Custom Made Coil Need Advice

Hi,

I'd like to hear opinions on how this setup would work. I'm trying to make hot water to heat a basement with baseboard heat and some domestic hot water. I know this is ambitous and a coal boiler is not doable. I'm trying to work something out with my Channing 3 stove. I never run the stove beyond half tilt and have plenty of headroom to work with and with oil at 2.50 a gallon... Even, If I could just generate enough heat for the basement I would be happy. Ideally I woud like the stove to help out so my boiler and hot water heater don't have to come on that much. I have also uploaded pics of the setup I'm proposing and a drawing of the coil. In the drawing I haven't added the temp/pressure relif valves, shutoffs or bleeders so keep that in mind.

The engineer from the coil company suggested a large spring like coil so there are no bends to restrict flow. The coil looks like a rolled up garden hose. He suggested 3/4" SS, 12 1/4" x 12 1/4" with 16 turns so the height of the coil, looking at the side, would be around 12 3/4". This is a tremendous amount of surface area and should produce lots of hot water. Here are my concerns:

1. The coil co. only has 316 SS with .065 wall. in stock. Is this the right material to handle the high temps of the stove?The coil will sit 6-7" on top of the grate and I'm afraid that the temps in the stove will get to hot for the coil. I'm still researching this and have a call in with an engineer who makes the S.S. product.

2. - I could always make the coil a little smaller and then weld a plate to sit between the coil and the fire with air vents on each side. I'm afraid this would cause combustion problems making the fire box smaller?

3. Should I bother with using coils in the boiler or just get a super store tank. My boiler is old and not the most effecient so I was thinking it would make sense to use boiler coils to help out the existing hot water heater and that the H.W. heater would still have a head to make its own heat and to help with quicker heat recovery.

I would have to change out the coils in the boiler since they haven't been used in 14 years and didn't work that well when we moved into the house. The boiler is about 30 years but should be able to handle any load I throw at it since it's only heating the basement while the stove does the upper 2 floors.

Thanks in advance

Trader ,

I don't think you have to worry about that 16 ga wall tubing bursting on you . I have designed many high pressure steam coils (40 - 100 psi) using the same tubing. The stainless won't break down until you reach about 2200* F. You want 316L for its corrosion resistant properties. The spring coil idea will work but I think your going to have to put space inbetween your loops. Stainless steel expands when it gets hot also I have been involved with the bending of stainless tubing for 10 yrs and you will never get those coils to want to stay that tight togeter without compressing them with a hydrauilc press and welding some sort of brace on them to keep them tight. I think you want to be about 3/4" to 1" apart on the loops. By doing that you will get heat 360* around the pipe and won't obstruct the flue gases. Is your stove a top vent or bottom vent. I have bottom vent channing III.

2200 is pretty hot. It sounds like you have a lot of experience. Do you have an idea the temps generated 6-7" off the grate at 3/4 tilt for a Channing 3.

I have a bottom vent by the way with a direct vent hookup. I see what your saying about restricting the flow gases. hmmmm. What if I design the coil so it sits with the opening in the "spring" facing the grate? This would allow me to retain the extra surface area (by not introducing spaces) and would also allow the exhaust gases to flow freely? I'm glad you pointed this out. I'm not sure they can fabricate this coil with spaces anyway. Expansion and retraction is a concern. The stove will never see major temp changes so I think that would be beneficial. If the turns are tight together could this pose a problem or would they simply push themselves apart without any damage? Any ideas on how to install a brace on this coil. I guess it would start to sag after a while but it still needs to stay flexible so the pipe can move. Do you think I'll get a decent amount of hot water to do what I want? Thanks again.

I agree that you need to have space between the coils. Let the hot combustion gasses flow all around the tubes, not just up the center of the coil.

The temperature of the flame just above the coal is around 1800*, but this is not a concern for the coil, as long as water is circulating through it.

In my boiler, with 1500* flames touching the water filled cross tubes, the tube walls have never gone above 350*, and the water inside the cross tube was 150*. So you should have similar results with your coil with circulating water.

I believe you want to have a storage tank for the coal heated water. This water will store heat all night long, while the coal stove is keeping the hous warm, then in the morning you will have a supply of preheated water for showers.

Greg L.

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LsFarm you wrote: "I agree that you need to have space between the coils. Let the hot combustion gasses flow all around the tubes, not just up the center of the coil. "

The coil co. suggested these specs. I went back to the drawing board and if I make the coil 8" wide with 1/4" spaces and about 22 turns I'll have about the same surface area (about 48') and it'll sit another 4" off the fire. This is a better design for sure. Thanks.

