Electric Water Heater Supplemental to Indirect Gas Fired

[Syt0x]

Just moved into a new house and had a question about adding an electrical water heater to supplement a indirect gas fired water heater. House previously had a solar water heating system that no longer works and was using a basically new electrical water heater as a holding tank. I might look into revamping the solar system sometime in the future but would like to plumb it with the indirect currently used. Here are my thoughts please let me know if you guys agree or would suggest doing it a different way.

The plan is to have the electrical water heater first with the power being plugged into a timer. This timer would only turn on from 4am-7am and 5pm-8pm (peak times for hot water in the household). The hot end would then feed into the cold end of the indirect fired which would remain on all day. The thought being that when not in use the electrical heater could allow the water to reach room temp and allow faster heating through the indirect. During peak times the hot water from the electric heater would pass through the indirect without needing to be heated and would allow almost double the amount of hot water. Thoughts? Would it make more sense to have them in parallel? How would that work when 1 of the heaters is off, wouldn't it introduce room temp water into the hot side? Thanks!

 

[Dana]

Why are you even contemplating this- is the indirect not keeping up with your hot water loads? (Or are you just looking for projects as an excuse to "avoid the ol' lady"? )?? Do you have excess net metered rooftop solar electricity to use up?

The electric heater will take days not hours to reach room temperature, dwelling a the prime legionella growth temperature range (85F-115F ). Standby losses of electric tanks are quite low, which renders timers nearly useless.

In most locations heating water in a standard electric tank would be more expensive per BTU-delivered than just about any boiler fuel except propane. Standby losses of indirect tanks are also quite low (much lower than standalone gas fired water heaters) , and can be made lower by improving the insulation of all the plumbing between the boiler & indirect if that hasn't already been done.

The BTU output of almost all boilers is WAY more than that of a standard electric tank or gas-fired standalone tank, more than enough to run a full flow shower 24/365.

How big is the indirect?

What do you have for a boiler serving that indirect?

What is the storage temp on the indirect? Most can be operated at up to 180F storage temps, if that's what it takes to serve up enough hot water. Rated standby losses for indirects are typically less than 1F/hr @ 180F, often under 0.5F/hr @ 180F, which is probably still cheaper standby loss with gas as the fuel than electric tank at a storage temp of 120F.

If it doesn't already have one, a thermostatic mixing valve or tempering valve at the output to mix it down to a safe temperature is now required by code for new installations, and a good idea for mitigating scald risk if you decide to bump the storage temp up.

 

[Sponsor]

 

[Syt0x]

Yes to both of your first questions . We don't seem to struggle during the week as the family has staggered schedules but on weekends and Holidays when everyone is using our 3 showers we go through water pretty quickly. I am also unable to fill our jetted tub with hot water fully due to the tanks capacity. The old solar system was a solar water heater that pumped the water up to the roof and across 2 enclosed trays that were painted black. They have since been disconnected and not used in some time according to the previous owner.

The indirect is a superstor ultra ssu-45 with a new yorker cg-50awp boiler that has been converted to natural gas to serve it on top of running heater water to the baseboard heaters. The water temp is set to 180 currently on the controller to the water heater. .

The electric is a GE ge40m06aag 40 gallon capacity.

The thought was to either have the electric turn on every day and heat the water in that tank past the growth window of legionella growth temperature range before sending it into the indirect. It would allow the family to have 85 gallons of hot water vs 45 on weekends and holidays. Not sure if it would be worthwhile to leave it on a timer throughout the week instead of a switch that could be controlled from upstairs. Even with the boiler firing and heating the wire the 45 gallons goes pretty quick and then it is unable to heat incoming water to a comfortable shower temp before sending to the house. Is there a better solution you can think of? Just thought with an extra almost new water heater why let it go to waste if it would help.

 

[Reach4]

Is there a better solution you can think of?

Do you have a basement? If so, a shower drain water heat recovery system may fit your situation.

 

[Dana]

Heating the electric tank to 160F for several hours a time would kill the legionella, but not 120F or 140F.

What you really need is a bigger indirect.

If plumbing them in series, have the cold water feed the indirect, and have the indirect feed the electric tank, with the thermostatic mixing valve at the output of the electric heater feeding your hot water distribution plumbing. The just forget about the timer. Putting 180F water into the electric heater set to 140F means the elements of electric tank would almost never be on (unless you went on vacation for several days), and you'd be heating all the hot water with (usually much cheaper) natural gas.

If you have at least 5 vertical feet of drain downstream of the showers Reach 4's drainwater heat recovery heat exchanger may still be worthwhile:

The 117MBH output of the CG50 is capable of supporting more than a single 2.5gpm shower forever, but not three, but a 4" x 48" or bigger drainwater heat recover heat exchanger would roughly double the showering capacity by returning half the heat back into the cold water stream (to both the shower and the input to the water heater.) If there's more than a half-hour of showering per day it'll usually "pay off" in energy savings in a reasonable time frame, even with natural gas as the fuel.

It won't do ANYTHING for filling the spa though, since the heat recovery only happens when the drain and potable hot water are flowing simultaneously, which is why you'd still want to use the electric water heater as a storage buffer.