Closed System Hot Water Coil and Circulating Pump

I have purchased a very small Taco circulator pump and have retrofitted pex tubing under some hardwood floors in my 1850's house. I have an electric water heater that I am going to stop using, hook up a propane water heater to replace it. My plan is to use a closed system and heat the fluid in the closed system with my Alaska stoker, circulate it through the propane water tank and then to my radiant floor loops (all are very small, no more than 200' total).

My goals are to keep the domestic hot water tank hot when not in use and heat some very cold floors. If I get enough heat from the coal stove to accomplish these tasks, I'll use the extra electic hwt as a preheater tank and perhaps add a heat exchanger in my propane furnace plenum.

I could use some advice on a few things.

How can I best heat the water in the propane/primary hot water tank using a closed loop system?

and....

Can I use a black pipe constructed heat exchanger in the coal stove?

My idea for an exchanger in the propane tank was to use a copper coil (unsure of type of copper and thickness of tubing) inside the tank where the baffles are located for the burning gases. Could I spot weld the coil to the baffles nearest the tank and allow the heat to exchange that way? Or is there a way to place the coil inside the tank so it is in direct contact with the potable water?

Can't wait to finish this project!!!!

you will need a lot of volume if you are trying to use it for radiant heat. The coils normally are not enough in the stove to heat a house, you might need quite a bit of tubing in the stove to get enough heat out of it. For doing DHW (hot water for drinking) you can hook the loop up for your hot water tank, since that is not used all the time and it can circulate thru the stove and back to the tank. The temp. drop/recovery may not be enough for heating.

Most use a stainless loop (1 piece) in the stove, I would not have any joints or use thin wall copper.

Dave,

Thanks for your insight....any ideas how I can use the closed system to indirectly preheat my hot water tank?

If I continue with the use of black pipe and many joints inside the stove (realizing that it may not work or will eventually spring leaks, but I already have the parts and it is mostly assembled) I have about 14' of coil that can fit in fairly small area (16" x 18" x 1 1/2"). Doubling that would be quite simple. My trouble is that I want to use a closed system and have not seen much info on how to efficiently transfer heat to my 40 gal dhw tank and 40 gal preheating tank.

I'm not looking for whole house heating through the radiant floors, just to take off the chill. So I will run the closed loop fluid through the radiant loops last before returning to the coal stove. Once I hook things up, I will isolate the loops from the system until I see what temp the fluid is after bringing the dhw and preheater tank up to the desired temp.

Brett

I am not sure how you would hook it up to your hot water tank and making it a "closed" system, most people just hook up the where the PRV and the drain valve on the Hot water tank and circulate it that way. Without having to unhook the hot water tank, however you have to drain it to hook it up.

http://www.diydoctor.org.uk/projects/domestic_hot ... ystems.htm

Without having a coil in your tank that you can circulate hot water thru to make it a true closed system, you would need a hot water tank designed for that (boilermate, or something for solar heating, etc..but they are probalby quite expensive).

Just make sure you take the necessary safety precautions with PRV's etc...

Heres a good article

**Broken Link(s) Removed**

Sounds like its worth a try to me. A couple of important points: First, on your closed loop you must use a temperature/pressure relief valve (like on a water heater) so your loop doesn't blow up as the water heats and expands. And second, you must use an expansion tank on the closed loop, or you T/P valve will blow off water as soon as the loop heats. These two things are not optional. As for using black steel in the stove, it will probably leak. Its hard to find thread sealants that will take the temperature. But as long as the stove isn't cast iron, what can it hurt? As for the heat exchanger in your water heater, codes usually require a double wall heat exchanger. A coil in direct contact with the water in the water heater is not double wall, but a coil in the gas heat baffles would be. Actually, if the water in your closed loop is potable, you only need single wall, buy my experience opening up closed boiler loops is that the potable water doesn't look very potable after a few months. On the other hand a coil directly in the water heater would be a lot more efficient, and as long as it doesn't leak, who's gonna know?

I would consider doing something similar if my stove was in the basement. But as its in the living room, I think I'd be asking for trouble plumbing a water coil in it.

Good luck with it.
Joe

braindead wrote:codes usually require a double wall heat exchanger.