LsFarm you wrote: "I believe you want to have a storage tank for the coal heated water. This water will store heat all night long, while the coal stove is keeping the house warm, then in the morning you will have a supply of preheated water for showers. "


My goal is to heat the basement at night and to get some domestic hot water in the morning with my setup. In my drawing the water flows through the boiler and is constantly being circulated through the stove's coil. The excess heat would be applied to the coils that sit in the boiler for domestic hot water. I would use a Taco 003 to circulate the water between the hot water tank and the coils in the boiler so there is constant circulation and heat transfer. There is only 8 gallons of water in the boiler but I'd have the hot water tank's water circulating through the boiler and water would also be going to the baseboard heaters.

I'm not sure about how the storage tank you are suggesting would fit in with the whole picture?

My other idea is to run the water through the boiler and then through a Super Store tank and create a loop. This might be a better idea since the coils are shot inside my boiler and instead of putting money into a 30 year old boiler - I would then have an inderect (super store tank) heater that would hold the heat better and allow me to replace my older old Bock hot water heater at the same time. The only problem with that scenario is that I would be dependent on the stove and boiler to make all my domestic hot water and in the summer I would be forced to run my boiler hot enough to make domestic hot water. With the first setup I could shut my boiler down and the coil to the stove, in the summer, and just use my oil fired hot water heater which runs much more effeciently.

I'm also concerned that if the power goes out for the circuit that the pump is on of if the pump fails that the water in the stove's coil would get too hot. I guess the temp/pressure relief valve would just blow but if the water was to sizzled out of the pipe the pipe could fail from the heat. Any suggestions to back this up?

If you have a power outage, is your home water system a well system or a municipal supplied system?? If you have water pressure, if the PRV opens on the loop, the pressure drops, then the water supply will refill the tube with water, so you shouldn't have a melt down of your coil tubing.

I haven't looked real close at your diagram, but if your coil is making lots of hot water for the basement, then when the thermostat is satisfied, you will need to keep this water circulating. I'd want it's heat to be stored in a water tank somewhere.

There will be times when the Btus required for the basement are low, and the coil will still be pulling btus from the coal stove, this heat should be stored either in a tank or your boiler's water..

I would have the gaps between the coils at least 1/2", you want to be able to clean this gap, because it will get clogged with fly ash in the firebox, any semi-horizontal surface will collect ash. And a layer of ash is an insulator.

Greg L

I don't think the coil is in any trouble of overheating as long as you keep the water circulating. Your coil material is an excellent choice, I doubt it would fail unless it was in continuous contact with the burning coal for a long period of time. Don't worry about the expansion as it will be pretty even throughout the coil. It won't grow that much anyway. The location isn't real critical as its pretty hot anywhere inside the stoves firebox.

One thing I would do is check your boilers HW coil, especially if the legs are in and out at the top. Old coils, especially with well water, can be plugged with sediment or scale which would reduce their efficiency severely. There are some potent (flourosilic acid? leave this stuff to the pros) and other, more safer things available at the plumbing house to clean you HW coil.

I would think bigger is better on the tank size, 60 minimum and 80 is better (teenage girls? +).

I also would look into backup power, @ minimum for your stove coil pump if you are in a place like I am. Losing power in the winter for 2-3 day clips is no fun (well, now it is. but the neighbors hate you ). I prefer a whole house solution in this area, and reccomend 6,000 watts or better.

I'm glad everyone is in agreement that the coil will be fine above the fire. This was my biggest concern. My next concern is design. The coil in my boiler is shot. It hasn't been used in 14 years and didn't work so great back then. That is why I put in a oil fired Hot water tank and we never run out of hot water and before I bought my stove the boiler was happier too.

To change the coils in the boiler will cost about 500.00 while an indirect tank is about 900.00. I'm against an indirect because I don't want to have to run my boiler at high temps to make hot water in the summer. I would rather use the coils in the boiler and use the stove to keep that water hot enough to make domestic hot water.

LsFarm: I hear what you're saying about another tank. If the water is circulating to the basement and it's blowing the relief valve I could always turn the thermostat on the 2nd floor up to steal any excess heat since there is a huge difference between the 1st and 2nd floors of my house. The boiler can also sustain 190 degrees without blowing off.

