Converting gravity to circ, to outdoor burner system

Betz

I heat approximately 4500 sq. ft. I have added a Central Boiler outdoor wood furnace to our existing heating system. The original system used a No. 1 Ideal Gas Boiler converted from coal to gas(Pre 1920's). I have 3" supply lines (manifolds?) and 2" return lines. I have tied the outdoor unit into this system. I am having a problem with my return water being too cold going back into my heated water outside. This seems to be a function only of the furnace as it doesn't matter how cold it is outside. I would love to talk to someone about how to keep this return water warmer so my damper doesn't open so often to heat up the water. I will try to include a sketch of what i have right now. I'm not sure whether to change my pumps to high volume/low head or low volume/high head or what to do. I get plenty of heat in the house but I lose a lot in the outside burner, which holds about 400 gallons of water. Please email me and I can give you my phone number or a way to contact me. my email in case it doesn't up is ckbetz@msn.com. Thanks anyone who can help.

Mike T., Swampeast MO

I don't know much about outdoor wood boilers, but I can probably tell you what's going on in your system...

(I assume it has large, standing iron radiators.)

While 400 gallons in your wood boiler may sound like a lot, your system itself probably holds a few hundred gallons.

The water in your wood boiler is likely MUCH hotter than your system needs most of the time. So...when your t-stat turns on the circulator your rads get quite hot quite quickly and the space temp rises surprisingly rapidly and the circulator shuts down. The water in the rads cools and in the next cycle much of it flows rapidly to the wood boiler opening its damper. The cycle repeats and gets worse the warmer it is outside.

Don't know what type of mixing valve you're using there for bypass or how it's controlled, but you should be having LOTS of bypass--particularly in moderate weather.

My guess is that you have too little bypass and that the three-way valve you are using might not be suitable.

Something that [might] work is to install a properly sized two-way proportional valve in the supply line between the wood boiler and old gas boiler. Operate it inversely with outdoor temp to provide outdoor reset for the system. This will DRASTICALLY reduce supply temp to the rads, the circulator will run for significantly longer periods and the return temperature will be much higher as well.

p.s. As this is a converted gravity system, a low-head, high-volume circulator (like B&G 100) is most likely appropriate.

Betz

I can add some information. The systems requires a pump to constantly circulate water in a loop. When my t-stat calls for heat the zone valve opens and an inside pump kicks on. This inside pump is an 8 gpm with 28 feet of head. When the t-stat is off the valve keeps the water outside. This part works well. I did install another loop from the return to the supply when the t-stat calls for heat. I can manually control how much of the return goes back into the supply inside the house by a ball valve. I can immediately feel the heat difference as i move the valve. I do have the large cast radiators. And yes, I have plenty of heat in them. Oh, the unit is no longer under pressure as there is a vent in the burner unit outside. So i wonder if that makes any difference on the pump head and so forth. Thanks again for any help.

Rudy

wood burner

One thing you might want to look into is the fact that you are running an open system now. I assume that you are using an oxygen inhibitor supplement with your system. A good idea is to isolate the 2 systems with a heat exchanger. This will eliminate the possibility of oxygen eating away at your existing boiler,radiators, and any other ferrous components in your existing system. It is often nice to pipe the wood boiler in parallel with the gas boiler whether it is parallel P/S or straight parallel with a 3-way zone valve controlled by a low temp aquastat on the wood boiler line. This will eliminate the cool water from the gas boiler circulating through the system. The gas boiler would only fire when the wood died out. The previous post about the 2-way valve hit the nail on the head about your existing system needs I just thought I'd talk about the oxygen factor. One other thing to remember is that with an open sytem you have to account for the static head that you now have that you didn't have with the closed system. (I assume your old system was closed??). By the way, what size pump do you have on the loop to the wood boiler, and distance to the wood boiler? Matt

Betz

The outside burner is about 30 feet from the house. I have a taco 011 which is 28 gpm and 30 feet of head. It is currently unplugged because it was pushing water through the new loop to recirculate the return back to the supply and the heat wasn't reaching the house. I am going to try a taco 007 which has the same flow rate but only 8 feet of head. I can put the pump where it shouldn't push through the house but am going to try it on the burner first. I have corrosion inhibitor in the system which is supposed to take care of those problems. The folks at Central Boiler are telling me I will lose efficiency with a heat exchanger because I suggested that, since the water won't mix directly and possibly cool it as much. My water is 185 degrees coming in and I really don't need it that hot, even on the coldest days. When we started remixing the water back into the supply the temperature in the unit dropped more slowly, but it stayed down longer. I'm going to reread the post about the 2-way zone valve, I can't picture what it will do for some reason. I have learned a great deal about this kind of heat lately but am learning a lot more every day. Thanks.

Rudy

heat exchanger

When you put a heat exchanger in you size it for the temp drop you want on the primary and secondary sides. If sized correctly for the application an exchanger will give a relativly uniform delta t. which would increase the performance of your stove (I'm assuming this is a 1-zone coverted gravity system).
Don't know the layout of your existing system, but sounds like you could take out the 009 and move in the 0011 from the wood boiler. You could eliminate the diverter valve and the bypass to the diverter valve as you are only 30' away with the wood boiler. Adjust the ball valve on the bypass after the check valve and before the pump to modulate supply temp. to the gas boiler.
Or... you could just eliminate the 009 and prioritize the zone valve for your future water heater. Either way there is a good possibility you probably don't need both pumps.
Just my thoughts-Matt

Betz

I did forget to add that I have a 2 zone system with one thermostat. It works pretty well believe it or now, although with my high head and volume pump, the upstairs tends to get a little warmer than it needs to.