I don't believe so. Unless you're talking about something else. Every domestic hot water (DHW) coil in a boiler or indirect water heater is single wall. It would not transfer heat if it were double. You will never get boiler water in the domestic water if there was a leak as the boiler runs at 20 PSI & domestic far above that.

Back to the regularly scheduled show.... If you have the iron pipe and want to give it a try, go for it. I don't think it'll leak. As long as there's water flowing through it, it will not get above 212*. IF it runs dry, then you'll have issues and have them fast. The iron will rust, might make your domestic water orange. Also it will corrode fairly fast. I'd give it a two year life expectancy. That being said, what you're trying to do is make a boiler from a stove. It will work, but not nearly as well as you hope. Air doesn't exchange heat to tubes very well. I'd either use the coil in the stove for DHW or infloor, but not both. Even if used only for the infloor it will be lacking. I know you said you just want to take the chill off, but I'd guess that instead of the floors going from 50 to 60, they'll go from 50 to 52. I'd stick with doing the DHW. As mentioned, make darn sure you have a pressure relief valve (PRV) on any and all closed piping. Also make sure that no matter what valves get closed, the PRV stays "online".
It'll be a fun project, let us know how it works. I'd just do one or the other though.

PS.... Infloor water temperature should be around 140F. Most use a mixing valve and the cool return water to achieve this. Pex should not go over 190. No Pex should be within 18" of a heating device.

Fred,

Thanks for the tips. Perhaps I'll reduce my plans a bit and switch to a stainless exchanger and heat dhw only. Or skip the dhw and try for the radiant floor system. I have small crawspaces under the floors in question that allow slithering only, and I wanted to do as much as possible while down there so I don't have to go under repeated times. I am replacing all my copper with pex and installing a soft water system and soon a new well with a more powerful pump and don't want to have leaks in the old copper everywhere when the line pressure goes from 43psi to 85 or so!

That being said, I may install the exchanger I've already built and plumb it into my spare tank and just monitor the temps it creates. I will have a prv and an expansion tank....

let us know how it works out. Good luck. I would htink with expansion and contraction on the threaded joints, it may leak over time in the stove, then you have steam if it doesn't put the flame out in the mean time.....

Freddy:
I stand corrected...the IBC says "essentially toxic" transfer fluids need a double wall heat exchanger. That wouldn't apply here. I must have had glycol on my brain!
Joe

I think what most people fail to realize is when they think about using a coil in stove for some type of hydronic heating is the simple fact that the boiler lets say oil that they are heat has a draft loss that they need toovercome before they can even use any of the BTU's for heating. I my case I have the equivalent of 17 linear feet of 3/4" pipe in my stoker and it is enough to keep the oil boiler from running to maintain itself and for one shower without a recovery period and that is it . The draft loss of the biler prevents the water from rising above 190* even with a stove body temp of 400* or higher.

I have a Swg Power vent so maintaining a steady draft is not an issue, but I can see how that would be a problem if I just had a conventional chimney. I was planning to hook up to one of our chimney in the future. If the water heating would not work nearly as well, due to reduced draft, perhaps I'll leave it just where it is!

Thanks for everyone's responses.

I have done prety much what you are proposing, my radiant floor zone pretty much absorbs all the heat before it (hot boiler water heated by coal stove) can get into my indirect fired water heater. If I plumbed differently I coulld get somewhat better results. The other guys are right, losses are high and results will be low. As far a placing a copper coil heat exchanger in place of the propane burner on you water heater to heat the water (if I understand you correctly), without a direct flame heating the tank you will not heat anything, it may heat if left standing for 12 hrs, but the daily usage of DHW will wipe out all heat transfer. (would most likely take just one use of DHW) The best would be an immersion type coil or an actual heat exchanger designed for such an application. On the positive side if your goal is to just take the chill off cold tile floors, my system does do that nicely, until it goes down to single digits.

frank

Frank,

Thanks for you input. I currently have only installed tubing under the two bath floors (about 60ft of tubing). I may not go beyond that. If I can preheat the water before it enters the propane dhw tank, and warm my floors, I will be very happy!

I am busy replumbing the entire house at the moment and will tackle this project when all other plumbing is complete.

Brett