SF-250 Hot Water Coil

SF-250 hot water coil

PostBy: Cap On: Tue Aug 08, 2006 10:08 pm

Hello Folks--

I talked about modifying my Harman SF-250 with a home made ss hot water coil last winter. I started on it a few months ago. See attched images.

I used 3/4 od x.065 wall 304 ss tube. I bent the tube with a bender in a vice. I have one hp Parker SS coupling holding the two sections of tubing together. The coil was too large to fit thru the door thus I had to cut it into 2 sections. Later in the summer I will tie the ss tube which is penetrated thru the firebox into my 80 gal hot water heater. The hot water heater is less than 10' to the left of the stove. More than likely I will relocate the valves further downstream so the teflon seats do not melt. The valves are ss Whitey valves made by Sweg-Lok. ( Extra stock laying around in the shop ). I plan on using a few compression fittings on the outside parts of the water circuit so I may take it apart in summer so not to jam up my corner where the well water system is placed. Hopefully this plan works out without the use of a pump. I'll post follow up images once it is completed.

The 4 bolt flange is the probe for the firebox thermeter in which I installed last winter.
Coal hot water #2.jpg
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The in/out ports. I can relocate the valves a few feet away to prevent overheating of the teflon seats.
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The ss coil. I welded a steel angle iron bracket to hold it in place.
Hand Fed Coal Boiler: Harman SF 250, domestic hot water loop, heat accumulator

PostBy: mswartz On: Thu Aug 17, 2006 12:10 pm

I was thinking of doing something similar. Have you considered the need for a pressure relief valve near the heated tube to help avoid a steam explosion?
Stove/Furnace Make: Standard Oak

PostBy: George-NJ On: Sat Sep 16, 2006 3:51 pm


Great Idea, I had wanted to do this last year with my hand fired unit, but didn't have the time to fool with it.

This year I got a stoker to do this to. I just ordered a 20' length of the 304 - 3/4" x .065 for my project. The stuff is very exspensive, but I figure worth it. I plan on making as big of a spring like coil that will fit through the door (which is pretty big on mine). I'm gonna hook it up to my boiler with a taco pump that will run all the time.

I'll do the pressure saftey blow off valve, then in & out the boiler chamber with the pump inline. The stove heats where we hang out very toasty with it's convection, I figure that with the coil I will be getting the further off rooms a bit more warm, not that they're too cold with out it. I'm also counting on this setup to heat my domestic hot water that runs off my boiler.

I'll have about 12-15 feet coiled up in the stove, I'm thinking that the water coming into my loop is already more than warm, with the heat inside the fire box I should be able to get 180 + water out of that loop and back to the boiler. The oil burner should rarely kick in.

Question for you is, how easy or hard is it to bend the ss? Do you think I could use an electricians emt bender? I've never worked with ss before, but have done alot of copper & emt work.

PostBy: Cap On: Sun Sep 17, 2006 7:24 pm


304 SS x .065 is very difficult to bend and I do not believe it is possible to bend using a conduit bender. Conduit benders are used on 1/2" rigid conduit & 3/4" emt. Conduit is measured in pipe, not tube. You are using ss tubing which measures outside diameter while pipe measures inside diameter. The 1/2" pipe is very close to the 3/4" tube, it's not exact therefore you will require a tube bender. A tubing bender mounted in a vice on a steel work bench works well. It's an extremely heavy duty tool. It took me alot of strength and a long cheater bar to bend the .065. Also, keep in mind, the radius of your bends will be quite large limiting the amount of loops you will fit in the fire box.
Hand Fed Coal Boiler: Harman SF 250, domestic hot water loop, heat accumulator

PostBy: Cap On: Sun Sep 17, 2006 7:38 pm

I finished the water connections today and will post images once I have the relief valve and bleeder valve in place. I continued using ss 3/4" tube x .035 for the lines outside of the fire box. ( used materials pulled off of a liquid hydrogen system! )

My water heater only has one cold water inlet and 2 hot water outlets one of which is occupied by the 150 psi releif valve. No matter how many times I drew it up, I couldn't find a way to feed cold water from the tank, to the fire box coil and back to the top or hot side of the tank (mechanically it is easy). The water would take the*path of least resistance* & feed directly from the well tanks and deadhead at the top of the water tank unless someone was running the hot water. Right?

Therefore, I ran a tee off of the well tank water line directly to the fire box coil and back to the cold water inlet of the the water heater. I have isolation valves so I can shut off the normally open water inlet and open the isolation valve to the fire box coil.

I'll be testing as soon as the temps drop below 50F. and report back. I'll also post some images soon.
Hand Fed Coal Boiler: Harman SF 250, domestic hot water loop, heat accumulator

PostBy: George-NJ On: Mon Sep 18, 2006 10:07 am

Hi Cap,

Thanks for the quick reply & info.

Can you post a pic of the bender that your using? This way I'll have a better Idea of what to look for.

