backflow in oil/wood system
I am writing because I can't find a hvac guy who is willing to figure this out.
I have an oil furnace and wood boiler both tied into a hot water baseboard system. The wood boiler is properly installed with its own expansion tank, relief valve, "normally open" valve (for power outages), thermostat to trigger water circulation at a certain temperature, and manual valves to isolate it from the system.
Both systems operate perfectly on their own -- however, I am more interested in being able to have BOTH running than running one or the other exclusively. In other words, I would like to be able to burn the wood boiler, have it provide heat, and have the oil furnace kick in when the water temperature in the wood boiler drops below a certain temperature.
The equipment is all present for this to happen...and it does work, but the problem is that the water in the wood furnace can never drop below that of the oil furnace due to gravity feed/backflow. When the wood boiler cools down, hot water flows backward into the hot water output on the wood boiler. I'm assuming this is due to a temperature difference between the hot water coming out of the oil furnace and the relatively cooler water in the wood boiler.
At any rate the oil furnace has to do the unnecessary work of heating the wood boiler -- which totally defeats the purpose of running both for efficiency.
To make a long story short (TOO LATE!)...I was wondering if anyone has had this problem with an oil burner/wood boiler installation or might have a solution.
My first thought was to add a circulator on the return into the wood boiler that allowed circulation only when temps were reached. But would that stop the backflow? Can't water flow backward through a circulator?
Then I considered a backflow preventer on the wood supply line before it enters the supply coming out of the oil furnace. That might work...but I'm sure there are other ideas out there.
Anyhow, sorry for the long post. I'm not getting any help from the local HVAC guys, nor the original designer of the wood boiler. I know it can be done because I had an almost identical system in another house. I just can't recall what was different about it to prevent the backflow.
Can you supply a diagram showing how everything is connected, where the pumps are, the elevations of the two boiler inlets and outlets, etc.? That would help tremendously. Otherwise, we're all pretty much shooting in the dark.
Good idea...here's a diagram of the system. I've left off the domestic hot water loops for simplicity. Also, the return does not actually run below the level of the oil furnace, I just drew it there to avoid having to cross pipes in the diagram.
I spoke today with the company that made the wood boiler and they said it should originally have been installed with some type of check valve. It has been this way since 1978...I can't believe no one fixed it before.
So, now I am thinking that putting a flow check valve in the hot line out of the boiler (between the expansion tank and the valve near the oil furnace) would do the trick.
But I'd rather bounce the idea around before I drain the system and start cutting and sweating things on there. :)
How much heat are you really losing to the outside environment when the wood boiler is not fired up but the water in it is hot? Is this all in the basement? If the heat from the wood boiler is entering the house then does it really matter if it stays hot? I'm thinking you may want to repipe the system so the wood boiler and oil furnace are in series, with the return going into the wood boiler, the wood boiler feeding the oil furnace, and the supply going to the house from the oil furnace. If the wood boiler is burning hot the water just passes through the oil furnace. When the wood heat drops the oil furnace will kick in and boost the temp.
More work than just installing a check valve, but will give you more of what you really want, I think.
Your current setup has the two boilers in parallel. This can be problematic in dividing the flow between the two if you want to run both simultaneously. You might have a situation where most of the water goes through the oil furnace and so it does most of the heating, and less going through the wood furnace where, if not much water is going through it, you're wasting a lot of heat out the chimney. If most of the water goes through the wood boiler and the fire goes out, you'll be circulating cooler water mixed with the water heated by the oil furnace. See what I'm driving at? In parallel flow water, just like electricity, takes the path of least resistance.
Is the circulating pump triggered by the water temp or room temp? Room thermostat is what I presume.
Thanks for the thorough response!
The system is all in an insulated room in the basement. I'm sure the majority of the heat "lost" into the wood boiler stays in the house, but some of it definitely goes out the wood chimney, some gets sucked up as air supply for the oil furnace, and more radiates out the walls. Any uncessary loss is too much at $2.79/gallon (and rising) :)
Funny you should ask about the circulator. It is tripped by either the room thermostat OR the sensor in the wood boiler. This is really annoying since the system will produce heat even if the house temp is above the room thermostat setting.
