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Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Moose, Jan 6, 2008.
Here's what my control looks like, and hopefully the aquastat pics can fit too.
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are the 5 circles in the center of the drawing the return lines???
Here's the actual piping. Valves 1 & 2 are to the left, valve 3 is on the right.
Ok i get it, just took me a minute
I see you have an expansion tank on both untits, should I do the same? Is bigger better?
Yeah those little circles have "Cs" in them for circulator. When I re-piped the system this fall, I eliminated flowchecks, and used the new Grundfos circulators with built-in check valves, I also eliminated the copper lines running to the boiler and used PEX.
A lot has happened since I started to write this, so much of this might no longer help.
This is a hard one to follow, but for what it's worth. First, I am not a professional, just a home experimenter, offering advice based solely on my experience. I always consult pros (and pay for the advice) if danger to me, others, or property may be present. That is true, here, so, Moose, take the benefit of my experience for what it's worth but rely on my experience solely at your risk.
It seems to me that Moose wants to retain an operable oil burning boiler but add a new wood burning boiler which will be the primary heat source, with a fall back to oil if needed or wanted. So, this probably should be a parallel hook-up with the hot output of the WB going to a T on the cold return of the OB. The other port on the T is the cold return from the heating zones. The hot output of the OB also should be a T, with one port going to the cold return on the WB and the other port to the heating zones. For Moose's purposes, the aquastat on the OB either is disabled or set high so that it will not fire the OB so long as the WB is supplying all needed heat. If disabled, it would need to be reconnected when the oil burner is placed back in service.
The aquastat on the WB is set as normal, often on at 160 (make on rise). This aquastat will turn on a circulating pump on the WB to circulate hot water in a loop through the OB. If system water temp falls below the differential point on this aquastat, the circulating pump will turn off. This would happen as the fire in the WB goes out or heat demand depletes the system. This aquastat could be one that clamps onto the hot water out on the WB (but another issue here discussed below).
Draft/fire control on the WB is a different issue. I don't have an immediate suggestion here, but on my boiler I manually turn on the draft fan when I fire the boiler, the fan operates until boiler water temp reaches 190, then the fan shuts off; fan comes back on at about 180, and this cycle continues until the wood burns out. When boiler water temp falls to 140 the draft fan shuts off, the assumption being that the fire has burned out. The control to do this was part of my boiler's control panel.
The heating zones operate off the OB loop. Assuming one heating zone, this zone could operate with a line voltage thermostat which turns on a separate heating zone circulating pump. This pump draws from the T on heat out of the OB and returns to the T on cold return of the OB. A similar arrangement can handle multiple zones with zone valves (and perhaps balancing valves).
The WB also should have an overheat loop in which to dump excess heat. I suggest a separate aquastat that fits into a well in the WB and is wired in parallel with the regular WB aquastat as well as operating a relay in parallel with the zone thermostat to turn on the heating zone circulating pump. The overheat aquastat might be set at 200 (make on rise). If the overheat temp is reached, both pumps start to allow heat dissipation. This works until there is a power failure and the WB is in full burn. Although the draft fan will go out, temps still can rise. and I suggest a separate gravity fed dump loop directly off the hot water out of the WB. To keep this from operating all the time, I suggest a normally open solenoid valve on this line. In other words, so long as power is available, this valve is closed and the overheat loop does not function. On power failure, this valve opens and the gravity fed overheat loop operates. Sizing of this loop I understand should be at least 15% of the BTU output of the wood boiler.
Now, the other issue mentioned above as to the WB. For this boiler to operate efficiently its water should be kept hot and cold water should not be fed into the cold return, except when the boiler is first heating up. This requires a thermostatic mixing valve on the hot water out of the WB. All hot water on initial firing (actually, not very hot), is returned directly to the cold water return of the WB. As the water heats, this valve gradually closes and hot water is diverted to the hot supply line for the heat zones. Return water then also goes through this valve, and the valve mixes hot water directly from the boiler with the cold return water so that warm to hot water always is returning to the boiler, resulting in high boiler efficiency.
