatinghelp.com/discussion/comment/1824481#Comment_1824481

I made that recommendation and got a firm “no interest” due to reliability concerns.
I know AO Smith had some issue which required power cycling the units to get the heat pumps back on line. Has there been any design updates to address those issues?

pecmsg

80 gallons is a lot of water.

Id go 2 HPWH 40 gal ea. it gives you a backup when needed.

Jamie Hall

I presume neither gas no oil fired is an option.

Heat pump water heater? I'd give it a firm no as well. They do have their place, but one has to be a little careful of sizing them, as their heat pump recovery rate is abysmally slow, and if you ask them to go faster they will — using electric resistance, just like the much less expensive all resistance ones. And use just as much electricity.

Now. Two 40s vs. and 80. Well, as noted, the recovery rate will be twice as high, since you have both heating elements running at once. If your electrical system will take it, that's great. Check. They will need independent circuits (usually 30 amp/240 volt). The problem — and it is not a trivial problem — is getting them to both contribute more or less equally. Unless the plumbing is identical between a source point and the reconnection point (for instance, the hot side of the tempering valve) they won't. One will take most of the load, and the other won't contribute much. And I do mean identical. Not just sort of similar, but identical. Not necessarily mirror image, but exactly the same pipe lengths, exactly the same elbows and Ts, exactly the same valves… the whole lot.

Me? I'd go with on 80 — if I didn't have oil or gas available.

hot_rod

pros and cons. Two tanks can be piped in series, parallel with balanced piping, reverse return, or add two balance valves. A small amount of additional standby loss with more surface area.

Two 30A required.

Is the 80 providing adequate water for the building? People tend to use as much DHW as you have available, so operating cost may go up with two tanks😏

PC7060

https://forum.heatinghelp.com/discussion/comment/1824498#Comment_1824498

agree, the house has 1990 era rarely used super tub. I though a 65 was more than sufficient.

EBEBRATT-Ed

Just throwing this out not sure if it is a good idea.

2 40 gallon piped in series you could set the thermostat as a lead lag system say 1 tank set at 130 and 1 tank set at 120. The 130 would be the first tank in line preheating the 120 deg tank which would be the 2d tank in line. with a mixing valve on the outlet of the second tank to maintain 120 degrees. This would give you 80 gallons of 130ish degree water (B4 the mix valve) and under light load would only require the heaters in the 130 degree tank to operate. If the 130 degree tank cannot keep up the 120 degree tank come on to boost the heat over increased load

Jamie Hall

Yeah I think a series setup like that would probably work well…

mknmike

I have two natural gas 40 gallon tanks in the house I bought. They were in a cramped utility room I ripped down to have them in an open basement. So that's probably why they weren't piped to be able to shut one down in the event of a failure and keep using the other. They are just in series, the 2001 install after the 2016 install. They crammed them right next to each other such that the relief valve on the 2016 can't open. If I did the install, I would have spent the few extra bucks and time on the 3/4" copper and valves to be able to bypass either one in the event of a failure. I think THAT would be the major benefit of 2 40's over one 80. Have a failure/leak? Go to the basement, shut off a valve and open another and you still have hot water. Take your sweet old time getting a new one installed instead of having an emergency where I'm heading out to Home Depot before they close and installing through the night before I leave for a work trip or something like that (or just paying out the butt for an emergency install).

ethicalpaul

https://forum.heatinghelp.com/discussion/comment/1824496#Comment_1824496

that's weird, mine just runs and runs with no problem. I think I must be the luckiest person in the world!

Kaos

For dual tank setup, I would go for one 40/32 gallon HPWH and a 40 gallon electric in series.

Install the electric downstream of the HPWH and set it a bit bellow the output temp of the heat pump. This way the only time the resistance heat tank runs is if you run out of water from the first tank.

This would get you much larger capacity, faster recovery and generally much cheaper to run as the bulk of your hot water use would be supplied by the HPWH. You can even get the 15A HPWH so you don't need all that much more power.

Add a couple of bypass and isolation valves so you can use either on its own just in case.

Jamie Hall

https://forum.heatinghelp.com/discussion/comment/1824637#Comment_1824637

but… how do you keep the heat pump water heater from engaging its own resistance elements? Unless they are locked out somehow… all the ones I can find have "backup" electric heating elements…

I'll ask again — I presume there is no ability to install either gas or oil fired? Or is your electricity cheap?

ethicalpaul

Come now Jamie, you must have seen me report multiple times about how my HPWH is set up to only use the compressor? Anyway, with Rheem, you can definitely tell it to only use the compressor. It's mysteriously called "heat pump only mode" 😅

And yes it does have resistance heating elements if you want to utilize a "mixed" mode, but I have never had the need. As the earlier poster said, there are also now 15 amp ones available that have very tiny or no resistive elements.

PC7060

https://forum.heatinghelp.com/discussion/comment/1824652#Comment_1824652

@Jamie Hall - the house does have natural gas but not in this area. Would require adding a B-vent through roof but they are already converting a ventless fireplace to vented unit. Water heater is conditioned laundry room adjacent to garage so may be doable; I’m not sure where meter is located.

Jamie Hall

In that case, check your operating cost for electric vs. natural gas very carefully — and the capital cost of a gas hot water heater and installation vs. the cost of an electric heater, especially the heat pump types.

Intplm.

https://forum.heatinghelp.com/discussion/comment/1824496#Comment_1824496

AO Smith was having a bad run some time ago. I'm not sure if the issues have been resolved.

Bradford White has had a very good reliable water heater. Both Conventional type and hybrid. And depending on your location, you can get a very generous rebate on the hybrids.

Larry Weingarten

Hi, If looking at two tanks vs one tank, I'd go with two in series, valved for isolation and bypass. Even with perfectly balanced plumbing, it will in time get unbalanced due to corrosion or scale. I like the two tank concept with HP in the lead position. Reminds me of two tank solar thermal. I attempted to be an early adopter of a 120 volt HP, but could not get the thing to behave. Wound up going back with propane. Hopefully the industry is maturing. 🤔

Yours, Larry