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Scavenging heat from an oil boiler

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Nofossil, Nov 10, 2007.

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  1. Nofossil

    Nofossil Moderator Emeritus

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    We use oil for heating our hot water when there isn't enough sunlight to do solar and it's not cold enough to use the wood boiler. It's always made me crazy that the boiler is still full of hot water and hot cast iron when it's done heating the hot water.

    I finally built a system that scavenges that leftover heat and uses it in a variety of ways. Here's a graph showing the process as well as a link to the writeup.

    As always comments and suggestions are welcome. If anyone is interested in more details about how to do this, please contact me.

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  2. elkimmeg

    elkimmeg Guest

    Interesting theory but what about removing heat from the boiler so that it x causes it to cycle to warm the stack I alway considered making boilers more efficient by holding the heat in and not extracting it I use a motorized damper to prevent heat escape up the chimney I also use a fire retentioner to burn the oil more efficiently I also change nozzels and nozel fflow rates and spray patterns to hit the optimum sweet spot finally T re gap the electrode slightly wider to create a hoter spark and a better more complete ignition of the oil

    What I'm saying I want to retain that residual heat so that I do not need as much next time to have to heat it i up more than usual You could be wasting energy and actually outsmarted yourself. Why not use real energy saving devices like the motorized damper?
  3. Nofossil

    Nofossil Moderator Emeritus

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    I use this boiler almost exclusively as a backup heat source for my indirect domestic hot water. When it runs, it's not going to run again for at least six hours, and more likely twelve or more. Even with a motorized damper, the boiler loses heat much faster than the hot water tank, hot tub, or storage tank.

    By dumping the leftover heat into the hot water tank, I also increase the time before the hot water tank will need heat again. I have a tempering valve so that the water at the tap is the same temperature even if the hot water tank is above the setpoint.

    Using the oil boiler is a last resort. When it happens, I want to do everything possible to make sure it won't happen again for as long as possible. Hopefully, the sun will shine or a wood fire will happen before there is a demand for heat again.
  4. cbrodsky

    cbrodsky Member

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    Neat page!

    I have contemplated something similar but haven't done so yet as I have heard varying stories about the stress placed on a boiler by cooling to basement temperatures. But given that it's regularly cycling at least 40 degrees, I'm not sure it's all that much worse.

    In our situation, I at most have two periods a day that I need to fire the boiler. In the mornings, sometimes we need it to run our bedroom zone when it's been a cold night. And since we have it on anyway, I use it to finish my solar HW up to temp. vs. using electric at the top of the solar tank. Then in the evenings, if I am out late after work, then I need it to come on and help maintain temperature as the woodstove output is declining after 12 hours.

    I know the standby losses are bad and would like to not have it running in standby all day - I'd ideally just have it shut down at ~8AM, come on if the temperature requires in the evening for a few hours, and then shut down overnight again until ~6AM when there is a call for heat by the thermostat.

    I think if I went that route, I'd probably just dump all the extra heat to my baseboards to at least leave the heat in the house rather than the basement - it would require a lot more complexity for me to dump that residual heat back into my solar tank, for example. I'd also be able to recover more heat this way - I could run the boiler back from ~150 to near house room temperature.

    Then it just comes down to how much heat that is. I think our peerless boiler holds about 12 gallons. So the recovered energy would be at best 12 gallons*8.3lbs/gallon*(150-75F) = 7470 BTUs of heat per cycle, or about 10,000 BTUs of fuel to make that. The cast iron would add a bit more, but the heat capacity is a fraction of water, so I suspect it is still a rather secondary factor. Twice a day, this might be a gallon per week of fuel.

    This compares against the raw standby losses which are somewhere around half a gallon per day in our house. By turning it off 2/3 of the day, I could save roughly 46300 BTUs of fuel in that case. I think for me, the more immediate payback may be shutting it down, and then if I did so, I could also try to pick up some of the residual heat through the baseboard later.

