Why don't gas water heaters have dampers to reduce standby losses?

Discussion in 'Water Heater Forum, Tanks' started by gellfex, Jun 20, 2012.

  1. gellfex

    gellfex New Member

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    I tried searching, and came up with nothing here, and google got a reference to an aborted non-electric product from the mid 90's http://www.homeenergy.org/show/article/id/1319. Is it only that the standard gas model has no electric components to control a flue a damper? surely that would be easy to make as a option, to make it run like a boiler that opens it's damper before firing.
  2. Terry

    Terry Administrator Staff Member

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  3. gellfex

    gellfex New Member

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    Thanks Terry. That looks great, but has quite a price premium over a standard tank. It's too bad there's no retrofit device. I use a water heater to provide hydronic & domestic for my little rentals, and wish it were more efficient, but I don't actually pay the bills (the average monthly total energy bill is only $60-70). I would get no capital recovery from investing in 5 of those, That's half the reason I didn't do a condensing tankless, the other half is where the wall vent would go is too close to the bathroom window.
    Last edited: Jun 20, 2012
  4. hj

    hj Moderator & Master Plumber Staff Member

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    The problem with a mechanical damper is that it is not "instantaneous", because the bimetal element has to heat up, and during that time the burner is backdrafting its combustion products into the room. With the new FVIR systems, that backflow would probably "false trip" the mechanism and require resetting.
  5. Dana

    Dana In the trades

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    By adding the space heating load, it IS more efficient than the EF rating of the beast. The steady-state thermal efficiency of an atmospheric-drafted tank type hot water heater is ~78-80%, and the more load you add, the higher the duty cycle, and the higher the average efficiency. The standby losses only occur while it's standing-by, and the less time it spends in standby mode, the lower that loss ultimately is. If typical use in hot-water heating mode is 250 therms/year, and you're adding ~500therms/year for space heating, during the heating season the total standby time shrinks dramatically- probably by half.


    The net efficiency improvement of thermal flue dampers in hot-water only applications is only about 4% . During the heating season the duty cycle of the burner is likely tripled or quadrupled, reducing the standby loss by more than half already, and reducing the net benefit of the damper. Bang for buck you're probably still better off spending the money on 5/8" wall closed cell pipe insulation for at least all the near-tank plumbing (including the 10' of cold-feed closest to the tank, and the temperature & pressure valve+ overflow plumbing), and any hot water distribution plumbing that is readily accessible. The near-tank plumbing insulation reduces the standby losses by high single-digit percentages, and insulating the distribution plumbing decreases the amount of heat going down the drain as tepid-water on short draws by keeping the temp in the pipes high enough to be useful for ~20-30 minutes, thus not fully abandoning the heat between subsequent short-draws. See: http://www.leaningpinesoftware.com/hot_water_pipes_pipe_cooling.shtml

    [​IMG]

    There were a number of retrofit thermally actuated dampers available in the '90s, some of which may still be around if you dig hard enough, eg:

    http://flairproducts.net/pdfs/StackPackST46specsheet.pdf

    (I picked up a similar unit for a wider flue on **** a half-dozen years ago to use on an atmospherically-drafted tankless.)

    Beyond what it does for the efficiency of the hot water heater, the damper reduces air-infiltration drives by minimizing the cross sectional area of the open flue when not in use. This has a (very) low single-digit percentage reduction on the total air-conditioning and heating loads too, but it's harder to measure. But there are often bigger, cheaper to fix air leaks in a typical home or apartment that have a larger net effect.
  6. gellfex

    gellfex New Member

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    Really interesting post. I've been at war with standby losses on my own indirect, I've blanketed it, insulated with 3/8 poly and used both loops and traps nipples to cut the standby. My next project is to put in a programmable thermostat that has a wider hysteresis than the stock Triangle one http://www.amazon.com/RANCO-ETC-112000-000-Digital-Temperature-Control/dp/B000LDF4Y8 . Despite my efforts it keeps cycling the boiler on summer days with no HW demand at all.

    The part I'm not getting about the vent losses is wouldn't the losses up the pipe be the same whether it's used for heat or not? is it just the loss proportional to the higher BTU's burned that makes it more efficient?
  7. Dana

    Dana In the trades

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    When the flue is actively burning, there's no standby loss, but that flue loss accrues to it's raw combustion efficiency- it's not a standby loss. When standing-by the flue temp is much lower and it's losing far less heat per minute, but the center-flue heat exchanger is still creating a convection force pulling cold air in at the bottom, passing warm air out the top.

