Building a OWF on the cheap

[mcguyver1]

Hi, I'm new here and I'm planning on building an OWF from two old propane tanks. I have several questions concerning the build. I've got a 250 gallon and a 500 gallon propane tank already cut in half. I'm just building a simple tank in tank boiler rather than a gassifier. I'll try something more complicated on the next build. The OWF I plan to build will hold around 100 gallons of water and I plan to put a radiator style heat exchanger in my down draft furnace. Also, I plan to heat my DHW with the boiler when I'm using the boiler to heat the house. I appreciate any help I can get.
The first question: When pumping water to a water to air heat exchanger, do you draw the water from the boiler at the top (where the water is the hottest) or the bottom and why?
Then do you position the pump to push the water to the heat exchanger or to suck the water through the exchanger so the pump see cooler water temps?
Thanks.

 

[BoilerMan]

First off welcome to Hearth!

To answer some of your Qs.

The first question: When pumping water to a water to air heat exchanger, do you draw the water from the boiler at the top (where the water is the hottest) or the bottom and why?

Typically water is actually drawn from the bottom of an un-pressurized boiler due to the higher pressure to prevent pump cavitation. You would return the cooler water to the boiler a bit higher and at some place a ways away to encourage good mixing in the boiler, because you cant rely on convection in this type of pumping situation.


Then do you position the pump to push the water to the heat exchanger or to suck the water through the exchanger so the pump see cooler water temps?

You ALWAYS want to push water to the restriction in a system, and in this case it is the piping and the HX. The ideal pump location is right at the boiler so it can push water through piping...through the HX.....and through the return piping back to the boiler.

Hope this helps.

TS

 

[Josh Carmack]

My Earth 405 pulls from the top, and returns to the bottom, you want cross flow no matter rather you pull from the top or bottom, as otherwise the water will stratify same as it does in a water heater. Something to remember, you want cross flow in your heat exchangers as well, but for a different reason. In your exchanger, be it water to air, or water to water, you want the boiler water inlet on the same side as the usable outlet. IE if the air enters from the left, and leaves to the right, then you want the water to enter to the right, and leave to the left. A cross flow approach in the boiler, and in your exchangers allow you to get the most BTU's out of your system in one pass.

 

[warno]

I just recently got my home built boiler into operation. You could check my build if you would like here https://www.hearth.com/talk/threads/im-building-a-homemade-boiler.145689/

Mine seems to work great heating my garage right now. I put my supply up high and my return down low. supply is on opposite side of the return to help mixing in the boiler. I run my circulater pump as a "puller" since I'm running a thermomix valve to keep my return temps above 140 degrees. That's how the valve manufacturer recommends to plumb it.

 

[mcguyver1]

Thanks guys for the info. The principles of a boiler seem so simple but suddenly it becomes complicated when you start planning the locations of different features such as water inlets and outlets.
Warno I've read your thread about your build. Very impressive. You're the reason I'm apprehensive about posting pictures of my build.
I plan to post some pictures but I'm going to have to reduce the size of the pictures before I can upload them.
My firebox is 29 1/2" in diameter and about 48" deep. I will be using a blower on the firebox. Besides blowing air directly at the fire, I've noticed some boilers also blow some air up in the top of the firebox. I read something about it helping combustion. Does anyone know if this is actually beneficial? Does blowing air up in the top of the firebox really improve the combustion?
Warno, what is the concern about the water returning to the boiler at a temp lower than 140F?

 

[warno]

The air over top of the is suppose to aid in burning of the gases produced while Benning wood. Problem is unless this air is really hot when it's added into the gases it won't ignite them. The idea is a basic gasification affect.

The problem with low return temps is you end up back in the condensation temperature range of the boiler which for me was until the boiler reached about 130 degrees. If you are running a tank in tank design the inside of your firebox tank will start like crazy until it's hot enough to burn it off. Basically like a cold glass of water sweats in the summer sun.

