Exterior tankless install with long head (long intro)

15 years ago

I'm looking at replacing a 40gal tank heater(NG) when it dies with a tankless unit, but I'm having a heck of a time figuring out if I even CAN since the installation constraints are pretty restrictive.

The house is a 3 level 1960's split on slab/crawlspace with the furnace and water heater near the dead center in the lowest level adjacent to the stair framing that connects all the levels. The current unit is open combustion and the original exhaust vent, grossly under-code for today, was shared between the furnace and WH, snaked up through the bottom of a closet on the top level, into the center level roof, and out. I've replaced the furnace with a closed combustion system that uses PVC pipes for intake/exhaust via the crawl space, but the WH still uses the old exhaust vent.

I don't think that I can facilitate routing of a dual-chamber vent pipe via the current route and I don't want to loose the upstairs spaces by framing it out straight up. The alternatives are to use a tankless system that allows PVC and fairly long runs similar to the furnace (which I can only find one of and it's expensive) or to install the tankless unit outdoors.

My gas entrance is at the far end of the house (middle level) along with the AC, cable, telco, etc. The only hot water appliances on that level are the kitchen sink and dishwasher. All the rest are in the top or bottom levels ~30 feet from the current tank location.

The outdoor location would actually make the plumbing, electrical, and gas work much easier as the crawl-space is easily accessible, but is the extra head on the hot side (probably 30 feet) too much to overcome for speed-to-heat on the appliances? How long would I expect to wait for the output (via 3/4 copper pipe from the tankless) at my most distant locations (master bath shower and sink ~45' straight-line, probably more via pipe) assuming pipes are well insulated, etc? BTW, the crawl-space is not climate controlled, but stays in the 60's year-round.

Thanks.

Dan

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Comments (6)

jakethewonderdog
15 years ago
Hey Dan,

A couple of things:

What heater did you look at that vented via PVC? Take a look at the Navien units. The "A" models also include a buffer tank and circulating pump. I've never used one, so do your own due diligence.

What are your outside temps?

Also, I don't understand how you would see additional head from the outside install compared to the basement install.

The thing that has me worried is the long distance from heater to faucet. The central location could limit that. Also, the buffer tank on the Navien unit could reduce wait times. Any chance of installing a return line for a recirculating pump?
thull
15 years ago
If you could, I'd try to modify the hot water feeds to be smaller home runs. You don't get the benefit of quicker hot water at the sink after a shower, but you probably get quicker hot water at each location.

Our tankless is outside and adjacent to one bathroom, but probably 20' of 3/4" to the other bathroom. Takes more water than I'd like for it to get hot there. Usually run the tub faucet, since the low flow aerator in the lav just takes forever.

Fixing that piping is on my before-I-die-but-not-much-sooner worklist.

You might also read an article in FH w/in the last year about a hybrid system of a tankless plus a small-volume (10-gal) electric WH. Seemed like it might solve some of the problems you're worried about.
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dgeist
Original Author
15 years ago
Answers:
- Was looking at a Noritz N-0841MC-DV for the direct vent PVC with 0-clearance to combustibles for inside mounting.

- Winter temps here (Atlanta) seldom get below the teens (It'll hit 50 today).

- Head isn't the best word, I guess, just increased pipe length (horizontal) to heat before any of the far-side appliances get water if it's mounted exterior.

Manifold conversion would be great, but I'd have to tear up a significant portion of the house to do that :)

My big reason for going with a tankless in the first place is space savings. We don't have much storage and would like to get some back from the closet where the current unit is (either with something more compact or moving it altogether).
jakethewonderdog
15 years ago
Okay, here is what I would recommend:

An outdoor unit will work, but I think that you would need to look at some type of recirculating pump. That can get tricky with the tankless. Consider one that is made to work with a pump. (some tankless have a built-in switch for the pump, the other that I mentioned has a built-in pump) If a return line is out of the question, look at a unit vented through PVC installed in the central location. Again, I would look at the Navien units as a competitor to the Noritz.

For the most part, replumbing to a manifold system is out of the question. A recirculating pump may be reasonable and there are some recent plans on the board that use a flow switch or a motion detector as a way of reducing energy usage.

Here is a link that might be useful: Navien spec sheet
thull
15 years ago
FWIW, we're also in ATL and our Noritz is outside. I insulated the outside pipe and installed a self-regulating heat cable (under the insulation) to prevent freezing. It's worked so far (~2.5 years).
dgeist
Original Author
15 years ago
Jake, it sounds like Navien is the Korean answer to the Japanese trio Noritz/Takagi/Rinnai. Their features seem to be on-par with the newest from the others and seem to be a lot more extensive in the condensing offerings. I do like the recirculation features being built in. Also, the 3" PVC venting is a feature I've not seen before. I can't identify any pricing. Know if the units are stocked at any online warehouses?

Thull, anti-freeze measures definitely sound good. What's your experience with delay for hot water at the most distant fixtures (and how far are they from the unit)?

Dan

Exterior tankless install with long head (long intro)

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Exterior tankless install with long head (long intro)

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thull

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