On-Demand Electric Water Heaters
woobagoobaa
| Posted in Energy Efficiency and Durability on
All Electric Single-Wide Renovation … hot water options.
Greetings all. Situation is a gut renovation of a manufactured home in zone 5. About 1200 sq ft. Foundation, sheathing, siding, window, insulation improvements etc. Formerly propane fired heat and hot water (nat gas not available). We are going back in with cold climate heat pump for heating and cooling. Space is at a premium, so its been suggested we use an on-demand ELECTRIC hot water heater. I have vague recollection that these were problematic. Anyone have direct experience with tankless electric hot water heaters? Any suggestions for hot water appreciated.
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The big problem with on-demand electric is that you need a really heavy electric supply to provide enough heat on demand. The secondary problem is they aren't very efficient, although all resistive heat is the same efficiency.
The gold standard for efficiency is a heat pump water heater, but they are somewhat inflexible in their space requirement and in a small space the noise might be an issue. An electric tank would give you more flexibility about the shape of the tank but would have the same efficiency as on-demand.
I just have to ask -- is it really cost effective to do a gut job on a manufactured house? Usually gutting only makes sense in places like cities where there are obstacles to tearing down and building new.
Thanks DC. Its a long story, but the property purchase price and reno budget work versus the comps in the area and purchase/install of a new manufactured home. And the reno will result in far higher build quality than my understanding of current manufactured home practices.
The noise, size, and expense of the heat pump hw heater is a concern. So I think are options are standard electric tank or electric on-demand.
I would look into rebates and incentives. Heat pump water heaters can be run as resistance heaters. I bought mine for the same price as a resistance due to rebates. If it were me, I would buy the heat pump water heater (which have been getting much quieter) and if the noise is really an issue run it as resistance, or just use the heat pump when no one is home. Take a look at Ao smith, their new one is 45 decibel .
Its too big for the space. Unless they make a small form factor?
To give a sense of the size of an electric service you need for an on-demand water heater --
Assume you need two gallons per minute at 110F, your incoming water is at 50F. That's a heating rate of 60,000 BTU/hr or 17.5 kW. At 240V that's 73 Amps. Circuits for continuous loads have to be sized at 80%, so you'd need a 100A circuit just for the water heater.
If you have space, an electric tank is best. Tankless has many large downsides and just 1 upside, space savings.
I agree with Paul about electric tankless.
I'm currently putting together an ATW heat pump-based system, but because of space limitations in my utility room, I've had to resort to using an electric tankless for DHW. However, I was saved by the fact that the ATW heat pump pre-heats a DHW water feed to 110-120F for me, so I can get away with using a significantly smaller electric tankless to bump it up to 120-130F, rather than lifting all the way up from 40-50F.
My first choice in my all-electric build was something like a Marathon electric tank fed by the same 110-120F feed, but I just don't have the right utility room geometry to fit both the DHW and the buffer tank. :(
Not to derail too much - but in the case where you preheat to 110F and store it there, isn't there a concern for Legionella to develop? 120*F water starts to kill legionella, but only if it stays 120 for 2+ hours. You need water at 160F to kill it instantly (like passing through an instantaneous water heater).
+1 on Legionella risks. I have heard that the practice in Canada is to keep the tank set high (165 or 170F) but mix the outflow with cold water downstream to prevent burns. Since the temperature difference is greater, the energy loss is greater for any given level of insulation. That said, a smaller tank can be used since less water needs to be kept hot to provide a set amount of what at 110F or 120F.
Back to the OP’s question, would a small tank set at a high temperature be small enough to work? Do not forget the downstream valve.
Legionella is a fairly low risk in this configuration, as domestic cold water goes through a heating coil in the 120F indirect tank, on its way to the input of the electric tankless. (or a regular tank, if only I had enough room)
So no particular need to heat the water up and then mix it back down again, as no significant amount of water will stay in the 120F environment long enough, in general, to develop Legionella, though if I feel paranoid about it I can set the tankless to lift to 160F from 120F, which is still a significantly smaller energy bump than going up from 50F.
Unfortunately, as stated elsewhere in this thread, all that doesn't really help OP, as if they don't have room for a tank, an indirect tank takes up the same amount of space, though the SunAmp as mentioned below conceivably might help with that.
This is a bad idea unless you are on chlorinated city water.
Storage tanks like this need to be periodically brought up to pasteurization temperature otherwise you risk bacterial growth.
The better way is to use a reverse indirect where the heat pump heats the tank directly and the DHW is run through the indirect coil. Bonus, you can use the tank as a buffer tank for your heat if needed.
The only reason here to run the tank at a lower temperature is to reduce standby losses, and the ~10 degree temperature difference you're getting from the on-demand heater isn't really gaining you any significant amount of savings compared to the very slight increase in standby losses you'd get by just setting the temperature on the tank 10 degrees higher. Basically that's a long way of saying you will never, ever make up the cost of that tankless water heater with the savings on standby losses you're getting with the tank set only 10 degreees lower than it would otherwise be set to. That's on top of all the issues with bacterial growth mentioned by the other posters here.
I'd just skip the tankless heater completely, set the tank-type heater a little higher as is normally done, and be done with it.
Bill
You might want to check out Sunamp Thermino hot water tanks. They use phase change materials to store heat energy for water, and so their tanks are about 1/4 the size of a similar water tank. They use grid electricity to store the energy just like a regular tank, so no additional power required.
https://sunamp.com/en-us/hot-water-thermino-overview/
Great recommendation!
Paul,
Isn't the OPs whole problem not having room for storage?
Smaller storage would be easier to fit than larger storage…
With numbers: a 60 gallon equivalent Sunamp is 44% the size of a 55G electric water tank. And it's a box, so the true foot print savings is 66%.
Exactly. I have a fairly small mechanical room on a new build that I'm going to put a Sunamp phase change tank in part because a full sized tank is too large. I'll let you all know what my impressions are once I get it!
Cranking the tank temperature up and mixing it down as suggested earlier works great, what I use at the cottage do stretch a 5gal tank. No problem to run a decent shower using a lowish flow head. This does increase standby losses a bit but since those stay inside your envelope, it is not lost.
Make sure to get a tank with a big element, this greatly adds to runtime and speeds up recovery.
Another option is drain water heat recovery, makes a big difference on how many showers a tank can supply.
P.S. Overseas, it is not un-common to mount a lowboy water tank above the toilet. If you have enough ceiling height, it doesn't get in the way and you can always put a cabinet around to. They also have horizontal mount tanks but those are very hard to find here.