Need help with an externally mounted domestic coils system.

Ok guys I have an oil steam boiler which heats my domestic hot water. I would like to use my alaska channing to help heat the water. I would really like to mount a coil in the air space in the back of the stove as I really don't care for the idea of drilling holes in the stove. I don't use the convection fan on the stove so this area gets pretty hot. I have the capabilities to bend either a custom copper or stainless coil. I would like to hook the coil up to the boiler directly, and circulate the water through the coil to the boiler with a taco 006 that I have. How big/ long should I make the coil to keep the boiler hot but yet not be blowing off the pressure relief valve constantly? Do I have to have a tempering tank( ie. 40-50 gal. water heater) to make this system work.

Ok I guess a little more explanation is necessary because my last post was as clear as mud . What I want to try is mount a coil in my stove's air space in the rear of the stove. I then want to put a tee in the drain of my steam boiler's gray water or non oxygenated water. Then I want to add a taco 006 or 007 after the tee and push the water up to the coil in the stove and then back to the extra top port of the boiler via a swing check valve. This is a very unusual set up and I need some input from everyone on whether this is a workable and mostly a safe setup. My hope is to be able to keep the boiler warm enough to keep it from running when hot water is not being used like in the middle of the night.

I think you will get some heat from the coil mounted externally behind the stove, but if the water is circulating I doubt if you will ever create steam in the coil if the only heat source is the stove. It is pretty hard to create steam in a coil even when the coil is mounted inside the stove. [providing water is circulating]. If I remember correctly from posts on the forum, only after a night of high stove temps.

You would be keeping the water in the steam boiler warmer than without the coil, but it will depend on many variables whether it will be enough heat to keep the water hot enough to prevent the oil burner from running.

The big question is what happens when the water in the oil boiler is heated to create steam? If the water is under pressure, and just below boiling temperature, it could possibly flash to steam in the coil, and what this would do to the water trapped between the coil and the return to the oil boiler is anyone's guess.

I'm inclined to recommend putting in a tempering tank prior to the hot water coil in the steam boiler, and let the coil in the stove preheat this water. It will greatly reduce the oil heat needed to heat the preheated water to a comfortable domestic water temperature. This would keep the steam system and water system separate.

I'd be cautious mixing the steam system with a water coil, hot water is dangerous enough, but steam can develope some high temps and pressures.

Hopefully someone with experience in steam systems will offer their expertice.

Hope this helps, Caution and safety first. Recovering from burns, even minor ones is not something to risk.

Greg L
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You may actually wind up pulling heat from your steam boiler.
I read this a few times and I am not sure what your up to. But whatever it is, do not connect a stove coil to the pressure vessel of the steam boiler.
Can we get a drawing?

Greg,

You are right if I were to do it the way I proposed I would have to Wire up the oil boiler to only run to heat the hot water to temp while the coil was in operation. I think a tempering tank is a good idea if I can get enough heat out of the exterior coil to heat a forty gallon tank plus the six or seven gallons in the coil of the boiler . Maybee a smaller tank is in order here any thought would be great. I think the un jacketed portion of my stove in the rear and top runs between 300* and 400* on average . I'll try and give a drawing or picture tommorow.

There is a book called "Handmade Hot Water Systems" by Art Sussman and Richard Frazier; ISBN # 0-932708-00-5 Library of Congress Catalog Card No.: 78-71915 . My copy is from 1978 but my small local library had it so it should be pretty easy to find, it has methods for making a hot water coil to fit in the chimney pipe of a wood stove among other things. I'm sure nothing is up to code but it's an interesting book.

Here's a diagram of my proposed water coil setup.

What are the setpoints on your steam boilers control and what pressure does it operate at?
Do you have a tankless coil or the pump and tank in the drawing are part of the current system?
Do you need the relief at the stove with one at the tank?

The boiler operates at less than 5 pounds steam pressure. The system I am proposing would run through my current tankless coil in the boiler. I currently have no pump or tank in the hotwater system. As for the presure relief valve that is a good question. In most coil in stove setups I have seen they have a pressure relief at the stove as well, but I am not sure if it is absolutely neccesary. Maybe LsFarm will have some input on that. I currently have one on the tankless coil that is in my boiler.

If that is a check valve where you line goes back to the boiler's coil feed, your drawing is fine. If you don't use the PRV at the stove you should have an air vent. Either should be at the highest point in the system.
I still have a problem with this, I would like to know what the boilers on and off temperature settings are. I have a feeling the only thing you will get is a big tank of hot water out of this.

The boiler's aquastat for the hot water coil is set at 140 off and 120 on . The whole reason behind the system is to keep the running of the boiler to a minimum. I have only used the steam boiler to heat the domestic hot water for the past five years. It has never come on to run through a heat cycle except for the once yearly fall test run. Since it has a tankless coil I figure I need at least a small tempering tank to keep the PRV from blowing after hours of non use. I' dont have to hook up the coil to the boiler I could just run it to an electric hot water heater and turn off the furnace. I didn't do so because I felt if I kept the boiler warm it wouldn't rust .

120* on and 140*, the deal sounds much better now. The trick is going to be to produce water temps at about 130*+ or so at the stoves coil. You may be able to do that by controlling the GPM through the coil.

I have two things here to think about.
One, your boiler will still fire when you use water as your tankless coil will see the cold feedwater first and it will quickly draw down the boilers temperatures. Think about feeding the replacment water to your stove coil first and then to your supply system and boilers coil. That may put your boiler into retirement so to speak if you have a large supply tank. You may want to run the water even hotter if possible and use a tempering valve. This would eliminate any chance of boiler firing for hot water use.
Two, the big problem I have with this is I don't think the coil outside the stoves firebox will put enough heat into the coil. It may, but I am not sure the exposed coil can transfer that much heat. If it doesn't you will need to enclose the coil either in a tank of water against the stoves surface or surround it with a matierial that conducts heat from the stove to the coil. By putting the coil outside, only about 30% of it's diameter will be exposed to the stoves high temeratures. More coils could help this process.
I would try it with the coil as you described, if it doesn't work real good you can add what I described in #2. I would put the coil on the top of the stove if possible and if there is enough area.

I thought about the surface area problem and I think I have a solution. I have 2.5" of thickness I can have. I can build a stainless box to enclose the coil. I will then fill the box with sand as this should help hold the heat in once it gets hot. I have seen one of these years ago on the top of an old wood burner. The top of the stove lifts off but getting the box into the airspace may require some modification of the stove. I gues I've just descibed a primitive water jacket .

That's it.

jpen1 wrote: I gues I've just descibed a primitive water jacket .

Sure beats a staight-jacket.

What do you think of changing the flow?

I think changing the flow would help with short cycling of the boiler. I'll have to map out the piping a little better and revise the drawing. I'm still not sure how big a hot water tank to go with. I think that will be the hardest part because I see the size as a balncing point of effect vs. non effective. Too big a tank and the boiler will run because the coil can't keep it warm enough. Too small a tank it will probably blow off the PRV. I think I might be able to mount the coil on top in the box. I should be able to get 4-5 feet of copper in the box without having to modify the stove.