Tankless in series with tank?

[mikeinkirkland]

In a vacation home that's only used a weekend or two a month, a tankless makes a lot of sense. But, in this case, the only fuel options are propane and electric. And, due to the electric service, the biggest electric tankless is probably limited to a 7.5KW unit which is relatively small. Interestingly, right now in the area, the price of propane and electric are almost the same per BTU/H when you factor in the losses up the flu with the propane heater.

So I'm thinking a possible solution is an electric tankwater heater set to about 60F which should us minimal energy most of the year when not being used (Whirlpool makes electric heaters with a 60F vacation mode) followed by a small 7.5KW tankless. That should give about 1.25 Gal/min at temps hot enough for a decent shower. So you show up at the house and have a limited amount of immediate hot water. If you're going to stay a while and want more hot water, you can turn up the tank. When the tank is hot enough the tankless probably won't even turn on.

My question is if the above scenario is code legal? Can you put a small tankless unit designed for point of use with 1/2" fittings in series with the main hot water line for the entire house? I've looked at the IRC 2009 code and it's not at all clear about water heater capacities, etc. It mainly says you have to use an "approved" solution and they're mostly worried about wiring, gas lines, venting, drains, expansion valves/tanks, wiring, etc.

[Dana]

Unless the thing is installed outside in the snowy passes, the standby losses of better electric tanks are so low it could take you a decade to recoup the cost of installing a tankless at Puget Sound Energy electric rates. Insulating all of the near-tank plumbing within 6-8' of the tank (including the cold feed and the temperature/pressure outflow plumbing) to R4+ with 5/8" wall closed-cell pipe insulation can take the standby loss down even further, and would be also be cost-effective. If you set the tank to 120F, insulate the tank with retrofit tank-blankie + 2" of rigid foam-insulation bottom pad and insulate the near-tank plumbing your standby losses would be on the order of ~$5 month, (assuming 12 cent electricity.) At 120F it won't kill legionella, but legionalla won't grow. Don't park it lower temp than 120F unless you take it all the way below 80F.

Insulating all of the hot water distribution plumbing in the house would make sense if the house were in daily use, and even here it makes more economic & practical sense than a tankless in series. Get the good stuff even if you have to buy it online (or at Graingers)- the box-stores only seem to carry the 3/8" wall pipe insulation.