 

[mcguyver1]

Warno,I ask this question about what you said out of ignorance. I won't know if I don't ask and I am by no means being a Smart posterior...and there is always a chance I misunderstood what you said. If I understand correctly, the water returning to the boiler from your heat loop has a thermostatic mixer that mixes some of the hot water from your boiler to the returning "heat loop" water to ensure the water is at least 140F. My question is, what is the difference between mixing the boiler water and returning water just before it enters the boiler verses just letting the water mix with the boiler water as it enters the boiler. Also, what pressurizes the mix water at the thermostatic mixer. I know there would only be a couple pounds of pressure on the returning water but the mix water would have to have at least a little more pressure to "mix in" with the return water.


I have an electric down draft furnace. The duct below the blower is a 12" x 12". The electric heat strips are about 6" below the blower. There is only about 4 inches of duct below the heat strips then the duct splits into two directions horizontally below the floor. I don't see any good way to put a HX below the blower. There is plenty of room above the blower to put a 24" x 24" HX. Would a HX above the blower be effective? What are the cons to having the HX above the blower?
Thanks.

 

[maple1]

Return water entering your boiler consistently below 140 will cause excess creosote condensation in the firebox. And increased corrosion risk.

Think you lost me on the pressure talk. Usually a pump pulls return water through the mixer (pump between mixer & boiler return tapping). [Or maybe in some cases pushed through with the pump being between the boiler supply tapping & the bypass T]. So whether it gets pulled/pushed through from system return or the hot boiler bypass or how much of each is determined by the mixer.

 

[warno]

Maple hit the nail on the head with your answer.

I didn't buy it from here but this is the mixing valve I used.

http://www.hydronicspecialtysupply.com/caleffi-boiler-protection.html

I used the 1" version because that's what all the rest of my plumbing was. If you look at the brochure it shows very detailed instructions on plumbing and actions of this valve.

If you have any other questions don't hesitate to post them up. Alot of help to be had here.

 

[mcguyver1]

Thanks guys, that clears a lot up for me on the return water temp.
Who would know about the HX above the blower? That's going to be an issue for me. It would a big hassle to raise the whole furnace and install the HX under it. I would have to extend both the AC lines and the electrical.

 

[warno]

I don't think it matters where in the plenum you put the HX. It does matter that you install it with the flow the right way though. In my research on the subject I have seen them every which way. So I want to say again I don't "think" it matters which spot you put it.

 

[mcguyver1]

Thanks Warno. I had two concerns about locating the HX before the blower, One was that the hot air might cause the thermo protection on the blower motor to trip, and secondly that the blower would cool the air as it passed through.

Now I have a question about heating DHW. I have seen the plate exchangers and way they work. I was wanting to use my hot water heater for hot water storage and not have a pump running all the time. Also in the warmer weather I want my regular hot water heater to start working when the OWF is shut down. I made a very crude sketch of what I want to do. I would love some feedback on the pros and cons of doing it this way.

The basic principle of operation is: There would be a potable water pump, a copper coil, a thermostatic mixing valve and a temp sensor from an electric hot water heater (my HWH is propane).
When the temperature sensor senses water temp is too cool it would operate a relay that energizes the pump.
The pump would draw water out of the cool water supply side of the HWH tank and push the water through a copper coil in the boiler. The water would return to the HWH through the hot water "out" on the tank.
I realize that there would be some differences in exactly where the water came from and went to if some one was using hot water when the pump kicked on but there is a thermostatic mixing valve to prevent scalding water from going out the hot water supply line to the house.

I hope I explained that well. It is easy to make a mess of explaining an operation. Maybe the sketch will help.
The next question is how long of a 1/2" copper line in the boiler would I need and how many GPM should the pump be pumping?

 

[mcguyver1]

I was hoping to get some input from some of the pros on here. Is there any reason this wouldn't work?

