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Community and Q&A

Should I go with chimney vent or power-vented water tank for cost savings?

AlanB4 | Posted in Energy Efficiency and Durability on

The current chimney vent tank needs replacing, its 8 years old, the anode was never replaced and its rusting internally 🙁

The current is a natural gas tank, 40gal, chimney vented, its a 6 inch outer diameter, and i have had no back drafting issues of any kind since the furnace was replaced with a high efficiency last year.

I assume there is warm air being lost all the time from the stack effect (though its in the basement which is not very warm), so if there is a $500 differential in price between power vent and chimney vent, would the power version (presuming it saves energy) save enough energy over a 10-15 year life to pay for the increased tank/installation cost?

Also would going from 40 top 50 gal make a energy efficiency difference (i would not use more water since i don’t drain it now, but just from a fuel cost perspective).

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  1. ranson | | #1

    If the chimney is inside your house, there's another consideration. You can run a power vent out a wall. If you stop using your chimney, you have the option of closing up the hole in your roof. That removes a large thermal bridge and potential leak location. Even if removing the whole chimney isn't in the works, it's very easy to just knock it down to below the roofline and patch the roof.

    My house had a pair of big brick chimneys, one venting the furnace, and one venting the water heater. By removing them, I got rid of two leaking roof pentrations and freed up a bunch of space on every floor.


  2. user-2890856 | | #2

    You will find that all newer water heaters whether they are atmospheric vent or direct vent are more expensive than you remember . They are also about 2" larger in diameter . Just because your heater has not backdrafted yet since the other appliance was replaced does not mean you shoudl ignore the fact that it could . In fact , it probably will because standing pilots are a thing of the past since the new Naeca rules went into effect this year .
    If you wanna install something worthwhile and save loads of money if you're going to stay with NG you should consider a modulating / condensing heater . They are pricey but the payback is not quite long . A stainless steel unit will require less maintenance also and last a good long time with minimal maintenance . If you store at 140* and use a mixing valve you increase the storage capacity and use less energy continually after first run . Differential can be maximized to make it the most efficient that it can be . These units should last well past the 10 year mark and probably longer if taken care of . Look at HTP Phoenix Light Duty , AO Smith Vertex has electronic problems that are a nuisance . The HTP unit can also be purchased from Westinghouse through Menards and other outlets .

  3. AlanB4 | | #3

    I would love advanced (and pricey) but my financial situation is cheap and low cost.
    I can't actually afford cheap, i need low cost and value (replacing something three times at half the price costs more then replacing it once with better quality)
    I'm located in Ontario Canada, if cost was not a consideration i would remove the chimney and replace with the tank Martin had posted a link on awhile back (phoenix with double pipe iirc)
    Direct vent is not easily found around here, so the choices seem to be chimney vent and power vent, and i do understand that indoor air is being sucked into the chimney, but both options use interior air for combustion.

  4. AlanB4 | | #4

    bump (hoping for Martin's advice)

  5. GBA Editor
    Martin Holladay | | #5

    I was scratching my head for a while at your first sentence, since I have no idea what a "chimney vent tank" is. I finally decided that you are probably talking about a water heater.

    No, I don't think that buying a fancy water heater will save you $500 in operating costs, especially if your fuel is cheap natural gas -- but the answer to your question depends on your fuel cost and the number of gallons of hot water you use every day, so there is no simple answer.

    For a thorough discussion of these issues, see The Water Heater Payoff -- also available here: The Water Heater Payoff.

  6. AlanB4 | | #6

    Its a hot water tank, and natural gas, which is about 21c/m3 at the moment. I am just wondering what kind of chimney heat loss (lower efficiency of water tank (?) and household heat loss) i can expect since i replaced the furnace with high efficiency so a powervent water heater would let me cap the chimney permanently but costs about $500 more then an equivalent chimney vent
    The house uses maybe 30-40 gal of hot water a day total.

  7. Dana1 | | #7

    If only the HW heater is vented into a chimney previously shared with a furnace you have what's known as an "orphaned water heater", which is problematic in several ways. It's FAR more than a parasitic heat loss problem!

    A flue that had been shared by a furnace or boiler is ridiculously oversized for the heat output of just the hot water heater, so the strength of the draft is much lower, and the temperature of the liner much lower. Under these circumstances:

    * Mildly acidic exhaust condenses on the liner all winter, rotting chimney mortar from the inside out and causing spalling damage of terra cotta flue liners (typically starting at the top, where the temperatures are coolest, and the condensation highest.)

