Outdoor Wood furnace to tankless water heater?

I did a lot of net research on those instant hot water heaters for a long time. Lots of hype, pitches and promises all over the place. Finally I got onto a professional plumber website where they really got into the in's and outs of these modern wonders. It seems that in a nutshell if you live in the place long enough it will probably eventually pay you back. The problem is that the number of years they came up with was well beyond 10 and if memory serves it was actually somewhere way beyond that. There were also serious problems with Calcium and hard water in general burning out or plugging expensive things inside the units themselves. The icing on the cake is location. If you live where its cold you need a Big Motha unit to heat the water the higher number of degrees between groundwater temp and useable. Again its no big deal if you live in the South but gets expensive quickly when you have to start running heavy dedicated circuits, heavy copper wire and adding electrical boxes in an older house to power that heater. At the end of it all the calculations came to a small savings over a well insulated standard type of tank heater like you probably have now. Sorry I can't shoot you the url for all of this but its out there somewhere if you look hard enough. They were mainly referring to Ohio, possibly Cincinatti ?? and the central US for most of their discussions. Possibly that will help you zero in if you go looking for that bad news.
I will say that you have to hunt out the "bad news " as it's so covered over by hype and promises. You can probably do much the same by hooking a preheat tank inline with your existing electric or whatever and get more bang for the $$$$. Gotta move now before the Greenies get my range with their rotten eggs & cabbage...................

Unless you're using an hx, you also need to fill the boiler system with potable water. Most boiler systems will not do well on this due to corrosion issues, pH and O2 in the water.

I've used only a sink supply tankless electric hot water heater and was not happy with the results. Could not heat well water at 50 to hot enough to do dishes at a flow rate much more than a dribble out of the faucet. And on and off and on hot water was a disaster, as initial hot water "over-heated" when water turned off, shut down unit, and then cold water when next turned on hot water faucet, until unit reset, then moderate hot water, etc. etc. Use it about a month and then took it out.

Whole house heaters may be better.

My turn (since I'm a "greenie" )!

Just for the record, I have a Rinnai "whole-house" unit, and I love it. Installing them isn't as easy as just swapping out your tank, and it takes a little thought, but when done right, most people like them. It's saving me about $15-20/month over my old tank heater (so a 6-7 year payback, since it cost be about 1200 to purchase/install by myself, after credits and rebates). Granted, it's somewhat of a long payback period, but it adds value to the houe (at least in my area, Ithaca), and I personally pay attention to the "externality" costs beyond the financial aspects of energy production. By the way, Bosch tankless units are a little hit or miss...its well worth investing in a Rinnai or Tagaki, or the other higher end units.

HOWEVER, your thread wasn't so much about pros-cons of the types, but how to make them work in conjuction with wood heat, which is a great topic, and could easily be applicable to ANY type of water heater, so I'm going to try to bring the thread back to that topic. One reason tankless heaters work well for this, is that they are supposed to be plumbed in with 3-way valves at the cold water and hot water connections (for flushing out deposits). These 3 way ball valves make it incredibly easy for DIY'ers to plumb in alternative heat sources. I'm looking into buildling a homemade solar heater to preheat my DHW, but may just use a length of black hose or plastic pipe for now during the summer time.

Regarding the comment about not workign well in really cold climates...that's mostly a smoke screen being blown by those who don't like the units. If someone drills the well and trenches in the pipes to your house correctly, your water temp should stay pretty close to the below-frost line ground temp (regulated by "geothermal" effect of the earth). The gas (propane or NG) on-demand heaters have no problem putting out enough hot water with 50 to 60 degree groundwater temps, and this is both from personal experience and from other owners I've talked to. Some of the manufactures make units designed to sit on an outside wall (outside the house), and those really should be used in the warmer climates...some people mistakenly install them in cold areas (no venting issues being the primary reason), and occasionally have some issues.

Here is a start of a list of items to research and incorporate into whatever your design is. If others can add items to flesh out the list, please do so.

By the way, Mother Earth News has covered these in various issues of the magazine, and is also a great resource (www.motherearthnews.com).

1. Is there a max temp for the cold water supply for your tankless heater (or any other heater)?
2. Make sure there's a temp/pressure relief valve prior to the heater, piped to an appropriate drain/location (there should also be one on the outlet of the heater already).
3. Make sure you're running the water only through things appropriate for potable water, like jebatty said (e.g. nothing that's been used before with glycol/antifreeze).
4. Some people using solar preheaters incorporate fittings to blow out the system in winter (using a compressor). If you're planning on using an outdoor boiler, you won't be doing that, but you'll need to make sure your pipes won't freeze (espcially if you want to travel at all during the winter).

What else, guys?

brentlee said:

Well...I'm not sure.

The wood furnace I am looking at has it's own domestic water line. Meaning no water to water exchanger. The domestic water could be ran up throught the furnace and then down to the house into the "cold side" of the tankless water heater.

But since the furnace is 150 feet away from the house the water in the line will cool. When I call for hot water the tankless would fire heating the 150 ft of water up to desired temprature and then shut off when the heated water from the furnace reaches it. I would assume that the furnace would be able to continue delivering heated water from that point out.

The other line of the furnace goes to a water to air exchanger in the blower of the house.

One closed system...one open system.

Am I missunderstanding the way this could work?

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That would work fine for the DHW, especially since it lowers the likelihood of getting superheated water into the supply of the tankless unit.

However, how will the furnace heat your house? A seperate loop hooked to a heat exchanger in a forced air furnace?

M1sterM said:

Regarding the comment about not workign well in really cold climates...that's mostly a smoke screen being blown by those who don't like the units.

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It's more related to certain manufacturers who rate their units based upon much higher inlet water temperatures than are realistic outside warm climates like Florida.

If the unit is rated for a proper temperature rise (eg, 50-120), there won't be a problem. If someone buys a unit that says it can supply X gallons per minute, but doesn't notice that the manufacturer rated it that way at a 50-degree rise instead of a 70-degree rise, then they will be unhappy, and go tell all their friends how horrible the tankless water heater is. Or, if they find out why they are having that problem, they will tell all their friends that the manufacturers and those who sell them are a bunch of crooks.

A few manufacturers make it hard for legitimate technology to gain ground. Same goes with solar, which is often rated based upon unrealistic conditions.

Joe

brentlee said:

I am planning to use a Bosch 1600ps. I now allot of people have had problems with the Bosch's but for the price I can't seem to find any other tankless that will except pre-heated water.

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I was thinking about using two of these models - one downstairs and one upstairs in order to get them as close to usage point as possible and allow me to run two appliances at the same time.

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For about the same price of two Bosch 1600PS's, you could buy one Noritz 0631 or 0751, which would give you a much better unit, and more capacity.

Joe