Heat recovery from oil fired boiler

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Krikkit

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I was researching the possibility of recovering some heat from the exhaust of my oil fired boiler to heat domestic hot water. Came across this thread: https://terrylove.com/forums/index.php?threads/harvesting-waste-heat-from-my-water-heater.43640/ which talked of recovering heat from a gas fired water heater. (along with many off topic discussions on nuclear, wind, shower drain heat recovery and PV power)

Currently I have an oil fired boiler for space heating (6 zones) with an indirect water heater tank that has it's own circulator pump, the water tank is treated as an independant zone as far as the furnace is concerned. My boiler has a power vent in the exhaust which I believe would eliminate the issues of condensation and back draft brought up in the previous forum as the power vent pulls air through the stack.

My intention would be to put a coil inside the first foot of vent pipe, before the automatic draft vent, as this is the hottest point, after the auto draft vent the gases are cooled somewhat with the fresh air from the room. The coil would be attached to a water tank much like the range boiler that stood behind my grandparent's wood fired kitched range. (water circulated by convection) See attached pic. Cold water sitting in that tank is heated by the coil in the stack, outflow goes into the existing tank. One of the issues I would have to overcome is finding a flexible tube to link the coil to the tank, as the vent pipe has to be removed to clean the furnace once a year. I don't want to have to drain the tank and disconnect the coil to do this. Need a material flexible enough and able to tolerate that kind of heat.

Another thought that was also mentioned in the other thread is to put an uninsulated tank in the same room as the boiler, no connections to the boiler stack, just the water sitting in the tank would have the chill taken off it by the ambient temperature in the room. My boiler, oil tank and the indirect water heater are in a room separate from the rest of the house, fresh air comes in through a vent in the outer wall, exhaust is vented out by the power vent in the stack. When we bought the house this room was like a dry sauna. Since then I've insulated every pipe that I can access as well as the expansion tank. The room still stays in the +30C range (86 F), the water heater is set to 60C (140F) so that should make a considerable difference with mimimal cost would it not?

Something else that has come up as a possibility is to capture some of this room heat by inserting a long length of pipe in the cold water line before it gets to the existing tank. Maybe a full 100' length of 3/4 pex in a coil suspended from the ceiling, or some baseboard rad units (minus the housings) strung together. How much pipe would it take to see a difference in the temperature of the water?

Maybe someone has already tried one or the other of these. I encourage sharing of thoughts, and as much as I'm interested in other alternate sources of energy, please try to stay on topic.
 

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Dana

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By extracting heat from an oil burner exhaust stream you will be condensing highly corrosive acidic liquids out of the exhaust onto your heat exchanger, and possibly dribbling into the boiler itself. The very reason for the air dilution prior to the flue is to lower the dew point of the exhaust gas mixture to prevent condensation in the flue, which can destroy a masonry chimney in very few years. By cooling the exhaust prior to the dilution point you'll definitely get heavy condensation there (where the dew point of the exhaust is higher), but also in the flue.

Most of the heat in a sauna-like boiler room is standby losses from the boiler hot, not the flue. An un-insulated tempering tank in series with the indirect would save some energy, but at the risk of creating a legionella incubator, since 30C is in the sweet-spot temp for rapid legionella growth.

If you're looking to save oil, set up the boiler for cold-starting (if it's cold-start tolerant) or set it's low limit to the lowest condensation-safe level (typically 140F, to keep the exhaust from condensing on the heat exchanger plates when it fires up.) Controlling the boiler with a heat-purging indirect-tank-aware economizer like the Intellicon 3250 HW+ (a $150-200 piece of hardware to bypass some of the boiler controls, DIY-able for those with electrician skills) it probably a good bet here, since it results in lower average boiler temp, lower standby losses.
 

Tom Sawyer

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It is, as Dana said,a bad idea that would cost far more than you would ever save.
 
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