Need Help With an Externally Mounted Domestic Coils System.

Jpen, I think what coaledsweat is suggesting is the same as below, just re-worded. If not here's my take on your plumbing schematic.

I would not use the oil boiler to heat a storage tank, use the external coal heated coil to heat a supply tank[also called a tempering tank]. The way you have the plumbing now, you will have a large quantity of oil heated water cooling-to-tepid in a storage tank, and no way to raise the temp of the stored water quickly. The effectivness of an external coil is still a big question, and you are gambling that it will keep the storage tank warm. But you will still be using oil all the time to heat any domestic hot water. If you shut down the coal stove the water in the storage tank will cool within a day, and you won't have hot water.

Use the external coil to heat a supply tank. This supply tank can be a discarded hot water tank, either 40 or 50 gallon is common and will work. This supply tank has the cold water supply to your domestic hot water tee'd into the loop. The outlet from the tank/loop goes to the inlet of your steam boiler's water coil.

The way this will work is that during a long night or day that the coal stove is producing heat, the coil will slowly heat the water in the supply tank, and store this hot water. When you use domestic hot water, this preheated water will supply the coil in the boiler, then on to the point-of-use in your house. If the water in the supply tank is hotter than the boiler's water, then some heat will be transfered to the boiler, can't be helped. This heat is still stored in the boiler water. If the supply water is cooler than the boiler water, then the boiler will eventually turn on the oil burner to keep the water at a desired temperature.

Your external mounted coil is not going to be very effective [my opinion], and the sand-box you described may act as an insulator instead of a conductor. I'm thinking that this location is going to make the coil almost ineffective at recovering heat. These coils don't have much surface area, and need to be in the high temp of the firebox to be effective. I'm thinking that at best you will be warming the supply tank, I doubt very much if you can get 40 gallons of water to 200* in even 12-18 hours of heating with no added make-up cold water. [no hot water use].

If you can find a discarded electric hot water tank with a bad electric element, and make up your own coil, you won't have much invested if the external coil is ineffective. Experimenting is a good thing as long as there isn't too much money invested.

Any amount of pre-heating the supply water to your steam boiler's coil will save you oil, even if the water is only 100*, it is better than the usual supply water temp of 50-55*. So I think it is a worthwhile experiment.

I think an insulated aluminum or SS box around your external coil with a highly polished inside surface would be more effective at transfering heat to the coil than surrounding it in sand. Most of the heat coming off of the back of your coal stove is radiant, and needs to be reflected back to the surface of the coil. The surface of the coil needs to be a dull dark color, to absorb as much heat and reflect as little as possibe.

Greg L.

Greg I can use # 7 finsh stainless ( mirror finish) to build the box over the coil. I had another thought . Do I even need the tempering tank. Why not just hook the stoves coil right to the boilers coil. Less water to heat up and if anything it will keep the boiler from running in the middle of the nite when no water is being used. I'm not sure but wouldn't the boilers wet base act as a buffer to keep the coil from overheating? I have a taco 007 laying around so all I would be out would be the cost of the pipe and the valves and fittings. I have seen external coils on radiant wood burners be effective but the wood stoves jacket was probably hotter. I like the idea of an internal coil, but I don't want holes in my stove if it doesn't work right plus straight stainless pipe threads which would be required to mount the internal coil are notorious for developing leaks. I could weld two 3/4" full couplers in the side of the stove but that would require removing the stove from the house welding it and putting it back in place.

I'm pretty sure you want the tempering tank. I think the external coil will be slow to warm the circulating water due to the lower temperature on the outside of the stove.

If you hook the external coil directly to the boiler coil, you may as coaledsweat said pull heat from the boiler water. Plus you will have the issue of what will happen when or if the oil boiler is called upon to make steam.

I would prefer to heat a tempering tank with the [expected] slow heat-gain from the external coil. This way you will gain the most from the coil. If the external coil proves to be very effective, then you may be able to change the plumbing to include the oil boiler.

The mirror finish SS sounds good, insulate it with a layer of fiberglass, it should capture as many BTUs as possible.

