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[MScott]

     First, I'll tell you what system I have. I use a Central Boiler CL 4030 OWF to heat a 1500 sq. ft. well insulated house and a 1000 sq. ft. attached garage. The garage is on a separate infloor system using glycol heated through a heat exchanger. In addition, I am entirely off grid solar with a 3 cyl. Perkins diesel powered generator backup that has its cooling system piped into the OWF system. The main system uses water only.
     On the whole, the system is working well even though this model OWF is probably considered marginal in size. My main concern is that I would like a backup system in case I decide to travel during the winter months and to ensure that the system does not freeze if I happen not to be available to keep the furnace full of wood.
     My thought is to add an inline water heater or boiler (size depending on whether I want it just to avoid freezing or to be a complete heating system if I get too old and feeble to handle the wood furnace) to heat the water at the point before it returns to the OWF. At present I am looking at Tagaki or Rheems models of tankless water heaters. My concern is the size of the inlets to these heaters and whether it is feasible to have the water flowing through them at all times but only be heated when the temperature drops to a pre-set level (ie. 140F if heating or 50F if only freeze protection.) Would there be too much resistance to the flow? I know some of the less expensive tankless water heaters are fired by water flow rather than temperature so I could not use this type of heater unless there was a temperature activated bypass valve and the heater was in parallel with the main water line.
     Does anyone have any thoughts or suggestions on this, or has someone already done this? I would appreciate any help or advice.

[slimjim]

It is done all the time, first you need to decide rather you want freeze protection or heat!

[juddspaintballs]

I think you already unknowingly described that you want the backup system to heat rather than be freeze protection when you said "if I get too old and feeble to handle the wood furnace." 

It sounds like your house and garage are currently heated entirely with water.  If that's so, do you already have some type of backup boiler or did you design your heat system 100% around the OWB?  If you don't have a backup heat source, I'd strongly recommend installing a propane/gas or fuel oil boiler capable of heating your entire house and garage.  You can keep it turned way down if all you want is freeze protection, but it's there in case you want or need to heat the whole house with it. 

[mlappin]

On the other hand if you just want freeze protection, maybe a tankless water heater with an aqua stat set to only run the water heater when water temp drops so far.

If you want actual heat out of it, then you need an actual boiler like judds has already suggested.

My father is nearing 70 and still cuts wood, albeit smaller stuff now.

[MScott]

slimjim: You are probably right, I need to decide. At this stage I'm just exploring the possibilities. Cost is an important consideration in my decision.

juddspaintballs: My house and garage are heated entirely with the OWF although the garage has glycol in the floor system and, at this stage I do not have a backup although there is a propane furnace (forced air and not used.)

mlappin: Freeze protection is the most important at this point although the actual boiler is a definite option if it is something that I can afford. Incidentally, I'm 74 years old and still cut, split and haul my own wood (and not just small stuff.)

I guess what I'm looking for is some advice on what type of heater/boiler others are using and how they are hooking it up. mlappin mentioned an aqua-stat. I have no idea how these are used. Are they just a temperature controlled switch to turn on the heater or are they used to control the flow of water. I'm quite new to this type of heating so bear with me.
If there is a thread that explains how a heater has been added to an OWF system I have been unable to find it although I did try searching.

[mlappin]

mlappin: Freeze protection is the most important at this point although the actual boiler is a definite option if it is something that I can afford. Incidentally, I'm 74 years old and still cut, split and haul my own wood (and not just small stuff.)

I guess what I'm looking for is some advice on what type of heater/boiler others are using and how they are hooking it up. mlappin mentioned an aqua-stat. I have no idea how these are used. Are they just a temperature controlled switch to turn on the heater or are they used to control the flow of water. I'm quite new to this type of heating so bear with me.
If there is a thread that explains how a heater has been added to an OWF system I have been unable to find it although I did try searching.

Dad was cutting a green black cherry that blew down in last weeks storm, cut the 18” diameter pieces only 16” long so he is slowing down a little

An aqua stat is a mechanical thermostat that you could strap right to one of your boiler lines and if the temperature got low enough to the set point  it would trip and run the back up heart source until the water temp rises enough to trip it back out.

[MScott]

Thank you. I have a lot to learn. The last tree I cut down this year was an old sugar maple (former sap tree well past its best before date) that was about 36" through the but. I had to quarter the bottom 8 feet to be able to lift the blocks into my ATV trailer, then split them up with my hydraulic splitter and piled them. I love doing it but I realize that I'm not going to be able to do it forever. I'm looking at a boiler for backup.

[slimjim]

OK now we need to think about how the system all gets tied in together, there are many options out there for plumbing, we would need some pictures or a drawing of your current setup.
Do you have any idea what you want to use for fuel for the add on boiler?

Propane is cheap, relatively easy to install BUT you are locked into buying from the company that installs the tank, Gas wall hung units will be the most efficient but require Primary Secondary plumbing

Oil fired is typically the home owners choice here in Maine as we don't have much Natural Gas, in most cases the oil boiler can be simply added to your existing plumbing and you have the ability to shop for the best price on fuel.

I hope I'm not confusing you but there are lots of choices and you should look at all of them!
Where do you live?

[MScott]

I don't know how to do drawings or post pictures, but there are two 1" pex lines to and from the OWF which travel along the ceiling of the basement next to the wall. They end at a manifold that disperses (through shutoffs) to a fanned heat exchanger for the basement (controlled by thermostat); a heat exchanger in the furnace plenum (not currently used); a heat exchanger on the propane hot water tank; the heat exchanger for the garage floor system and tied into the diesel generators cooling system. My idea is to hook the new boiler into the line just after the manifold (ie between the manifold and the OWF.)
I would be using propane as I already have the house plumbed for it and it is used for hot water (summer only), stove, fireplace and clothes dryer. You mention secondary plumbing. Does that mean a parallel line that only diverts water to the heater when it is called for? Is this required for a heating boiler or just for a tankless water heater? I am actually looking at a used dedicated boiler that might work and is within my budget.

As I have noted below, I live in Eastern Ontario, Canada. Thanks for your response.

[juddspaintballs]

If you have a propane furnace already and your boiler is plumbed into the duct work with a coil, why don't you use your propane furnace as your backup heat source for freeze protection? 

[MScott]

As I mentioned earlier, I am off grid solar and have to somewhat monitor my electrical energy usage. I found that, for some reason, the propane furnace was a real energy hog so I have turned it off. I have found that using the OWF uses much less electricity and my only problem has been that I have to physically tend it to keep the system warm. If the boiler works as a backup/replacement heat system I will probably just replace the furnace with a small fan to circulate the heat through the heat exchanger and my ductwork.

[AirForcePOL]

I have thought about doing this as well.  If I did it, it would be set up as a complete backup.  I sometimes have to travel for work and I hate to leave the wife with the responsibility of keeping the stove loaded. I would rather pay for propane for a week than put the burden on her.  I keep an eye out for used propane boilers on Craigslist in case I find a good deal.  They can be had for around $500-$1000. I have a propane tank for the garage heater so I could hook it up to that without any problems.  My boiler is my only source of heat in the house though, so it would give me peace of mind. 

[MScott]

I'd like to thank everyone for the input and for getting my mind straight on what I actually need. I looks like I have located a used boiler on Kijiji that is within my budget. It's a Triangle Tube Prestige Solo and I will be going to pick it up tomorrow. I have to have my heating contractor order a conversion kit for NG to Propane and hook it up after I get the water and exhaust completed. Apparently the company will only sell the kit through a registered installer. I think this is a much better option than trying to use a tankless water heater and should solve my problem.