    I've not yet been 100% convinced those shutdowns are a good idea but curious if others are doing anything like that. Since oil/elec are getting to be about cost equivalent, I could now do a complete oil shutdown and use electric for water availability 24x7.

    -Colin
  5. Eric Johnson

    Eric Johnson Mod Emeritus

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    I have an old Weil McLain gas-fired boiler that I use as the distribution vessel for my wood system. The zone pumps (4) are all hanging off the return manifolds on the WM, so I don't have much choice. I shut the gas boiler down, turn off the gas, disconnect the power and plug up the chimney when we're using the wood. With the new gasifier and heat storage tank, I doubt that I'll fire up the gas much anymore except when we go on vacation. I should probably figure out some routine where I crank it up a couple times a year just to verify that it's still working.

    Keeping wood and fossil-fuel boilers and electric water heaters running in parallel is a fine idea, but I've never had any luck with it. Usually, for whatever reason, I'm set up for one or the other and trying to comingle the two always seems to produce unexpected results, like trying to heat your house with an electric water heater.
  6. Nofossil

    Nofossil Moderator Emeritus

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    My oil boiler doesn't 'standby' - if there's no demand, it cools all the way down to basement temperature eventually. I just help it get there more quickly.
  7. cbrodsky

    cbrodsky Member

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    What model is it, and how long have you operated it that way?

    -Colin
  8. Nofossil

    Nofossil Moderator Emeritus

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    It's a Burnham V-14A, and it's operated that way since it was new in 1989. We have indirect hot water, so there's no need for it to stay hot.
  9. JohnnyBravo

    JohnnyBravo New Member

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    so if it is in the basement, are you not geting the heat anyway? as it cools t heats the house, no?
  10. cbrodsky

    cbrodsky Member

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    I found a picture online - are these also cast iron boilers? How often does it fire it back on during the winter from a cold start? I assume you're also running longer burn cycles with it this way which is good for efficiency.

    Since you're quite into controls, here's another question you might know the answer to...

    One of my complaints w/the standard Honeywell aquastats are that they can short cycle easily. If there is a call for heat, it will fire the boiler unless it's above the high limit. I imagine this is to ensure the tankless coil can still supply water at the same time there is a demand for heat, but in our case, we aren't working the tankless coil very hard since the water is solar preheated.

    There is an aftermarket device (Beckett heat manager) designed to help reduce the short cycling, but it's not cheap - and is more elegant than what I need. I'd prefer to just make a poor man's version by letting the aquastat hi/lo settings dictate when the boiler is on or off, knowing the temperature may swing futher outside that range. In the end, I'd get less frequent, longer firing cycles which can help overall efficiency. Simply disconnecting the thermostat trigger into the aquastat doesn't work because then my zone valves don't respond to the thermostat signal anymore.

    As little as we use the boiler, I do like the idea of completely idling it more frequently when it's not really needed, but to keep my wife happy, it'll need to be automatic. Seems like today about every 2-3 hours, it runs for 3-5 mins even though I've had no heat demands on it. I think these run a gallon or so per hour of firing - which isn't cheap these days when you start adding it up.

    -Colin
  11. cbrodsky

    cbrodsky Member

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    Unfortunately no... it's an uninsulated full basement seperate from the living area.
  12. Nofossil

    Nofossil Moderator Emeritus

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    Wait a minute - I think that question was aimed at me....

    Some of the heat is lost up the chimney - I don't have an automatic damper on the oil boiler, so it draws air through itself and up the chimney. Second, we use it almost exclusively in the late spring through early fall to heat our indirect hot water tank, so leaking heat into the basement is at best no benefit, and at worst adds to the cost of air conditioning.

    When we go away during the winter, I turn off most of the scavenging logic - I still scavenge to the hot tub, because it won't get heat at all otherwise.
  13. Gooserider

    Gooserider Mod Emeritus

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    An oldy but goody, just found in the Green Room, but really think it's more appropriate for the Boiler Room, so I've moved it...

    This still the way you do it NF?