 

[notshubby]

on mine I drew off the top (where the hot water is). with the pump in the cold return at bottom (so the pump didn't run as hot). but I didn't have to worry about cavitation as my boiler was lower down the hill in the backyard. and I had an atmospheric expansion tank at the highest point in my system. my understanding as explained to me was that water in a loop cancels itself out so to speak because the rising water will be helped along by the gravity siphon effect of the falling water. so the pump just has to overcome restrictions and friction. and since the pump really isn't doing much can be installed on either feed or return. not like a well pump that has to pump the water up with no help from down flowing water (ohh wait deep well injection pumps do work that way).

 

[mcguyver1]

notshubby, I would have a similar situation as far as my boiler would be below the HX. In my mind I though that the pump would be better on the cool side and work exactly as you described. How did you get an atmospheric expansion tank? Did you just get a regular bladder tank and drill a hole on the air side?

 

[Heat1285]

mcguyver1,

i would take into consideration the size of the boiler and how many square feet are you going to be heating??? 100 gallons of water is not alot for a traidtional OWB build that you are speaking off...i say maxx you will be able to do 1500sqft and DHW. also, for that style boiler, just being a tank inside a tank i would just do a natural draft style door with a solenoid and a flapper......you are also going to want to either create some sort of baffle or diverter in the firebox so you dont loose all your heat right up the stack....some places even just push the stove pipe way down into the firebox. If i were you i would go check out (broken link removed) , they manufacturer all their stoves out of old propane tanks and make a very very solid product from what i hear. you may want to try and mimic their simple design.

 

[maple1]

notshubby, I would have a similar situation as far as my boiler would be below the HX. In my mind I though that the pump would be better on the cool side and work exactly as you described. How did you get an atmospheric expansion tank? Did you just get a regular bladder tank and drill a hole on the air side?

A used electric tank water heater should work.

 

[maple1]

Thanks Warno. I had two concerns about locating the HX before the blower, One was that the hot air might cause the thermo protection on the blower motor to trip, and secondly that the blower would cool the air as it passed through.

Now I have a question about heating DHW. I have seen the plate exchangers and way they work. I was wanting to use my hot water heater for hot water storage and not have a pump running all the time. Also in the warmer weather I want my regular hot water heater to start working when the OWF is shut down. I made a very crude sketch of what I want to do. I would love some feedback on the pros and cons of doing it this way.
View attachment 168221

The basic principle of operation is: There would be a potable water pump, a copper coil, a thermostatic mixing valve and a temp sensor from an electric hot water heater (my HWH is propane).
When the temperature sensor senses water temp is too cool it would operate a relay that energizes the pump.
The pump would draw water out of the cool water supply side of the HWH tank and push the water through a copper coil in the boiler. The water would return to the HWH through the hot water "out" on the tank.
I realize that there would be some differences in exactly where the water came from and went to if some one was using hot water when the pump kicked on but there is a thermostatic mixing valve to prevent scalding water from going out the hot water supply line to the house.

I hope I explained that well. It is easy to make a mess of explaining an operation. Maybe the sketch will help.
The next question is how long of a 1/2" copper line in the boiler would I need and how many GPM should the pump be pumping?

You're still using a pump there.

A sidearm heater can heat with no pumps, by convection. You'd have to plumb it right. The cool water supply from the hot water heater tank to the sidearm would come out the bottom of the tank. The drain port can be used if it's a top feed tank.

 

[notshubby]

notshubby, I would have a similar situation as far as my boiler would be below the HX. In my mind I though that the pump would be better on the cool side and work exactly as you described. How did you get an atmospheric expansion tank? Did you just get a regular bladder tank and drill a hole on the air side?

at first I started out wth an old oxy bottle with the top cut off but that was a little to small. eventually I switched to a hundred lbs propane tank with the top cut out. I built a closet around it and put a cheap door on it for access. you don't need a bladder in the tank at all just tee into one of the lines as close as you can get to the highest point in your loop. from the bottom of the tank. and add an overflow drain down a cpl inches from the top. water will rise in the tank when the boiler temp rises and fall when it cools down. in my case the lines came uphill from the boiler into the house along the floor joists,then back down to the heat exchanger so where my lines ran along the floor joist I teed in and put my tank on the floor above.