    * There is a high risk of back-drafting/spillage of the water heater's exhaust into the house.

    To run an atmospherically drafted HW heater without these issues one could install a narrowing chimney liner appropriate the BTU-rating of the burner, thermally isolated from the old liner with blown rock-wool. Since a proper flue retrofit is almost always more expensive than the up-charge for a power vented hot water heater, most opt for the power vent.

    Sealing up the flue ports at the bottom is important, more important than sealing the top. If you seal only at the top you will get moisture condensation inside the flue over the winter, potentially resulting in freeze-thaw spalling of the brick at the top of the chimney. Seal off access to indoor air to the extent possible before capping it at the top.

  8. GBA Editor
    Martin Holladay | | #8

    More info on orphaned water heaters: When a Flue is Too Big.

    If you expect energy savings from abandoning your brick chimney, you have to think about the cost of addressing the chimney. A quick & dirty approach would be to air seal the cleanout door, air seal all of the thimbles, and install a weatherproof cap. A better job would involve demolishing the chimney down to a level below the top-floor ceiling, so that none of the chimney penetrates the thermal envelope. That work is expensive.

  9. AlanB4 | | #9

    So it seems the consensus is go with the powervent, however i have a metal chimney that runs up the middle of the house so its kept warm most of the way up and has no mortar to damage

  10. AlanB4 | | #10

    I put it into Hot2000 40 gal tank:
    Conventional tank pilot, 6in flue, natural gas
    Estimated Annual Space Heating Energy Consumption = 25256.38 kWh
    Estimated Annual DHW Heating Energy Consumption = 4615.30 kWh

    Then induced draft fan (non pilot, correct?)
    Estimated Annual Space Heating Energy Consumption = 24450.50 kWh
    Estimated Annual DHW Heating Energy Consumption = 4079.10 kWh

    So the savings are 785.88kWh in heating and 536.2 kWh in hot water, or 1322.08 / 10.5kWh/m3 of natural gas times $0.25m3 makes $31.48 savings per year (assuming the software is accurate)

  11. Dana1 | | #11

    It's not about the cost savings- it's about your indoor air quality/safety and the integrity of your house. A 6" flue even with a liner is ridiculously oversized for the heat output of an atmospheric drafted tank, and prone to backdrafting even if the chimney is full inside conditioned space for most of the run.

    With an oversized flue the flue velocity is so low that you'll have condensation in winter, which won't wreck a stainless steel liner, but WILL corrode an aluminum liner (or galvanized B-vent). For the 30-40,000 BTU/hr burners on most atmospheric drafted water heaters a 3" flue would be appropriate, and a 6" flue is more than 5x oversized for the BTU capacity. No matter how you might feel about building codes, you can't break the laws of physics. Without the input of the bigger burner on furnace to keep the liner hotter in winter, it's going to have problems properly drafting a water heater.

  12. AlanB4 | | #12

    @ Dana
    Somehow i have had no problems with physics, no backdrafting, and i have multiple monoxide detectors which have never gone off, and last winter we went about 15F below the design temp for weeks (approaching -40F) without any problems (except the 40% price increase on natural gas).

    I completely agree that better and more efficient technology is worth more money, my concern is that i have very limited dollars to work with, otherwise Martin had posted a double piped Phoenix(?) tank i was very impressed with and would jump at if it was available in Ontario and i had the money!

    I'll see what i can do, my 15ACH50 will cost lots of money to remedy but Hot2000 says $200 saved per year in heating costs, but i have to dig and encapsulate the crawlspace enough to reach the leaks, and who knows what that costs to fix, and i have to redo electrical before i can insulate the attic which only has some fiberglass between the rafters...

  13. GBA Editor
    Martin Holladay | | #13

    I'm not sure I understand your reference to Hot2000 -- are you saying that this software predicts that air sealing measures will save you $200 per year?

    I don't think I recommended the installation of a Phoenix unit.

    The savings that can be attributed to any particular method of heating domestic water are limited, because most North Americans don't spend much on the energy used to heat water. It sounds like your funds are limited. If you are happy with your atmospherically vented water heater, go ahead and install another one -- even though it's not the best approach. It's your house.

    Or just install an electric resistance water heater. That would be inexpensive, and it wouldn't have any backdrafting concerns.

