I use a 40 gallon indirect hot water heater that relies on my boiler. This provides sufficient hot water 95% of the time. I am contemplating installing an electric on demand hot water heater just after the indirect tank with the thought that the electric on demand unit will not turn on unless the water flowing into it is less than the target temperature. On the rare occurrences when the indirect tank runs out and starts sending out cold water, the electric on demand unit will pick up the slack. I am looking at an electric on demand unit because I do not want to deal with the venting of a gas on demand unit. I would appreciate any feedback on why this might not work as intended.
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Is there a reason a 60 or 80 gallon indirect tank isn't an option? Keep it simple.
The electric add on option is a little less expensive than replacing the indirect tank with a larger model. However, more importantly, I thought from an efficiency perspective it would be better to constantly heat the 40 gallons in the indirect tank and only occasionally use the electric on demand unit to help out rather than constantly heating 60 or 80 gallons.
How old is your indirect tank? Is it possible that it is full of sediment or the heat exchanger has scaled up? I only ask because my (limited) experience with boiler fed indirect water heaters has been that they will provide virtually unlimited hot water.
another solution might be to get a mixing adapter and set the temperature higher on the existing water heater.
https://www.google.com/search?q=water+heater+mixing+valve
I would be afraid that between cycles, and first thing every morning, that on demand water heater would come on for a bit at the first use of hot water.
It also will be expensive to wire any kind of unit capable of picking up the slack in demand
My dad put in an electric on-demand heater for a remote bathroom, and you had to control the temperature by reducing cold water flow to let the heater keep up.
Thanks for the advice. I will look into the mixing valve. I agree that the on demand unit might kick on first thing in the morning when the bit of "hot" water in the pipe leaving the indirect tank is not so hot. I would expect it to turn off as soon as the water coming in is already at or above the set temperature so I do not think that would be a major inefficiency but I am not sure.
I agree the electric instant on heater would only be on for a short time when hot water is first turned on. Likely not a huge energy use, but not to be dismissed. These things take a pretty good slug of electricity.
I missed the indirect part in your original post. What temperature is your boiler set for?
And what model indirect tank and what size boiler do you have? The reason to know is to estimate what the steady state flow temperature rise your existing equipment provides at your high demand. (how cold is the cold water you get after high use)
This is needed to set design parameters for the instant hot water heater. Hot water demand flow and required temperature rise will inform your choice of product.
How long will the electric wire run from the service panel will the water heater be?
Is there room in the panel for the large breakers required? Are you capable of doing this installation or will you contract this work?
Thanks for taking the time on this. Boiler is 125,000 btu input, condensing set to about 180 degrees. Indirect tank is 40 gallon set to about 140 degrees. After high use the hot water gets pretty cold, too cold to shower.
Unfortunately the location for the proposed electric on demand unit is about 40 feet from the panel. The 18kw unit I was looking at would require two 8/2 wires. I have space in the panel. I have electrical and plumbing experience and would do the work. All in, I expect the cost would be about $1000.
Venting a gas water heater can be greatly simplified if you use a condensing unit.
I have a condensing boiler now and a gas on demand hot water heater would require the same vent. The area where an on demand unit would go is a little busy already which would make it a little difficult to vent. Because I do not expect the on demand unit to be used often, I thought it might be easier to give up the higher efficiency of the gas unit to get the benefit of the ease of installation of the electric unit.
So the mixing valve would not help much (but might provide good scalding protection with the water set for 140 degrees)
It would cost a little more, but you could consider replacing the 40 gallon indirect heater with a larger one. (which is what the first response here suggested)
for example: https://www.supplyhouse.com/Burnham-AL80LT-80-Gallon-Alliance-Light-Transport-Indirect-Water-Heater-w-TPI-Thermostat
Electric on-demand water heaters will also have some maintenance needs, and may have a shorter service life than a new, larger indirect unit would.
Another option might be a heat pump hot water heater, as there may be large rebates on these units depending upon where you live. Also, you can program the units based on usage, and if you need an occasional morning boost from the electric elements when the heat pump is too slow.
I recently installed one for my daughter and son-in-law and they love it. Previously they heated their domestic hot water with an oil fired boiler with a tankless coil insert...
Consider this. If you were to install a small electric tank heater in line with the indirect tank and set the thermostat to a slightly lower temperature than the indirect heater. The electric tank heater would never have to heat water but only maintain its temperature. Even then it would only have to do that when the water temperature fell below its setting. (Insulate the hell out of it.) You would always have on hand hot water in the amount of both heaters. The last bit of the indirect heaters water would go into the secondary tank and if it were used up before the indirect heater recovered then and only then would the secondary heater be called upon to actually heat water. You would essentially be increasing the capacity of you indirect heater. A small electric tank heater would have the advantage of requiring a smaller circuit than a demand heater and would be less expensive. Just a thought.
Thanks for the advice.
getting in a little late on the conversation - how far does the water have to travel to it's final usage ? might be worth installing a pump to run the hot line "cold" water back to the hot water source, that water should be warmer than the incoming cold water from your water supply
good luck
The max distance is probably 25 to 30 feet.
so you've got 25-30 feet by 1/2 inch of "warm " water that could be fed back into the indirect tank, probably only a couple of gallons but might help(?)
other thought is how much water is being use - maybe a low flow shower head/
is the indirect water heater actually holding temperature as designed? what's the recovery time? 40 gallons is a fair amount so running out sort of begs the question of both usage and recovery
just a few thoughts
good luck
30 ft of 3/4 inch pipe holds 159 cubic inches or about 0.69 gallons.
Not much additional storage for hot water.
But, if you could insulate all the hot water pipes, and install returns to a circulating pump, you would have hot water at each faucet right away. Of course you would want tempering valves where the 140 degree water could hurt folks, which would then extend the hot water volume available.
This option would loose heat along the way, take energy for the pump, and require quite a bit of retrofit.