 

[mcguyver1]

You're still using a pump there.

A sidearm heater can heat with no pumps, by convection. You'd have to plumb it right. The cool water supply from the hot water heater tank to the sidearm would come out the bottom of the tank. The drain port can be used if it's a top feed tank.

Maple1, If you use a sidearm do you run the pump constantly to supply hot water to it?
I was trying to make it so I only run the pump long enough to heat the water in the HWH tank then the pump would shut off. I know you could also do this with a sidearm heater but I thought the sidearm was a slow inefficient means of heating the water in the tank
.
Is there a downside to making a setup like in the sketch? I know you are experienced so I would appreciate your input. If you see something about my proposed setup that won't work please point it out.

I have a little common sense but I'm not real experienced in this sort of stuff. I don't know the math when it comes to heat transfer and such.

I wanted to heat the water in the HWH tank to roughly 140F then have a thermostatic mixing valve set to temper the water going into the plumbing down to 130F. I like my water hot and don't have any youngsters running around.

This going to be hard to explain but I'm going to try
.
My concern is: if someone is taking a shower and the pump kicks on to heat the water in the HWH tank, some of the water coming back from the boiler will go strait into the plumbing and to the person taking a shower rather than going in to the HWH tank. That means that the water that the water returning from the boiler (that was heated by circulating the water through a copper coil in the boiler) needs to be at least 130F (the temp the thermostatic mixing valve is set to) or the shower will suddenly get cooler.

If the boiler is at a minimum of 170F.
And the pump was moving water from the HWH tank, thru the copper coil in the boiler and back to the HWH tank at a rate of 5 or 6 GPM,
And assuming that under certain circumstances at least some if not most of the water going to the copper coil would be coming strait from the cold water supply at roughly 58 to 60F,
How long would the copper coil inside of the boiler have to be to ensure the water temp returning from the boiler was at least 130F?
I hope I expressed my question in a manner that could be understood.

 

[mcguyver1]

at first I started out wth an old oxy bottle with the top cut off but that was a little to small. eventually I switched to a hundred lbs propane tank with the top cut out. I built a closet around it and put a cheap door on it for access. you don't need a bladder in the tank at all just tee into one of the lines as close as you can get to the highest point in your loop. from the bottom of the tank. and add an overflow drain down a cpl inches from the top. water will rise in the tank when the boiler temp rises and fall when it cools down. in my case the lines came uphill from the boiler into the house along the floor joists,then back down to the heat exchanger so where my lines ran along the floor joist I teed in and put my tank on the floor above.

notshubby, does the atmospheric expansion tank completely empty when the circulation pump is not running?

 

[maple1]

Maple1, If you use a sidearm do you run the pump constantly to supply hot water to it?
I was trying to make it so I only run the pump long enough to heat the water in the HWH tank then the pump would shut off. I know you could also do this with a sidearm heater but I thought the sidearm was a slow inefficient means of heating the water in the tank
.
Is there a downside to making a setup like in the sketch? I know you are experienced so I would appreciate your input. If you see something about my proposed setup that won't work please point it out.

I have a little common sense but I'm not real experienced in this sort of stuff. I don't know the math when it comes to heat transfer and such.

I wanted to heat the water in the HWH tank to roughly 140F then have a thermostatic mixing valve set to temper the water going into the plumbing down to 130F. I like my water hot and don't have any youngsters running around.