  14. AlanB4 | | #14

    I got the house file from the auditor so i can play with upgrades. It says my heating load from 15ACH50 to 5ACH50 cuts the annual heating requirement by 6262kWh, at 10.5 kWh/m3 times 25 cents/m3 is $149.09. Again who knows how accurate the software is. The $200 must have been with the attic insulated.

    Perhaps the high air changes has allowed the tank to not backdraft, who knows.
    I do agree that powervent is probably the best choice, i'll see if i can make it happen.
    Thanks everyone

  15. Dana1 | | #15

    At 15ACH/50 you're at pretty low risk of backdrafting induced by exhaust fans, but you're not immune from wind-induced backdrafting.

    But just fer yuks, close all the windows & doors, turn on all of the kitchen & bath exhaust fans, turn on the dryer, and check the direction of air flow at the draft hood of the water heater. Before you turn all that stuff off, turn on the hot air furnace and check it again. If it's still drawing well under those conditions, your risks are pretty low, for now. As the house gets tightened up things will change.

    At air leakage rates of 15ACH/50 there is usually quite a bit of low-hanging fruit on the air sealing front. Even if 5ACH/50 may be expensive, 7ACH/50 might not be- it just depends. In your climate reducing the air leakage to under 10ACH/50 will make a significant comfort difference in winter due to the more comfortable humidity levels, and getting it to 5ACH/50 would be even better.

    Simply because none of the CO alarms went off doesn't mean you're not getting backdrafting. (Just because you're not dead yet isn't an indication that you're not mortal! :-) ) Carbon monoxide isn't produced in quantity unless the burner is seriously messed up. At the combustion-air /fuel ratios of an ~80% efficiency burner like an atmospherically drafted HW heater the CO levels are very low. But the PM 2.5, NOx and other gas burning exhaust components aren't healthy to be adding to your indoor air.

    The energy savings estimates from air sealing based on blower-door tested leakage numbers are little more than a WAG, with very large error bars (in both directions.) A blower door test only tells you how big the aggregate hole size is, but not where the holes are. The location of the holes relative to one another make a large difference. A single large hole in a wall has very little actual leakage, where as concentrations of holes a both the bottom and top of the house are subject to significant stack-effect drives, causing air leakage in far greater proportion to their size than holes of equal size half way up the walls. When attacking air leakage prioritize big holes first (any location), followed by sealing off both the bottom & top of the stack.

  16. AlanB4 | | #16

    I have no kitchen or washroom exhaust fans, but i will check it while doing the laundry. The furnace is high efficiency double piped (the old one was not), the installers got lazy and tried to talk me into one pipe (we have no more drill bits...) but i kiboshed that.

    I don't disagree with you at all, i would like to have low air leakage and double piped hot water (and heat pump dryer) but cost is a significant problem, I can't seal any more air leaks (was at 17.7ACH50 before) until i dig out the crawlspace enough to find the air leaks, i have been in there a few times and its too shallow to reach most of the walls :(

    I also understand that its not a perfect science determining air leakage heating loads, especially since software does not know where the leaks are just the total, but i do know that one of the heater ducts separated in the crawlspace and thats why it was warm all these years (back room was ice cold). I was able to remove the end piece and reach in and block the vent but i can't reach it from in the crawlspace to reconnect it, but since its been stopped up the crawlspace and floor above get ice cold in winter instead of the room temperature they used to be.

    According to the software (again assuming accuracy) before any upgrades the -20C design temp load was 54975btu/hr and 37303kWh/yr (heating).
    Since then i reduced the air leakage where i can, got the high efficiency furnace and filled the walls with cellulose reducing it to 44864btu/hr and 24337kWh/yr.
    If the water tank had not failed prematurely (and a tree shared with the neighbour didn't get damaged in storm recently) i was planning to dig out and encapsulate the crawlspace and seal all the air leaks i could find (hopefully reducing the air leakage by more then 5ACH50, during the blower door test a very high percentage of the air was coming from the crawlspace), but that may have to wait another year or so now.

  17. Dana1 | | #17

    Duct imbalances & or duct leakage are a far greater factor in pressurizing/depressuring parts of the house relative to the outdoors than whether the burner is sealed combustion/direct vented. The efficiency of the furnace is "in the noise" relative to the design & implementation of the ducts.

