Outdoor Wood furnace to tankless water heater?

[brentlee]

The Bosch 1600ps is made to except preheated water from solar or wood furnaces.

"This water heater is specifically designed as a backup to preheated water produced by solar-based systems or from woodstoves. Its tankless design saves floor space and will save you money, as it only operates when your preheated water needs a boost in temperature."

Rehabing a 1918 farmhouse and wanting to use an outdoor wood furnace and thinking of the posibility of using in conjunction with a tankless so that in the summers i can not fire my furnace but stay eco and economic without having to "change over" anything.

I understand the basic concepts of a traditional DHW system with wood heat DHW line but how would a system work with the tankless and at what costs or advantages.

Also my furnace would need to be approx. 150 feet away from the house and the tankless could be fairly close to the shower ect.

Any input would be appreciated as there is not much info on the net about this setup.

 

[drizler]

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...................

 

[jebatty]

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.

 

[M1sterM]

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]

Thanks for the input. I have done allot of research and have read the definate pros and cons on tankless. I think that using with my outdoor wood furnace would give me the best of both worlds and allow me to make easy changes between systems in seasons and when I am away.

Also with a 150 foot run the and the tankless close to it's usage point I could get almost instant hot water and a continued supply of hot water but use actually very little gas. Relying mainly on the wood furnaces heat except for the intial hot water.


I will research more into the issues that were mentioned but I guess my remaining questions are would i have to put a circulator on the DHW line of the furnace or any sort of storage tank or could I just run the DHW line straigth from the furnace into the house to the tankless and what kind of results could be expected from this concept (assuming everything functions correctly).

For those apposed to the tankless....doesn't it make more sense in my case to have a hot water heater that only (and instantiously) runs when the furnace isn't delivering the desired heat than to have to use energy to contiinue to keep the water that the furnace arleady heated warm?

 

[brentlee]

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?

 

[M1sterM]

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?

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?

 

[brentlee]

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?

Yes that is correct.

The wood furnace I am planning on using actually has three differnt lines. Two closed and one open. One of the closed will run into a water to air exchanger in my forced air furnace and the open line is for DHW.

If I am going to be gone or want heat (or hot water) but don't want to fire my wood furnace I could use the gas on my forced air furnace and use just the gas tankless if hot water is needed.

Basically the wood would be my primary source of heat and DHW but the gas furnace in the forced air unit and tankless gas water heater would be back-up.

But everyone I talked to about using a tankless with a wood furnace looks at me like I am speaking a different language.

 

[BrownianHeatingTech]

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.

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]

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.

I will be heating Missouri well water...which I think should be about 50 degrees (53 is the average air temprature in my area) out of the well and shouldn't give the Bosch any problems.

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.

 

[BrownianHeatingTech]

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.

...

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.

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