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Flue heat recovery / economizer - Oil

jeepmanblair Member Posts: 4
edited December 2020 in Oil Heating

I have a oil fired hot water tank (Bradford white tank & beckett burner) that heats domestic hot water and also feeds a hydronic air handler for house heating. I'm in northern Canada and it gets very cold here! When the scales meet at -40 that burner runs ALOT!!! I've done some reading on flue gas heat recovery and am aware most people (professionals) strongly advise against it due to the condensation issues that can arise. Regardless, i would like some more information before I can put this out of my head. The tank vents into a 7" selkirk style chimney that is around 18' long and runs outside the house inside an insulated chase.

I've mounted a temperature probe just above the tank (in the breech) and inside at the top of the chimney (about a foot down from the rain cap).

I'm getting around 640F in the breech and around 220F at the cap.

I just had a contractor out to test the system:
CO2 9.2%
O2 8.5%
CO 0
Flue 320C
Inlet 14.5C
Nett 305.5C
Eff 76.1%
Loss 23.9%
Xair 68.3%
He did a smoke test and there was basically none.

I had just changed the nozzle and cleaned the inside of the tank "flue" so all he did was dial the barometric damper to achieve around -0.04" draft. I have since bought the same Dwyer draft gauge he had. The chimney draws around -0.07" while firing with the damper closed at around -10C. Prior to the adjustment the damper hardly ever moved now it is almost wide open while firing. The side effect is the utility room was usually pretty warm, now the excess heat is being pulled up the chimney. If my understanding is correct, more of the burner heat should be working to heat water rather then getting sucked up the chimney with the damper set correctly.

Of the posts I've read, the condensation point of the flue gas varies from as low as 136F to between 350-400F. Seeing as my flue gas is only 220F at the cap and i don't have any issues, it must be lower than that. It is also my understanding that the room air added to the flue gas by the barometric damper dilutes the flue gas and lowers the condensation point.

I would really like to pull as much heat as possible from the flue gas without causing issues. What I'm thinking of doing is using a Magic Heat exchanger and running the fan from a variable speed temperature controller that reads the flue temp at the top of the chimney and keeps it above the condensation point.

Now to my question: How do i determine the condensation point? If it is around 140F I could pull around 80F out of the flue gas and add it to my house!!!


  • Intplm.
    Intplm. Member Posts: 2,009
    Selkirk. I truly like their chimney selections.
    Is your Selkirk chimney stainless steel and cement-lined? Or lined with another filler?
    You seem to have all of your questions correctly at the ready.
    What I would do is contact Selkirk technical support directly.
    They should ( hopefully ) be able to give you some [ SAFE ] info on heat transfer.
  • jeepmanblair
    jeepmanblair Member Posts: 4
    edited December 2020
    It's a stainless steel inner and outer with a fibrous refractory type insulation, like rock wool. 
  • Intplm.
    Intplm. Member Posts: 2,009
    That's good. Great actually. I don't know if you can get much better than that.
    Contact Selkirk to see if they can give you what you are asking for with safe, non-hazardous flue gas and condensate issues.
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    First you are using a water heater to heat domestic and provide some space heating. That's not the proper use of that appliance and voids the warranty. It's just not going to give you want you want as far as heat.
    Please use the water heater for heating domestic water, and get something else for space heating.
    Or get a boiler that is made to do both.

    Hopefully you have the domestic water separated from the hydronic water.

    You post is kind of hard to follow as you as I think all your draft measurements are negative.
    What are you calling an 'economizer'? One of those things you put on the flue pipe to extract heat?
    Oil-fired water heaters run with high flue temps, not much you can do about it. And the draft at the breech needs to be in that -.04--.06 range for the burner to operate properly.

    Extracting heat from the flue is not a good idea, especially as I don't really know of any type of control you can put at the top of the chimney to monitor temps to control your fan. You could just use the onboard controls of the fan, and check it against the temp you ultimately get at the top of the chimney.
    But you're going to kill that water heater trying to do space heating with it. And any damage is on you for not using it properly per the manufacturer.

    There was an error rendering this rich post.

  • Jamie Hall
    Jamie Hall Member Posts: 23,447
    To all of which, especially @STEVEusaPA 's comments, may I add one more?

    Whatever you do, do NOT allow that stack gas to condense. It may right at the beginning of a firing, but no more than perhaps a minute, if that. While the condensed flue gas from a gas burner is pretty innocuous, relatively speaking -- basically nitric acid with some carbonic -- which can be handled by a chimney such as yours or, if cool enough, some plastics, the condensed flue gas from an oil burner contains -- even if it is ultra low sulphur -- sulphuric acid as well. There are a few high temperature glazed ceramics -- and glass itself -- which can resist that, and that's about it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • jeepmanblair
    jeepmanblair Member Posts: 4
    I have not touched the heating system since I bought the house 10 years ago. It is the same loop that does space heating and domestic hot water. It's an all non-ferrous loop inside the air handler. A bronze taco pump, copper radiator and all copper piping. Judging by how much the thing runs in the winter I have to agree with you that it can't be good for it.

    As far as the economizer goes, I would say the high flu temps of the hot water heater make it the perfect candidate for recouping some heat.

    My draft measurements are all negative. I will edit the post.
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    While I will say that what you want to do is a novel idea, and will work to extract the extremely high temp flue gasses.....

    The problem is that:

    A water heater is not efficient at heat transfer

    Spend your money on a good boiler, then you will have a flue temp in the 300F to 350F range. You have done a great job analyzing this, surely you know that is a better investment. 

    Keep in mind that your water heater is likely ready to fail with that much use anyway. Glass lined tanks with a constant burner running in an open system... will fail. 

    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • 426hemi
    426hemi Member Posts: 79
    edited December 2020
    This sounds like a terrible heating setup you would be better off with a oil furnace for heat anyway. We had stack economisers on a boat I was on the worst piles of **** I ever seen we had stack fires all the time, cleaning them was worst job ever the deck was always covered in oily sticky black carbon afterwards, they didn’t save any measurable amount for fuel and keep in mind they were on rather large diesels EMD 12-645 turbos with a 600-700 deg stack temp. The amount of heat your going to recover from a residential oil boiler stack is not going to be a measurable amount. If your trying to save money you would be better off spending the money on insulting the house and maybe not heating it as hot I can’t understand why people heat there house to over 60 deg to me this is very unpleasant I can’t sleep in a room that’s over 60deg I wake up in a puddle of sweat!