On my boiler I have a surface aquastat on the return line to the boiler AFTER the thermostatic mixing valve. This is the aquastat mentioned in the second paragraph above. I set this to come on at 150. The return line to this point is operating on gravity, hot water out, through the mixing valve, and right back into the cold return. Because of this gravity feed, the aquastat comes on quite quickly, turning on the WB circulating pump. But as stated, initially all of the output of the boiler is fed right back into the boiler by the mixing valve until the boiler fully heats, then the mixing valve starts to close, diverting hot water into the heat zones. As the whole system comes up to temp, the mixing valve closes more and more, until potentially diverting all output into the heat zone.
Hope this helps. You have quite a project on your hands.
Yes, I actually have 3, I have 1 on the DHW also. Originally I had to big ones, now I have a big one on the wood boiler and a small one on the oil boiler.
Jim's got a handle on the how the system should work, and his description is damn near the same as my system, except that my control system is a little more complex and is only going to get worse. The only thing I don't agree with is a parallel system, but that's a matter of choice. A series system narrows the control of water temps throughout the system, and I'm not for shutting down any electrical controls, all controls should be intergrated with safeties in place. Back when I designed my system, nobody had wood or alternate heat, I was on my own, and it took me 2 weeks to design it going back and forth adding possible scenarios. Back then I was 29 and pretty sharp, now I'm 50+ and pretty dull. In all the years I've had this up and running, I have never had one bit of trouble with the control, only circulators, oil pumps, and burners. It's fun to see other's with the same interests and similar and different ideas.
Here's my power failure zone valves. Originally they were mounted on the boilers, and wired in metal flex conduit. Now they are mounted to the PEX and "float".
Why do you recommend putting the hot supply from the WB into the return of the OB, jebatty?
He doesn't have what could really be classified as a boiler. I think it's more of a wood furnace/stove with some pipes running through the firebox. As such, probably no place to insert aquastat probes. The bimetal draft control sounds like it might work alright, though. The normally closed zone valve I have is called an Automag. It opens up when the power supply is cut.
Instead of a mixing valve, I have a Taco 007 pumping supply water into my return at low boiler temps. It's wired to the controller.
What size are the fittings on the boiler/furnace Moose? Forgot to ask that one, if the "hubs" are small (3/4") the boiler should go in parallel, if they are large (1 1/4"-1 1/2") it should go in series.
If you put the boilers in series all of the hot water goes through the oil boiler and will satisfy the aquastat, if you run in parallel, the oil boiler will never see wood heated water and will try to run independantly. Parallel is fine if you're going to shut down the oil unit, but then you have no automatic backup, (without elaborate control).
donno I'll check tomorrow.
I would have serious doubts about your stove being big enough to replace your oil boiler. What is the BTU rating of your oil boiler? Comparing pipe size on boiler and stove should be a good indication. My system would be somewhat similar to yours (less the oil boiler) my heat coils are outside the firebox, water circulates constantly, aquastat controls combustion air. I've followed your chimney issues somewhat and I'm a little concerned about chimney pipe going into stone chimney and cooling with cresote build up. How easy will your chimney be to clean frequently? I think I'd try to plumb through the boiler when burning wood (power off and flue blocked on oil unit) then use ball valves to isolate wood stove so oil boiler is left in it's current configuration. I don't want to discourage you in any way, but keep in mind complexity tends to feed on itself. I'll bet you'll have dreams of pipes, wires and pumps tonight. Gook luck Mr. Moose
Honestly I don't expect the wood burner to out perform the oil boiler, but since I've never used the oil boiler other than DHW I don't think I would ever know. If I get nothing else out of the wood boiler other than DHW then I would be satisified. Anything more that that I would consider a bonus. I know the thing is going to produce heat. the firebox on that thing is huge so I'm going to put it in and see what happens just want to do it safely and efficiently and I'd rather not have anyone say what the ____ were you thinking : ) I have decided to install an new chimney and just run it straight through the roof I think eric and bartman talked me in to that. Besides the oil boiler goes through the masonry chimney also. And to be honest I have dreams of two pipes one to to connect the wood stove/boiler to the existing system, a couple of new aquastats, and one more pump. trust me I like to "keep it simple stupid" as they would tell me in the Corps. And I will definatly need all the luck I can get.