Adding another tank would give me more reserve hot water for the system but it would also take more energy to heat this water. I could always add another tank if I see that I just have too much BTU's to displace. Even if the baseboard isn't calling for heat the water in the hot water tank will be circulating through the coil in my boiler which will also grad some heat. My other plan which is more costly would be to create another zone off the boiler and run the water from the stove's coil through the coil in the indirect hot water heater and through the boiler. The only thing I would gain is more surface area to dissipate heat form the indirect tank.

You may want to poke around and look at some of the heat storage systems they use in Europe with wood and coal to see how they are setup. Fuel cost so much there no matter what it is they always use very efficient methods.

I don't think your boilers HW coil needs replacing, and if it does, $500 is way too much.

I'll do a search and see what I can find about European heating systems. That is a good idea. The coil in my boiler is definetely not working properly. When we moved in to our house 14 years ago it barely made enough hot water for one shower. It's is customary to get the line flushed to remove calcium, etc. which acts like an insulator.


I was quoted 140.00 at the supply house to order a new coil. I'm thinking a plumber would want about 300-350.00 to change it out. I live on Long Island where prices are crazy. I'd like to change it myself but I wouldn't know where to start. I use to work with my dad doing plumbing but I was very young and we never did one of these.

traderfjp wrote:I'm against an indirect because I don't want to have to run my boiler at high temps to make hot water in the summer. I would rather use the coils in the boiler and use the stove to keep that water hot enough to make domestic hot water.

The primary reason for having an indirect hot water heater is have an endless supply of hot water. They are also very well insulated. Hot boiler water is pumped to a heat exchanger that's an integral part of the indirect hot water heater. Usually the boiler water is heated with oil, natural gas or propane. It's instant on, instant off with a huge BTU capacity compared to what's needed to heat the hot water. That instant on, instant off makes it an economical way to heat water. You avoid the standby losses in the poorly insulated boiler. An indirect water heater is far more economical to operate than a water coil in a boiler. Equipment costs are higher. Coal not being an instant on, instant off fuel is at a comparative disadvantage. It however can still be the cheapest fuel cost. I have an Weil McLain indirect hot water heater that can be heated with either my oil or coal boiler. In summer mode I use 0.8 gallons of #2 oil per day for heating domestic hot water only. The equivalent amount of coal (BTU basis) would be about 8 lbs. But because the coal fire must be burning all the time the daily coal consumption is much higher. For my coal boiler I estimate about 15 lbs per day just for domestic hot water. At that coal consumption it's still cheaper than oil.

In your proposed design if your domestic hot water use is similar to mine you will be burning 8 lbs. or more (perhaps double) of coal a day. In summer that much heat added to you home will adversely effect your air conditioning. You may have a system that is only practical to use in the winter. If I understand your installation you would continue to use your oil fired hot water heater for summer use.

Yanche: The system would only be used in the winter when the coal stove is up and running. During the summer I would bypass the stove's coil and let the hot water heater run on its own. I burn about 30 gallons a month in the summer for hot water. That seems to be about what you're are burning. A bit more I guess. I know that an inderect tank would be more economical with a traditional boiler setup. However, I would also lose my ability to run my Bock hot water heater independently from the boiler. It is much more effecient. So what I would gain with the indirect tank I would lose in the summer because I would have to run my boiler that is a 30 year old dry base Utica. Also, if I find that my stove can't produce enough heat than the boiler will be working even harder to heat the basement and the hot water. Not to mention that this would add 1k for a storage tank and the cost to plumb out another zone on my boiler which is about 500 plus parts according to one quote I got.

I just wanted to post an update and ask another question. This forum is awesome and I appreciate the help. I finally ordered my coil. It's 8" in diameter and about 20" long. I'm using 316 S.S with an 0.65 wall. It'll sit in the upper part of my Channing stove and about 8 inches off the fire. I also made them space the turns in the coil to get more surface area. I actaully reduced the size of the coil by about 6'. However, the coil is still about 40' long. It should definetely, generate some heat. I also checked the coils in my boiler and they were working fine. I wanted to know if I can use copper pipe with silver solder or should I use galvenized pipe. THe glavenized pipe would be much harder to work with but if it's safer I can plan it all out and then have my cuts made. Thanks

You cannot use galvinized pipe.

I assume your custom coil brings the plumbing connections to outside the stove. Right? There is no need to use high strength silver bearing solder. You should use tin/antimony (95/5) solder. It's lead free as required by most plumbing codes and has a melting temperature of 455 deg F. I would plumb everything with type L copper tubing using solder joints, avoiding treaded joints where possible. Use solid copper unions where you might need to take things apart for servicing. If you have really bad water (acid, hard, soft, etc.) there might be some special considerations needed for long life.