By pass re-circulating line

Dan13

I have a wood boiler (in garage) in line with a oil boiler back-up (in basement, swiched off 99% of the time).  Normally, the hot water flows through the oil boiler and then through the different zones requiring heat, then back to the wood boiler.  During warmer times when the zones don't need heat, the water sits in the line and boilers, and the water in the wood boiler gets too hot and the water in the oil boiler starts to loose heat.  I would like to recirculate the water through both boilers when the zones are off so I don't overheat the wood boiler and improve reaction time when a zone is needing heat.

I want to put in 1/2" tee in the 1" main line, before the oil boiler pump and go to the wood boiler return line.  When the zones are off, I believe the wood boiler pump will push some water through the oil boiler and back to the wood boiler, keeping the water flowing, preventing overheat.  The question is when the zones need heat, will the by pass tee cause a significant flow of hot water through the by pass instead through the zones?  I hope the 1/2" reduction and right angle would cause 80-90% of the flow through the main 1" line and to the zones, but would like an experts advise.

Thanks!

Ironman

Bypass

I think that I've got what you want to do pictured in my mind, but a little more info would help.

Is the wood boiler pressurized to normal system pressure?

Are the boilers piped in series or parallel or some other way?

Is there a separate pump on each boiler?

Where is the pump located on the oil boiler, supply or return?



A picture or a diagram would be very helpful.



I install a lot of wood boilers, but your piping description is not the norm that's why I'm looking for some more details.

Dan13

Response - details

Thank you for responding and sorry for the confusion.  There have been a few additions to this house (before and after I bought it), so there have been at least 4 different plumbers working on this system.

The wood boiler is in series with the oil boiler, and has about 40 feet of piping between them (one way) with a 8 foot drop (garage to basement).  There is one pump coming out of the wood boiler, another pump coming out of the oil boiler, and 3 pumps for 3 zones (in floor heating with long runs).  The system is pressurized to 18 lbs., flowing through the oil boiler, even though I don't have that burner turned on (only used if a back up is needed).  During normal running process, the wood boiler heats it's water tank, the initial pumps pushes the water down to the oil boiler where is flows directly through without any additional heating, then the oil boiler pump pushes the water to the zone manifold where 3 of the 5 zones have their own pumps.  The return from the 5 zones then comes to a manifold in the basement and goes up and back out to the wood boiler to start the process again. 

When the zones don't require heat, the water flow stops in the entire system and the water in the wood boiler continues to heat.  The wood boiler will eventually shut the fan blower off and will result in less wood burning, but the water continues to gradually heat up until a zone requires heat and the water starts to flow.  During early Fall and Spring when the time between zones needing heat is sometimes very long, the water in the wood boiler will get very hot and will "dump" into my overheat line, which is a waste of heat.

What I would like to do is to pipe a by pass just before the oil boiler pump to after the return manifold so if all the zones are off, the wood boiler pump will push the water through the oil boiler, through the by-pass, and back through the return so the water in the wood boiler will always circulate.  Hopefully the wood boiler pump will be strong enough to push is back up through the loop, but the flow doesn't need to be fast as long as I have some flow.  I attached a rough drawing.

My concern with this by-pass is when the zones do need heat, the some water will continue to flow through the by-pass instead of the zones.  To minimize this flow, I'm planning on using the small line for the by-pass and use a tee so most of the flow will go through the 1" pipe and straight versus right angle and smaller line.  I've though about installing a normally closed valve on the by-pass to prevent water flow during the heat cycle, but I was told that's another few hundred dollars in valve and controller.

Do you think the simple by-pass will work for continued circulation, and also not impact the heat cycle when heat is needed?  Thank you again for the inputs!!!

Dan

Ironman

Bypass

It looks like it should work fine. You could install a valve in it if you wanted to throttle the flow, but that shouldn't be necessary.



Since this wood boiler is within your house (garage), it should have a fail- safe dump zone. The reason for this is that solid fuel appliances can easily over-fire even with good control systems. This would most likely happen during a power failure. All your controls would cease to function as well as the circulators. The wood in the boiler would continue to smolder and give off heat even though the draft fan or damper is off. Without water circulation, the heat has no place to dissipate and the boiler becomes a potential bomb that could explode.



Two things should be installed at the wood boiler to avert this situation:



1. A properly sized pressure relief valve on the wood boiler (in addition to the one on the oil boiler) with its stem vertical. The one on the oil boiler would be useless in the above scenario.



2. A fail-safe dump zone. This can be accomplished with a buffer tank of sufficient size that must be correctly piped to the wood boiler. Or, an easier method is to use a 6 to 8 foot section of fintube baseboard, piped right above the wood boiler, with a normally open zone valve that closes when there's power and opens during power loss. Again, it's a normally open zone valve; that's the exact opposite of the standard zone valve used in a hydronic loop.



The attachment shows piping the dump zone into your house radiation, but if it's easier you can just add another piece above the wood boiler as I mentioned. The key is that it must be piped so as to induce gravity flow during a power outage.

Dan13

Response

Thank you for the advice and manual.  I have the dump valve already, and have tested it.  Also, I have a pressure relief valve on both boilers and both appear to be working.  I like your idea of the valve in the by-pass to control flow, just in case.

Thanks again!

Dan