Your "Series" piping idea makes sense. I would rather have the wood boiler run a secondary circulator that just sends water through the oil furnace, and have the oil furnace controls do the job of deciding when to send heat through the baseboards. This way the wood boiler would basically be doing the job of keeping the oil furnace hot...not necessarily acting as a primary heat source.
Obviously this would also require an "upper limit" setting to circulate the wood boiler through the baseboards to prevent overheating.
But I'm just thinking out loud at this point.
At this point I will just install the check valve to see if that stops the issue. I think it will at least reduce the problem enough for me to be happy this winter. But next spring I will probably re-pipe the system and add a secondary circulator.
I've been thinking about the "series" piping some more...and started having concerns about the water capacity of the wood boiler being added to the return. But the more I think about it, this really seems to make a lot of sense for my needs.
The series method actually still DOES heat the wood boiler (albeit with water returning from the boiler). BUT it is more likely that the return water will stay warmer in the well-insulated wood boiler than in the baseboards and pipes.
In other words, when the oil furnace starts pulling return water again it will actually be getting warmer water (from inside the wood boiler) to start with and will have to work a bit less to get it up to temperature. I think that this probably offsets much of the the loss up the chimney.
I have also added a manual damper in the wood boiler flue, so I can further reduce any loss up the chimney.
Also, the last benefit is that when I DO light a fire, the wood boiler will actually be preheated a bit.
So, anyway, I am really pleased I found this forum, and thank you dmaceld for this elegant solution. I hope that I can repay someone here someday!
Just a final update for anyone who might have stumbled across this with a similar problem.
Last week I re-piped this heating system using the suggestions from here and some slight modifications along the way. While the BTU savings might be minimal (I believe wood burning is more efficient than before), the result is that I now at least have options for heating that I didn't have before.
Here's a rundown of what I did:
First, I teed into the baseboard return line above the shutoff valve (#1) that is located just above the circulator. I ran this to a ball valve (#2), and then teed it into the wood boiler return line. There was an existing valve to isolate this line from the oil system (#3)
I then ran a line from the wood boiler hot line to a ball valve (#4), and then teed it into the baseboard return valve just below the shutoff valve (#1) but above the circulator. There was an existing valve (#5) that isolates the wood boiler's hot supply from the oil side.
There is also an existing valve on the return just before it goes into the oil furnace. (#6)
So here's how it works...
Primary Wood + Oil Backup: Valves #1 + #3 + #5 closed, and valves #2 + #4 + #6 open. Water returning from the baseboards is routed into the return (bottom) on the wood boiler, then out from the boiler to the circulator back into the oil furnace and back into the system. If the water in the oil furnace goes below 160 degrees the oil burner kicks on as necessary. This is how I run the system when I am only burning small fires a couple times a day.
Wood only: Same as above but with oil burner switched off (circulators still on). This is how I will run the system once I start keeping the fire going all the time. This prevents the oil furnace from running at all -- which at $3.00+ per gallon is very desirable! Of course, I've got to remember to switch on the oil when I'm leaving the house for a while. :)
Primary Oil + Wood Backup: Valves #2 + #4 closed, and #1 + #3 + #5 + #6 open. This runs the wood boiler in "passive" mode where it only circulates by gravity feed/hydronic flow...how it was installed originally. I honestly don't know why I would ever run it like this.
Oil Only: Valves #2, #4, #3 + #5 closed and #1 and #6. The wood boiler is completely isolated from the system.
Power Outage: With the power out, a 7th valve (electronic normally open) opens and allows gravity circulation directly to/from the baseboard loop. The hot water goes out through a dedicated pipe from this valve and returns through whichever return valve happens to be open. This takes place automatically, thankfully.
The weather here has turned cold (20's at night, 40s daytime) so I have been running it in the primary wood + oil backup configuration with great results!
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