    For instance, with the separated duct in the crawlspace, what was the path the return air, and what affect did that have on the pressure of the room with the water heater relative to the great outdoors? It could very well be that the disconnected duct pressurized the space, preventing backdrafting the water heater while the air handler is running, but duct leakage in different locations can have the opposite effect. It's capped off at one end, but what is the other end doing when the air handler is running?

    To be sure, fixing the broken ducts and sealing the crawl space is going to have a much bigger utility cost benefit than anything discussed so far. The crawl space is (apparently) a large air leak at the very bottom of the stack-effect stack.

  18. AlanB4 | | #18

    @ Dana I'm assuming the sum of all internal pressure differences is zero since the indoors will equalize pressure with the outdoors, especially in a leaky house.
    That said, the water tank is in the concrete floor/brick wall basement, the crawlspace shares a wall with the basement and is not at all airtight, though the gaps between it and the basement/rest of the house add up to maybe half a cupboard door. The ratio of basement to crawlspace is probably 70:30 (basement:crawlspace). All ducts are indoors and the furnace is adjacent to the water tank.
    The furnace has a main trunk going into the crawlspace feeding 5 rooms (well 4 since i blocked the broken one) and the plenum has three 4 inch connections feeding other rooms.
    There are two air returns, one going into the crawlspace and leading to the room above (relatively tight, little to no crawlspace air suction) and the other sucking air out of the living room, there are no basement returns though the above the crawlspace one is only a few inches from the basement door.
    I don't think the crawlspace is being pressurized/depressurized very much by the ductwork now, but of course not being able to access all of it who knows, plugging the broken duct did change it from room temp warm to ice cold.

    The crawlspace is a royal headache, you had answered a previous question of mine saying that the dirt may be adding a couple pints of water in summer humidity to the air but the rest of the 20 pints/day was coming from air leakage.

  19. Expert Member
    Dana Dorsett | | #19

    The sum of all the pressures will NOT necessarily be equal relative to the flue stack pressure at the water heater, even when using the great outdoors as the return path. Even balanced duct systems create room-to-room pressure differentials that can't be completely nulled, and an unbalanced duct system will exaggerate those differences.

    Using the outdoors as the pressure equalizer for the heating system is both a huge energy problem, and not a reliable solution for the backdrafting potential, which still has to be measured both with the air handler running and while off.

  20. AlanB4 | | #20

    "Using the outdoors as the pressure equalizer for the heating system is both a huge energy problem, and not a reliable solution"
    I like that, its funny.
    This house is a poster child for how to not build a house, but i'm stuck with it so i can only do my best with what resources i have, i discovered even the wall between the basement/crawlspace is basically a chimney to the attic, since the house is balloon framed you can see the attic from the basement headers with a mirror. If i win the lottery i will either fix the house, sell it, or tear it down, you can guess which option i would favour.

  21. GBA Editor
    Martin Holladay | | #21

    You wrote, "you can see the attic from the basement headers with a mirror."

    If you have enough access to stick a mirror in that location, you have enough access to try to stuff the air channel with something. Even a piece of fiberglass insulation in a garbage bag will improve the situation.

  22. AlanB4 | | #22

    Fiberglass in a plastic bag, never heard of that one! :)
    I was planning on screwing in a piece of wood and spray foaming the edges, the top of it is an interior wall in the attic (addition is lower then the original house) so i will need to handle that at some point as well
    I was thinking if i block the top and bottom i could fill it with cellulose, not sure if there is any rationale for doing so (except maybe as a mouse deterrent).
    I have not measured the radon levels in the house yet, again quoting Dana "Using the outdoors as the pressure equalizer... (is) ...not a reliable solution" but with 300 sq ft of dirt generations of owners may only be alive because of this (also a roof leak is being saved from wrecking the house because of the dirt, i accidentally discovered when it rains water goes from the plumbing stack protrusion all the way down to the crawlspace and is absorbed into the ground, something else that needs fixing that i can't afford.
    Did i mention the house basement header is at ground level on one side of the house, they came up with an interesting solution but missed a corner and part of the header has rotted away and i have not solved that one yet, i'll have to post a pic as another question, maybe tomorrow.

  23. GBA Editor
    Martin Holladay | | #23

    The "fiberglass in a plastic bag" approach is an old trick used by weatherization workers. It's a quick-and-dirty solution that isn't ideal -- it still allows air leakage, but it cuts down on the airflow if the situation is egregious, and there isn't enough time or good access to do a better job of air sealing.

    For better solutions, see my article, Air Sealing an Attic.

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