    Gooserider
  14. steam man

    steam man Minister of Fire

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    hey nofossil-ironically I have been driving myself nuts thinking about the same thing on my own boiler. Please don't take offense but what we're trying to do gave me a good laugh by making me think of a Rube Goldberg device. I recently installed an Intellidyne heat manager (they make them for Beckett). Got it on ebay <$100.00. My boiler is a Vaillant, maybe 6-7 gallons, cold start with an indirect. It will go to room temp except it is assumed with the indirect it will never reach that point for fear of condensation inside the boiler. As we discussed in another thread oil techs generally hate boilers that have gone cold to to the soot hardening and corrosion in the boiler so a low temp limit may be prudent in some cases. The heat manager does delay any burner firing if it sees heat available in the boiler and there is a demand for heat. I think it wants to see the temp down to 140 deg before allowing the burner on. Anything lower and the burner fires. It also looks at the rate of heat output to decide if the burner needs to fire sooner. It really does reduce cycling however I think there could be a sweet spot in determining nozzle size since the indirect heat exchanger can't absorb the heat as fast as the boiler makes it. I also think a boiler with more mass would benefit more. I have a monitor/timer and gallon useage device installed. I average about a 1.25 gal/day (at low outside temps) so if I get the 10-20% claimed reduction in fuel use I will be happy. I only heat DHW with wood right now and will be closer to zero once my solar panels go up this summer and boiler in for next winter. I have been monitoring the oil boiler temp closely and think maybe I could just put a an off-delay timer on any particular load zone to run its circulator for a certain amount of time to dump any excess heat from the boiler since the boiler may shut down with the temp on the high side. I know that Taco makes controls that do just that so it should work. The System 2000 boiler and I understand others now have controls available that do scavenging. I don't really notice the standby losses are that bad since my Reillo burner shutters closed when not in use and I don't have a stack (power vent) so there shouldn't be much chimney effect. I would note that I think an instantaneous water heat may be the simple answer for the "teenage girl" syndrome. No standby losses at all. But that would take all the fun out of it.
  15. Gooserider

    Gooserider Mod Emeritus

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    I have to say I'm somewhat confused by the whole boiler temp thing, ditto stack temps... :-S

    I get that having the combustion chamber cold (defined as less than ~250*F correct?) while firing is bad, as you get liquid condensing on the boiler walls, combining with other combustion byproducts to form acids that attack the firebox walls...

    What about the rest of the time? Is it OK or not to frequently do thermal cycling on a boiler, (either wood or fossil fuel) allowing it to cool, or even deliberately cooling it to near ambient temps and then firing it back up again on a fairly frequent basis (as opposed to once or twice a season) - does this adversely affect boiler life? If so how much? Does the energy recovered / saved this way balance against any possible reduced lifespan?

    What about the seldom-used backup boiler? Does it do any harm to a boiler to be sitting there most of the time full of cold water, presumably seldom fired but uncleaned?

    Another random thought - I understand why one wouldn't want stack temps to go below condensing temps if the stack is shooting straight up, so that any condensing liquid would drip back down into the firebox / heat exchanger, but what about a setup where you have a rear exit into a "T" ? Is there any reason why it wouldn't be a potentially good idea to do a second heat exchanger on the stack to try and get every last possible BTU out of the exhaust stream? :coolhmm:

    If the stack is SS, and feeds into a "T", then why couldn't you encourage the exhaust to condense and drip into the "T", where you had a drain?

    It would kill the draft on a naturally aspirated unit, but if you have a forced draft, do you really need the exhaust gas expansion to drive it out of the stack?

    In a low efficiency fire, it would make for major creosote buildup, but I thought the whole idea of a gasifier was no creosote because everything that was condensable gets burned? (I could see putting a delay on the stack heat exchanger, or maybe having a bypass to allow for startup smoke)

    The reason I'm wondering is seeing numbers like NF's 56% efficiency, and wondering if there wouldn't be a benefit in getting more heat out of the exhaust gas? Isn't that the whole secret of how they get those 90%+ numbers out of the latest mod-con fossil burners? Is there latent condensation heat in the exhaust we send up the stack that could be recovered?