If Bartman feels fogged at 50+, then my lights are about to go out 'cause I'm nearing 61 -- and just installed on my own my first wood gassification boiler. What this means is that one doesn't need too many watts firing to see the light or that intelligence is fully failed to allow one to do stupid things.
Why the parallel hookup suggested as I did? My logic tells me the answer lies in an assumption of two operating systems (both can be fully operable with all safety controls intact for each). In Moose's case, leave the OB controls intact but set the "on" aquastat at a temp above the WB aquastat if Moose wants the WB to be the primary heat source or disable it if Moose wants to heat only with WB. The OB aquastat controls firing of the OB and OB circulator. If disabled, the OB never fires, so I don't think there is any OB safety issue. If set higher than WB (maybe a WB 150 - OB 160 setup), then OB fires only if WB is not providing sufficient heat. Because WB hot is feeding OB return, OB is only providing "makeup" heat and will run minimally, such as when demand exceeds supply capacity of WB or WB fire out.
I mentioned the aquastat on my boiler is mounted on the return line AFTER the mixing valve. With this 2 boiler setup, I think it should be mounted directly on the heat supply side of the WB. Then the WB circulator will come on anytime temp reaches its setting, such as when WB is up to heat or WB is down and OB is supplying up to heat. I suppose by using a relay set up the aquastats on both the WB and the OB could turn on both boiler circulators anytime either one reaches the setpoint. This would provide unrestricted flow through the boiler loop whenever either boiler is operating. [this is where the fog sets in -- any pro's have advice? I think water passes through a circulator with little restriction when it is not operating, so maybe both don't have to be on.]
Critiques by others is welcome. Again, good luck on this.
Well here are the pics Ive started with the water to air heat exchanger since I think this is the one I"ve had the hardest time explaining.... this is the forced air side of the system. air is brought from the living space across the radiator heated by boiler water then pushed back into the living space.
outside of water to air heat exchanger... whole unit with some ducting...
close up of the radiator inside the unit
radiator with blower motor
Here is the existing oil boiler (in working order)
return lines with valves
supply lines with taco pumps
pump control hub
in floor radiant heat
Only been in the house since sept. I wouldn't know
now my plans are to put a Tee in at the elbow most shown in the pic below and another tee at the elbow just below not shown and place three valves basically the same thing the bartman did (I think) I think it should be simple enough the aqua stat for the over heat dump will be by valve 3. Seems simple enough to me.
You've got both the supply and return from the WB going into the return of the OB. I don't think that will work. This drawing would work if there was a heat exchanger on the OB's return line, but you're piping direct, which means you need to use both the supply and return on the OB, I think.
Here's a very basic diagram of my piping arrangement. I piped the DHW hx ahead of the gas boiler vessel because I want consistently hot water at the tap. Higher WAF. Also note that I piped the sidearm heat exchanger backwards in the diagram. I always do that, for some reason. Actually, the flow is top to bottom, but you get the idea.
I know the drawing is crude, I did it as I was running out the door, but those thiangles with the apexes together are going to be valves. and valve #1 will be closed with valve 2 & 3 open with the WB in operation and then just the opposite when the WB is not in use. with the WB in operation the return would feed the return on the WB heat the water then the supply on the WB would feed the return on the OB then the supply on the OB side would not change. I too would like the DHW first on the route but I think I will leave it like this for the rest of the season and tinker with it in the summer (basements are cool in the summer.)
Your valve 2 has to be on the return side of the wood boiler, valve 3 on the supply of the wood boiler. By the picture it seems reversed, but valving is in correct orientation.
That's a nice looking air handler, I haven't seen one here with the built-in heat exchanger. Usually I buy the heat exchanger separately, then have 2 transitions made by a sheetmetal fabricator.
I have never seen one of those heat exchangers plug up, doesn't mean it can't happen though.
That Weil McLain is fairly new, and is a good performing boiler, it just needs a better burner.
So I can't just disconnect the return from the OB and connect it to the return on the WB then connect the supply from the WB and connect it to the return of the OB?
Yes, that's what I do in my system, and that's what the plumbing diagram showed. In your diagram it looked like you had both boiler returns connected together, and the zones coming back into the wood boiler supply. Maybe it was just the way you drew it, usually in a block diagram, the return is always on the bottom of the boiler, supply is on the top.