ECN Forum Forums Code Related Discussion NEC & other Code issues Instant Under counter Hot Water Heaters

A lot of new homes here have electric heat because they're outside of city limits and there is no natural gas available. Those that want to add an electric on demand hot water system usually back off when we tell them that a 200 amp service won't be enough. At just over 7¢ a kWh, the payback period in energy savings could take decades to cover the additional cost for a 300 or 400 amp service.

If you are using electric heat there is no wasted energy from a regular water heater. It is still heating your house. Add a recovery unit on the A/C and you are getting free hot water in the summer(minus the recycled heat into the space from the tank)

Another factor in the equation is .... increased prosperity.
Where there was once but a single master bath, now homes often have several baths. As are the larger jacuzzi tubs. There are more appliances - like dishwashers - and they're larger. Finally, folks don't want to wait for that hot water to arrive.

So, the old way - having a single big tank of hot water - is looking inefficient. Having multiple heaters, located much closer to the user, starts looking attractive. Even though the new homes are simply massive - the last one I did was 14 times larger than my home - folks also like the compact size of these instant heaters.

Here's one way to look at things .... imagine taking an extension cord into a home, and using a generator set just outside a door. In my place, a 25 ft. cord would let you reach nearly everywhere. In these newer homes, a 100 ft. cord would be needed.

With more and more items be electrified, I do not see that multiple insti-hot water heaters being viable. Even on a 200 amp service. The one on the tub alone will likel be 100 amps for the large amount of hot that does not leave much for the rest of the house. The HO will not hesitate to use hot water while some one is in the shower and boom, out goes the lights.

Load calculations are one of the things that separates real electricians from the pretenders

We ran into this recently. Home totally renovated, 200 amp service already existing. Customer wants electric heat (not central) and he had already bought an on-demand water system. The hot water alone was 24kw (100 amps @ 240 volts)and because the hot water had to be calculated at 100% demand, that left us with 100 amps for the rest of the calculated load. We reduced the total heating to 7kw - not enough (in my opinion) for a 2-storey (600 ft² per floor)with a full basement which also needs to be heated.

Owner is convinced that the on demand system will save him money and that the electric heating for the house will be adequate. We also tested the on demand system and at full volume, it heated the water to luke warm from 36ºF. Owner had to cut back the input valve in order to get the output temperature to an acceptable level. There's no way this tankless heater will be able to keep up with a shower, dishwasher and washing machine at the same time, and space heating had to be sacrificed to boot.

This tankless heater needed a feed of #2 copper (125 amp breaker), which cost the homeowner over $500 for the wire alone (and wire prices have gone up since then) because the tank was closer to the bathroom and kitchen and quite a ways from the panel. This is something else to consider when calculating the payback period.

If a customer wants a tankless system I will recommend a gas unit, not electric. But not everyone has that option when gas isn't available.

They make propane fired on demand gas hot water heaters.

I was just about to ask why we don't see anything like that on the market! Carnot heat pumps are about 2-3x more efficient than a straight resistance element. And it would essentially do cogen (AC) in the summer, too. The unit in that photo looks far too small to be effective, though. My guess is that the average household simply doesn't use enough hot water to justify the cost.

As for the condensate pump option... water tanks are all required to have catch pans and drains, and they're ALL properly maintained, right??

I actually have one. Works great, never pay for itself:) but it does dehumidify my walk out basement and I'm ducting it into my living room for the AC. I can't run it all year as I'm in Maine and it would freeze my basement solid in the winter:) Why folks down south aren't using them is baffling. less baffling than folks down south not using solar though, that drives me crazy
BTW it has a COP of 2.5 and cost me 800 bucks

The thing they sell a lot of here is the heat recovery unit on the A/C. That is pretty much free hot water. I still think a 30 gallon tank in the attic would just stay hot enough for most people's needs 6 months of the year.