Rinnai connected to existing water heater?

1. We recently remodeled our bathroom and installed a soaking tub (capacity 70 gal) Our present (and almost new) 50 gal electric water heater does not have enough capacity for the new installation. We are considering a Rinnai. Two different installers have recommended that we install the Rinnai and have it feed into the existing water heater to use it as a holding tank. They claim the existing heater would not use too much energy because it only needs to keep the water hot in both sections and we will always have the option of bypassing it when it breaks, etc. We have adequate gas supply to the house, so the Rinnai can be installed with no problems. What do you think of this installation method?
Does it make sense?

2. The Rinnai will be installed in the crawl space under the house. Do we need and indoor or outdoor model? We live in NC.

Electric tanks have very low standby losses, and maintaining them as buffer tanks will eliminate the most egregioius of tankless "personalty problems).

DO insulate 100% of the distribution lines from the Rinnai to the tanks with 3/4" wall closed cell foam pipe insulation, or there will be substantial amounts of energy abandoned in the lines. Set the output of the Rinnai to at least 5F above the setpoint of the tanks.

Setting up the tanks with a recirculation loop, but controlling the pump with the aquastats on the tanks (disconnect the elements and drive a pump instead) would make it an all-gas-fired system. Done right it would keep the Rinnai from short-cycling on small draws, extending it's service life and operational efficiency. (insulate the return line as well.) With a recirculation loop you'd set the output temp of the Rinnai fairly high so that it hits something like the maximum modulated firing level at the beginning of a burn with the cooler water of the return line entering, dropping to half or so when the tepid water at the bottom of the tank hits. That way it'll modulate nicely during bigger draws, but still have decently long burns when in temperature maintenance mode.

Whether the crawlspace is ventilated or not, it should be an indoor model if it's inside the structural walls. FWIW, in NC you're better off converting any crawlspace into a "conditioned crawlspace" from a humidity & wood-rot control point of view. This would also protect the Rinnai from freeze damage. This involves sealing any ventilation (or converting them to operational windows/hatches), putting down a ground moisture vapor retarder (10mil poly sheeting) and foam-sealing the foundation sill & rim joists. (Even better, put a couple inches of rigid foam insulation against the foundation with Z-flashing at the top at the foundation sill for ant/termite control.)

Dana,
thank you for your reply. That's excellent information.
We have already conditioned the crawl space except for the foam sealing. Will consider that also.
Thanks again!

Dana,
I got a follow up question. We talked to one of the installers yesterday and he suggested we could save a lot of $, even long term, by installing a 40 gal electric tank as a holding tank for the existing 50 gal tank and forgetting about the Rinnai. He said that would give us all the hot water we need without any of the maintenance problems, etc. Plus the payback on the Rinnai would 15 years or so even with the tax credit.
What do you think?
Thanks.

Sure- if the 50 gallon electric tank is in good shape, the installed cost of a second one in-line is minimal, and would give you the tub-filling capacity you're after. Whether it cuts it long term compared to a (much cheaper than a Rinnai) 40 gallon gas-burner is a function of your utility rates, anticipated HW use, and the installation costs. Electric tanks are a much cheaper installation, with no gas-plumbing or venting issues to contend with, but in most markets heating the hot water with gas is a lot cheaper (even in a 0.55 EF tank) than with electricity. If the venting issues are easy, a power-vented gas-fired tank with an 0.60+ EF rating would probably cost half that of a Rinnai.

Thanks, Dana. It seems the second heater is the way to go. The 50 gal tank was installed late last year so it is in good shape. We are shower people with the very occassional tub soak. So other than that and when we have company in the house we don't need that much more HW. Just want to make sure we do have the capacity when needed. If I read you right that means a second electric tank would be best and cheapest. Is that right?

If you still advice the gas heater, we'll check on the gas heater installation to compare costs. Any problems connecting a gas heater to the electric heater? Would you use the gas heater as the main tank with the electric as holding tank or the other way around? (I wish we had thought all of this out when we replaced the heater last year!)

Thank you for your help.

The problem with having a ondemand feed into a water heater is the flow to the house would be determined by how fast the on demand could heat the water. Check your incoming water temp and the charts for the unit before doing this. Another option is to use the 50 gal as a preheat turned to its lowest setting then to the ondemand before the hot demand. This gives good flow and doesn't work the 50 gal very much.
If you go with a addition tank type if you match the tanks they could be plumbed in parallel but if they are different in size should be plumbed in series.

buhrly
http://www.metoliusriverplumbing.com/

I am so excited to come across this post.... I have been thinking over this exact situation for the past couple months. RECAP: I am also renovating our bathroom.... and need something to be able to keep up with our hot water demand/flow. We currently have a 40 gallon/38,000 btu NG water heater.. which works just fine, but won't be able to keep up with future demands. SO... I have been looking at lots of different tankless hot water heaters... and my main concern with those is....here in the midwest, our water inlet temperature can get as low as 40 degrees in the middle of the winter, so the tankless would struggle with that much temperature rise while keeping a good GPM. I have been looking at an Eternal GU 195 unit, and at a 80 degree temperature rise, it maxes out at 4.9 GPM.. which is fine for a regular shower, however, my future shower will flow around 15gpm (ridiculous i know, but i want lots of jets and whatnot).
So I think the tankless will work great in the summer, and less impressive in the winter. So...I have been thinking of keeping my current 40 gallon tank, and using it as a storage tank along with the tankless. My only issue is... I am not sure the best method of install. Should I plumb them in series...the cold water come into the tankless which then feeds into the tank... which then goes out to the house? OR... should I plumb them in series... the cold water come into the tank.. which then goes to the tankless... which then goes out to the house? I'm not sure which method would be more efficient or would supply more/longer hot water. More input on this topic would be great. Thanks!