This going to be hard to explain but I'm going to try
.
My concern is: if someone is taking a shower and the pump kicks on to heat the water in the HWH tank, some of the water coming back from the boiler will go strait into the plumbing and to the person taking a shower rather than going in to the HWH tank. That means that the water that the water returning from the boiler (that was heated by circulating the water through a copper coil in the boiler) needs to be at least 130F (the temp the thermostatic mixing valve is set to) or the shower will suddenly get cooler.

If the boiler is at a minimum of 170F.
And the pump was moving water from the HWH tank, thru the copper coil in the boiler and back to the HWH tank at a rate of 5 or 6 GPM,
And assuming that under certain circumstances at least some if not most of the water going to the copper coil would be coming strait from the cold water supply at roughly 58 to 60F,
How long would the copper coil inside of the boiler have to be to ensure the water temp returning from the boiler was at least 130F?
I hope I expressed my question in a manner that could be understood.

What you roughly drew will work, sure. But it will use a pump on the DHW side, and have to be controlled. If that's Ok with you, that's OK. And that's what mine does, although it uses a sidearm and a plate exchanger - I started with the sidearm, unpumped, then added the plate & pump for better performance when heating DHW solely from storage in the off season. (And now I rarely do that - go figure). Rather than replace the sidearm, I just left it where it was.

Sidearms heat slower, yes. And they don't transfer as good as a plate or maybe also not as good as a coil. But if you have a fairly big DHW tank (mine is 80USG), you will make enough hot water at night or in periods of non-DHW use for the rest of the time. The flow through of hot water isn't a big deal if you plumb things a bit different. Cold out at the bottom of the DHW tank as I mentioned, and rather than hot into the top, plumb the hot into the blow off tapping - my blow off tapping is down from the top of the tank a few inches. Use a T there (but keep in mind there is a stub about 4" long or so in the blow off valve, so orient for that). I had all kinds of hot water with just the unpumped sidearm during the heating season - the supply side of the sidearm usually gets plumbed in series with your other loads, so whenever those loads are being pumped by the system pump, the sidearm is heating your DHW tank. Or, you could plumb it as a separate zone and control your main system pump to start whenever the DHW tank wants heat. (Not knowing the rest of your system details).

You can also buy coils like you have in your diagram. Might be able to find a used one somewhere? New ones are like $4-500 from what I remember.

Something else you could do is plumb the coil in series with your DHW heater. Cold water would go through the coil first, then into your DHW tank. No pump there either, and if the DHW heater isn't hot enough from that or the wood fire is out, the elements will come on automatically. Put a couple of coils in and you should have lots of hot water - depending on how much heat the tank loses between DHW uses.

 

[mcguyver1]

Maple1, That's why you're the minister of fire, you have a lot of good ideas. There was a couple of other considerations I had that I didn't share.
We are in the process of building a new house. The one we are living in is old and small but good enough for now. The current HWH is old and I'm a bit concerned that if I try to do things like remove the blow valve it might just break off rather than unsrew. Also, the HWH is only a 40 gallon propane fired tank. I keep the temp down because it's not efficient and really uses the propane when you turn it up. It's fine in the summer because generally I take cooler showers but in the winter I like to take long hot showers. If I planned to stay at this house the HWH would be the first thing I would upgrade. I know that buying a new HWH wouldn't be the end of the world but I hate to buy a new one for house that we will demolish in a couple years.

I am going to put a wrench on the blow off valve and try to feel it out. I would rather have the water return to the side of the tank instead of at the top where it could head strait into the plumbing.

I would like to use the boiler to heat the water in the HWH to a much higher temp than I normally do so we would use a lower ratio of hot water to cold water and still get a nice hot shower. Also, the thermostatic mixer will stretch the hot water out. Hopefully everybody can take long hot showers and no one will have to wait for the HWH to recover.


I don't mind using a pump to circulate the water to heat the HWH I just didn't want the pump to run 24/7 like some of the systems I saw with plate X's. That seems like a waste.

