Build external bumpout for internal tankless?

[Dana]

With drainwater heat recovery you want it to feed BOTH the hot water heater AND the cold feed to at least the shower (but the whole-house is still OK, if you don't mind room-temp water coming out the cold taps.) The heat exchanger presents some restriction, to flow, and some are better than others in that regard (PowerPipe is the least restrictive) but the shorter they are the least restrictive and higher the flow. (I have a 4x48" and don't have any problem getting 6gpm+ through the thing, and that's with some half-inch distribution plumbing and a pressure reducing valve at the meter.)

They make some pretty stubby ones that fit in crawlspaces, and creating a wide section to manage a 3" or 4" version doesn't diminish the drain function but enhances the heat exchanger efficiency. The fatter and longer they are, the better the efficiency, and the labor to install a tiny one is about the same as installing a large one, so if you do it, get the fattest & tallest one that fits. Even a 4" x 24" PowerPipe delivers better than 30% return, and a 4" x 30" returns 40% @ 2.5gpm, according to Natural Resource Canada's third party testing. (At lower flows the return is even higher.) Even if it's not subsidized by the state, if you can fit even a 2x24" in there it will extend tank capacity measurably, and have a return on investment for families with long-shower addictions. It has to be on the drain, not the stack, but it can be located anywhere downstream of the shower. The closer it is to the shower it is the better, but not dramatically better, and even 50 feet away is fine, taking only small hit in measured efficiency.

A split-system heat pump water heaters (mini-split style) work where the tank-top versions won't, since it has the capacity & controls for de-frost. By not presenting a load to the heating system it's true efficiency in a heating dominated climate is higher, independent of it's EF rating.

The Vertex gets good reviews and tests very well on efficiency. The ~75KBTU/hr burner makes for excellent recovery times (it's 2x the burner of typical 40 gallon conventional tanks), and is enough burner to run continuous 1.5gpm showers at Portland's summertime incoming water temps (but maybe not all winter) even without a drainwater heat recovery heat exchanger. Marathon's are great for an electric tank heater, but like all electric tanks (including the heat pump variants) recovery times are poor and the first-hour gallons ratings are quite low compared to even the smallest burner gas-fired tanks of similar volume, so you have to go with a big 'un to go electric. Heating with a gas fired condensing water heater will be literally half the cost (or less) of heating hot water with electricity in most areas. (What are your gas & electric rates?), so even if you're paying $500-800 more up front, the payback on the difference for a typical 4- person family water usage would be under 10 years, maybe under 5 years.

[justinae]

Just got official rates this morning direct from suppliers:

Electrical: $.11039/kwh
Gas: $1.09/therm

Here is a matrix I made. The numbers tell a better story than the theory. It's interesting that an 80% efficient water heater is the best bang for buck.

EDIT: Ignore the first matrix. I added 5 and 10 year and can't figure out how to delete it.

[Dana]

The numbers for the Airgenerate are misleading, since they show the extra natural gas burned in your furnace spent supporting the load 8-9 months out of the year.

It's not too surprising that an 80% tank heater is the better financial performer at buck-a-therm fuel rates. If that goes to a buck-fifty the Vertex will suddenly start looking a lot better.

But there's more to it than raw net-present-value of utility savings. In a multi-person multi-showering household first-hour performance counts. A tankless takes that hands-down, but not necessarily with multiple simultaneous flows. The Vertex would be second, but has higher peak flow and fewer quirks.

The EF rating of the tankless also exceeds as-used efficiency (by at least 0.08) since A: They lose a lot of efficiency to flue purges and ignition cycles on short draws and B: you end up using more hot water due to ignition delays, etc.

The venting costs on the 0.82 tankless are likely to be 1.5-2x what you have there, and for the condensing tank it'll probably be less. The gas piping to the tankless units will also be measurably more expensive than to the gas fired tanks too- you're looking at 1-1/4 gas piping compared to 3/4" for the tanks.

And if you're using more than 63 gallons/day per the EF test, (which would be the case for the long-showering folks) the payback of going with a condensing unit pulls in by quite a bit, and the effective as-used EF of all tank heaters goes up a few notches. You probably won't hit 100gallons/day for an average but 80 is pretty common for a 4- person household.

BTW: Internet pricing for the Vertex with the 76K burner is running around $1600 these days- does it really cost another $498 to get it to your house? I think you probably priced the Vertex with the 100K burner (the GDHE rather than the GPHE) which is way more burner than you need. At 76KBTU/hr input and 95% combustion efficiency your getting about 72,000BTU/hr out of it, which is good for an endless 2 gpm shower with a 72F rise. That means even with a smokin' hot 108F shower you're good down to ~36F incoming water temps, as long as you don't have any other big draws while showering. With a 1.5 gpm low flow shower you can even take some other pretty big gulps off the 50 gallons without flinching- it more than keeps up with the shower alone. The 100K burner may make sense if you have real gusher-showers with sidesprays or a monster tub to fill, or if you're using it as a combi-heater to run the radiant floors in a McMansion, but it's another $500 YOU probably don't need to spend. Subtracting that $500 out of your matrix puts it closer in net-cash to the lower-end gas tanks, but you get a LOT more first-hour gallons out of it. (Given current hourly rates of divorce attorneys it might pay for itself in the first month by never running out of hot water just before SHE steps in! ;-) )

[justinae]

Good stuff Dana. Some of those costs are estimates, others are based on my contractor pricing. In terms of use we do have the guilty pleasure of long showers, though we've agreed to curb it. Other than that we aren't huge users. We've lived with a regular 50 gallon electric heater for quite a while and never had issues with running out or anything so I think we're fine there.

I think you might be right about the GDHE pricing. I'll have to call my supplier back and confirm.

I should have a decision in the next few days. I'll be running the pipes tomorrow and will need to get something connected.

The other concern with gas is that the heater will be installed in a 2' x 2' closet. I called one manufacturer today and asked if that size closet with upper and lower louvers would be enough for combustion air and he said yes.

[Dana]

If you made out OK with a 50 gallon electric you'd be in HOG HEAVEN with the small-burner Vertex, (or even a bare-bones 40 gallon gas-fired tank.)

Also in-re the Airgenerate, I had meant to say "...they DON'T show the extra natural gas burned in your furnace..." (self-editing isn't my strong suit, eh? :-) )

[justinae]

Yeah my thoughts exactly with the 40 gallon.

I thought that's what you meant with the heat pump.

I found a few tanked heaters that will work with the small closet application. I'm definitely going with a direct vent or power vent though. It's so much money, but I just came home from a workshop regarding building systems and I don't want to take any more chances of backdraft than I have to. I suppose the safest bet is with an electric, but I think a direct vent will be safe. Reliance makes a skinny 21" that I can get for $1100.

I'll update my matrix and post.

[wassermeister]

justinae,

Retail pricing for the AirGenerate AirTap ATI50 $1,699 and ATI66 $1,999.

It should be 10-20% less expensive if you can get it through a distributor or contractor.

The Energy Trust of Oregon is now offering a $500 incentive, there is a $400 Oregon residential tax credit, and there is a $1000 rebate from the Northwest Energy Efficiency Alliance.

Thanks.