I'm surprised that your oil boiler runs in the middle of the night. Without hot water being heated by the coil, what other heat load is on the boiler?? Maybe some insulation on the oil boiler jacket would help. Is the chimney pulling the heat up the flue??

Greg L.

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LsFarm wrote:I'm surprised that your oil boiler runs in the middle of the night. Without hot water being heated by the coil, what other heat load is on the boiler??

If you have any steam leaks it will fire fairly frequently. Fix 'um if you got 'em.

What kind of control is on the boiler? Does it fire on pressure or temp?

Ian, I don't think he is using the boiler for steam at all, just keeping it at 140*-ish for domestic hot water...

I'm thinking the chimney is pulling cool air off the basement floor over the boiler and taking the heat up the chimney... maybe an electric flue damper tied to the oil burner would save some oil and $$ ?

Greg L

LsFarm wrote:Ian, I don't think he is using the boiler for steam at all, just keeping it at 140*-ish for domestic hot water...

I'm thinking the chimney is pulling cool air off the basement floor over the boiler and taking the heat up the chimney... maybe an electric flue damper tied to the oil burner would save some oil and $$ ?

Greg L

That's a good point, they do draft. A candle by the air intake should prove what the chimney is uptaking. But if he is losing steam anywhere, the boiler will fire to replace it when it loses enough. Steam leaks are like leaking hot water faucets, it's just a lot more heat being lost.

I think jpen said in an earlier post that the boiler hasn't been called upon to make steam heat for a few years, just an opps-check for steam each season.

I'm guessing that the chimney's draft is pulling heat off the boiler all the time and the oil burner runs to keep up.

If the system is still set up to run when pressure drops, then that may be an 'easy' fix, rewire to fire only for temp, not pressure.

Greg L.

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Sorry guys I have benn pretty busy. I haven't used the boiler for steam heat in over five years other than a yearly test run in the fall. Greg I think you are right about the draft issue. As I am typing the boiler is running to maintain the aquastat and I haven't run hot water for about four hours. My baro on the furnace opens quite frequently when the boiler isn't running. Both my chimney draft very well. I can keep the draft with about 1/2 an inch of fire in my channing III bottom vent even with the temps near 70*. The boiler is set to run on a thermostat and only uses the pressure switch to shut it off and on when the t-stat is calling for heat. I'll have to check to see if one can be added to my boiler. That may be the best first step to cut the oil usage. We only use about 2000 to 2500 total gallons of water a month, with the hot usage being less than a third of that. That little usage and I still use 180 gallons to heat the hot water, however that is better than the 1100 to 1200 galloons I used to use for heat and hot water. In winter the boiler will run atleast 2 or three times to make the aquastat from about 9pm to to 7am. Where can you find auto pipe dampers with controls that allow it to open when the boiler ignites.

Here you go, most contractor supply houses would have them in stock.

http://www.fieldcontrols.com/gas-vent-damper?page_id=93
**Broken Link(s) Removed**They work very well.

I took the surface thermo and temped the pipe on the boiler. The pipe was 117* before the baro which is at least 2' from the boiler.

jpen1 wrote:I took the surface thermo and temped the pipe on the boiler. The pipe was 117* before the baro which is at least 2' from the boiler.

Do you have a manometer or draft gauge? If there is any draft it is pulling heat from the boiler. A lttle draft is a little heat, a big one...... well you get the picture. The damper just shuts the pipe off, no draft, no loss to the chimney. My oil fired boiler will fire because it is losing heat too. I may look into this myself.

My neighbor owns a large plumbing and heating shop I cheked with him on those dampers and he said they are much more trouble than they are worth. The field controls brand comes with some of the high efficiency gas boilers they sell. He says that the damper are very prone to failure and result in a lot of service calls . He thinks I 'd be better off with a coil in the stove and draft reducing chimney cap on the boiler. We have to put a manometer on the chimney and get its average natural draft and go from there. Only a slight breeze here this morning and the draft will hold up a piece of printer paper over th baro. #-o