Also; Where would I find one, plumbing supply or a pro tool shop?

I just got home from picking the 20' ss, I was surprised how flexible it was. I had it threaded & bowed through my Navigator's back side vent window, through the cabin & out the front pass side window. So I was thinking that it would be an easier bend...till I just read what you wrote. Oh well, nothings easy...


PostBy: Cap On: Mon Sep 18, 2006 4:17 pm


I borrowed the tool from our truck maintenence shop. It is a kit that bends 1/2" thru 1" maybe more. I think it had 5 sets of changerable componets. I would suggest finding a rental tool location to see if you can rent one for the day. Maybe the tube supply house can point you in the right direction.

The tubing should not have bent easily in your SUV. Are you sure it is .065 wall? yes, it will bow, but when you attempt a 180 degree bend, it can kink without the proper tool. By the way, 316 ss is easier to bend than is 304. But the cost is higher!
Hand Fed Coal Boiler: Harman SF 250, domestic hot water loop, heat accumulator

PostBy: George-NJ On: Tue Sep 19, 2006 6:47 pm

Hi Cap,

Yes, it's .065. They gave me a bunch of paper work certifying it's make up & size (like it meant anything to me). BTW it cost me $125 for the 20 feet. I bought an extra 20' for my friend who has the same stove I have.

I hope I can find a hydraulic bender for rent...

My coil loops I think will be kind of squarish 14"x12", 3 high so the bends will be four 90's. Sounds easy in my mind anyway...

This should be an interesting project.

PostBy: George-NJ On: Fri Sep 22, 2006 11:22 am

Hi Cap,

I had a hard time locating the proper bender, I ended up ordering one (not the right one, it's for copper $125, the right one was $375) but am going to kill the order. I dug out my 1/2" emt bender and thought that I'd give bending the ss a try, I have enough pipe to spare. The emt bender did it pretty easy. There wasn't even a hint of kink or any other problem.

I just did a 90, that's all I need being that my loops will be squared off and not 180's. So I'm a very happy man now.

Do you know if you can solder copper to the SS, or would compression unions be enough, or thread the SS?
Last edited by George-NJ on Fri Sep 22, 2006 6:32 pm, edited 1 time in total.

PostBy: George-NJ On: Fri Sep 22, 2006 6:31 pm

Got it bent up this afternoon, no kinks or distortions.
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SS COIL 1.jpg
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PostBy: George-NJ On: Fri Sep 22, 2006 6:37 pm

It doesn't look like much, but there is about 17' of SS tube that will be cooking inside the stove, should make for lots of heat.

PostBy: Cap On: Fri Sep 22, 2006 8:17 pm

Thant's a nice job George. I'm impressed the EMT bender worked for you. Yes you can solder SS to copper but you will have to use silver braze filler material. But I do not believe copper fittings which are measure out at 3/4 od are available. 3/4 od is caught in between 5/8" and 7/8" ( 1/2" & 3/4" nominal ). I have always used compression fittings such as Parker, Hoke, Sweg-Lok and there are other knock offs available. Try checking with an industrial pipe & tube supply house.

I wrapped up my water ssytem today. I'll post images later this w/e.
Hand Fed Coal Boiler: Harman SF 250, domestic hot water loop, heat accumulator

PostBy: Cap On: Tue Sep 26, 2006 9:13 pm

I finished running the tubing for my hot water loop over this past w/e. I am now able to heat my domestic 80 gallon hot water tank using the Harman SF-250. If anything, the water is too hot . I haven't tested it using a really hot coal fire yet but I did get the outlet water temps up near 200F with a wood fire. We'll see later in the season when temps fall into the 30's. I have attached images below. Currently I have a small wood fire burning. The temperature on the outlet tubing feeding the tank is at 140F. I can hold the inlet as it cool to the touch.
Temp Gauge.jpg
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Shows outlet water temp, screwed into a well.
Relief Valve.jpg
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Generant brass relief valve and bleed valve. Bleed valve really not necessary.
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The blue tanks is well water piped in parallel. The ss lines is heating the hot water tank.
Hand Fed Coal Boiler: Harman SF 250, domestic hot water loop, heat accumulator

PostBy: George-NJ On: Wed Sep 27, 2006 2:00 pm

Very cool! Good idea to have all the gauges. Can't wait to get mine going.

Larger Tank

PostBy: endinmaine On: Mon Oct 02, 2006 3:44 pm


You mentioned that your hot water tank is 80 gallons and if anything the water is too hot near 200*.
Do you plan on doing anything if the water stays too hot like going to a larger tank for a 2nd tank ?
When I move next year and setup my system using my Harman III I want to have all my "ducks in a row".

Hand Fed Coal Stove: Margin Gem Cook Stove and Harman Mark III
Stove/Furnace Make: Harman and Margin Gem
Stove/Furnace Model: Mark III and CookStove