    Just a thought for those more expert in the subject than I am.....

    Gooserider
  16. Redox

    Redox Minister of Fire

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    It would seem to me there is a tradeoff to every aspect of boiler operation. Most of the old school boiler guys I know woild shudder to think about thermally cycling a boiler this much. Most of them want to run it hot to keep condensation down and prolong the boiler's life. However, with the price of fuel being what it is, it is worth exploring the possibilities. I suspect that cycling a cast iron boiler multiple times a day is going to shorten its lifespan somewhat, but how much fuel can you save in that time? Might a steel boiler stand up better under these conditions? Inquiring minds want to know!

    Regarding the condensation issue and boilers, I might point out that the first applications for condensing burners were gas fired. There is a measurable benefit to condensing that steam in a secondary heat exchanger, and the corrosion is a manageable thing, usually. These were regarded with suspicion for many years, but time has proven the technology sound. I didn't hesitate when I had to replace the furnace in our home. It is really cool seeing an icicle hanging off the vent for the furnace; it really is that efficient! I haven't heard about a lot of condensing oil fired applications, as I suspect the conditions are much more severe. I can imagine a big mess if the burner were to ever go rich and fill the heat exchanger with carbon. Realizing the varying conditions that wood heaters have to operate under, I'm not going to be putting an external heat exchanger on the Quad any time soon, but the idea is interesting. It would seem to me that if the fire was burning clean and hot, you might be able to extract more heat from the flue, but you would want to have some kind of bypass in case the burn goes "bad". It would be interesting to see what condenses out of the flue stream, though. Maybe not creosote, but other things?

    Sorry to get OT. Just thinking outside the box again!

    Chris
  17. steam man

    steam man Minister of Fire

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    My experience with large marine boilers is that it is a no-no to let the stack get below app 250F due to the condensation in the stacks. We use a damper to bypass the heat scavenging air preheater to control this. Believe me, I have seen the large stacks rotted and ready to fall over from continuous low operating temps. The moisture is sulphuric acid and it is nasty. Boiler designers have allowed for the trade off of higher stack temps for the sake of longevity of the boilers. Interestingly, where the stack is exposed to the lowest temps, i.e. outside and high up, is where you see the corrosion starting.

    Small boilers are essentially the same but not as extreme due to better quality oil. The acidic moisture will eventually rot out the boiler and stack materials. The soot that builds up in the boiler and stack can turn rock hard and require difficult cleaning not to mention reducing the life span of boiler from the corrosion. I've seen pictures of an oil burner chimney almost plugged with rock hard deposits. Though I am not a home heating tech I know they especially hate to clean pin-style boilers that have operated this way.

    My take on it this is that the relative humidity of the boiler enviroment is a factor in the dew point temperature and maybe not just the combustion process. I have a cold start boiler that at times cycles way down to room temp and comes back up to temperature. I have a power venter with a short vent and typically see a 400 deg rise in the stack temp when the boiler runs. I haven't had any problems with it from cycling. It runs app 85% efficient and is 18 years old so the cycling issue is not a problem. I just inspected the power venter and noticed that the metal on the outside (lowest temps) is gone from corrosion and will need to be replaced this summer.

    As far as modulating gas boilers-yes the efficiency goes up into the 90% range but aren't they more expensive? Not too familiar with them but I believe the sulfur content in the NG is virtually nill compared to fuel oil hence less corrosion. Makes me think that low sulfur home heating oil is on its way. Its hard to say how the modulating oil burners will work as far as life span. With the materials available and paying a premium for them I am sure they will work. The higher oil goes the more we will see them.

    I've thought about trying to scavenge more heat out of my wood burning stack. We've talked turbulators here and scavenging. How much money do you want to put into it? I burn my wood burning masonary stack (35') with high temps and it still looks almost new after 18 years with a little cleaning.
  18. steam man

    steam man Minister of Fire

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    I just noticed my avatar picture and it made me laugh. I am squeezing into a large oil/gas burning boiler furnace. App 25 years old and still looks good.
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