I was concerned when you said that an (in the boiler) coil could cost upwards of $500. I didn't even know they made a special coil to go in the boiler. I was planning to just use a 1/2" copper coil from Home Depot. I didn't know if 10' was long enough...or perhaps 20'. I don't know the transfer rate of heat through the copper. I think that the flow rate needs to be fairly slow but I don't know how slow. There simply isn't much info about this out there.
Is there a formula or "Rule of Thumb" to figure out how long of a coil I need?
Thanks for the help. Having an experience point of view goes a long way.

 

[maple1]

Don't think I can help with the coil length question. My old wood/oil boiler had two 'proper' coils in it. Don't know how much tubing were in them, but the coil itself was about 1 foot long & maybe 6" diameter. The pipe in it was finned to help with heat transfer. They didn't work too bad - if the boiler was above 140°, we had enough hot water. But that came at the cost of having to keep the boiler hot all the time - very costly.

Used coils can usually be found on Ebay. Or you might find a boiler someone is getting rid of for cheap on Craigslist that has coils in it, that you could pull the coils out off, cap or plate the holes, the re-sell the boiler without coils.

Depending how you mount the coils in the boiler (top or side?), and if the boiler is pressurized or not, I would think making them might be tricky in getting the joint leakproof where the pipe passes through the plate. But if it's going into the top of an open boiler or tank, that might be easier.

My DHW pumps only run when the electric hot water tank needs heat.

One more thing I did was stuff two 100' coils of 3/4" pex inside my storage enclosure, most of it laying on top of the storage tanks. The cold water that is heading for my hot water heater goes though that first. It helps a lot. It has slow heat transfer, so if using a lot of hot water all at once it won't make much difference once the water runs for a while, but it makes for a couple of gallons of water that is as hot as storage after a period of no hot water use. So if that's 160, by the time it gets mixed down coming out of the hot water tank it's about enough for my shower - with no other heating. It does decrease pressure a bit out of the tap, compared to how the cold comes out, but not enough to worry about.

And, if you have to get a new water heater, you could always get a new electric one and take it with you to the new place. If you have room in your existing panel to wire in an electric hot water heater.

 

[notshubby]

notshubby, does the atmospheric expansion tank completely empty when the circulation pump is not running?

no think of it as the whole system is sealed boiler, heat exchanger, piping, except for the expansion tank which is wide open and at the highest point. when you fill the system with cold water it takes up less volume. as you heat the water it expands and level in tank rises. as it cools it contracts and the level goes back down. it just gets teed into the return line from the bottom of the tank to a tee in your pex line. the water does not flow through the tank. most of the time the water in the tank is at room temp. my boiler started out with about a 26 inch diameter round high pressure air tank 5/8 inch thick but I didn't like it so about halfway through its life I cut that out and put in a 1/2 plate firebox. however the outer water jacket was 1/4 inch. with flat plates a few pounds of air would make the sides bow out (I put like 10 lbs air in it to test for leaks and you could visually see the sides bow out) no big deal cause I wasn't running a pressurized system.

you have to have room for expansion. my boiler was lower than my basement because the backyard was downhill. I put it on the first floor. if your boiler is higher than the rest of the system then you can put an expansion tank either above your boiler or incorporate it into the boiler itself. if that's not an option than you build to pressurize and add an expansion tank, makeup water valve and safety blow off valve.

I put an overflow line in a cpl inches from the top of the tank which simply ran down through the trench next to the incoming pex and out onto the ground behind the boiler. since I had no additional storage capacity to act as a buffer it boiled over. once was because my solenoid stuck open and even though the fan shut off natural draft took over and the fire just kept roaring. and another cpl times when we had some really sub arctic cold snaps I turned my aquastat up and forgot to turn it back down when it started warming back up. you also need to remember that unlike gas or fuel oil boilers which shut off when its up to temp wood boilers keep going when the air shuts off till the fire dies down. so if you don't have a lot of extra water storage and you set the aquastat high it shoots up to that boiling point faster.