    But with the added duty cycle of space heating comes lower absolute standby loss:

    Just for yuks, say the burner the tank is 35KBTU/hr in, at 80% efficiency it's delivering 28KBTU/hr to the water in the tank, the other 20% is going up the flue.

    Let's say the design condition heat load of the small rental is about 15KBTU/hr, give or take. The average winter load tends to be about half the design condition load, so you're looking a 7-8KBTU for an average load, which is (7/28=) 25% duty cycle on the burner. An 80% (steady-state) atmospheric-drafted burner running at 25% duty cycle will be delivering more than 75% efficiency.

    When in hot-water only mode the duty cycle on the burner is about 4-5%, call it 5%, so add another 5% to the on-time duty cycle, you're at 30%, that's usually good for about 78% average efficiency.

    The standby period has gone from being 95% of the time to 70% of the time, with active burn time rising from 5% to 30%. This reduces the absolute standby loss, since the standby TIME has shrunk from 3420 minutes/day to 2520 minutes/day, a ~26% reduction in standby time (and standby loss). You still have ~74% of the original standby loss, but the magnitude of that loss is being distributed over more fuel use- the as-used efficiency numbers continue to climb as the on-cycle climbs. The absolute standby loss is actually smaller by a good bit, but the loss as a fraction of the total fuel-us is much lower. So while it's the proportional loss that's responsible for most of the efficiency uptick, the absolute number does come down.

    Still, flue losses are only about half the total standby loss of a typical gas-fired tank heater. Insulating the plumbing (and sometimes additional tank insulation) is economic, even at buck-a-therm gas prices.

    With indirects it's important to insulate the boiler loop plumbing as well as the potable plumbing to cut standby loss. Heat traps stop convection (or at least should, if they're working correctly), but that doesn't touch the conducted loss up the pipe, which is still substantial. The sidewall insulation on most indirects is pretty good, and most of the loss is plumbing related. You could triple the R-value of the tank and it won't affect the heat loss up the plumbing. The cheap 3/8" closed cell foam pipe insulation is only good for R1.5-R2- (better than nothing, but not huge.) It's worth getting 5/8" or 3/4" wall goods, which will cut that portion of the loss in half.

    How often does the boiler actually cycle just for temperature-maintenance burns? What's the storage temp? If you've been keeping it at 180F (to keep up during winter demand with colder incoming water) you may be just fine in summer with 130-140F water which would inherently cut the standby loss by more than a third.

    If you're a showering (rather than tub-bathing) family, a decently sized drainwater heat recovery heat exchanger would add enough apparent capacity to keep it at 130-140F (or lower) even in winter. (My entire system runs on 130F water, and with a 4" x 48" DWHR we can still take pretty much endless 105F showers with less than 50KBTU/hr of boiler output, even in January.) As long as the indirect has about the same capacity as your largest tub you'd be able to handle tub-fills with a 130-140F storage temp too, but there would be a real recovery time after a tub fill, and would get zero back from the heat exchanger (since for the heat exchange it needs concurrent drain & potable flow.)
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    For a very long time, people were only interested in the cost to buy a WH, not its operating costs as energy was relatively cheap (and still is in the USA compared to many other places). Only when the government got involved and prices started going up was efficiency an issue. A damper costs more, adds (some, but not much) complexity, and may involve additional interlocks for a not huge efficiency gain. If you want a real efficiency gain, a closed combustion burner may end up better - you won't be using already conditioned air and sending it up the flue while pulling in exterior air which then needs to be conditioned.

    Retrofitting has its pitfalls...if it doesn't open when needed, you could end up dead or sick.
  9. Dana

    Dana In the trades

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    All of the thermally operated versions had more than adequate flue opening in the "closed" position to handle standing pilots, etc. and have proven over time to be very reliable (a much longer mtbf than gas fired hot water heaters), but haven't been the greatest economic investment for gas fired equipment, which is probably why all of those products disappeared from the market. (There's a much better financial argument for propane-fired heaters though.) It's nowhere near the ROI of R4 pipe insulation.

    Retrofitting the damper above the dilution hood has a much lower uptick on efficiency than at the output of the tank flue, since mounting before the dilution hood slows both the convective loss through the flue and the idling stack loss to the outdoors. But it's easier to mount it in the stack, since you don't have to rig a method of mounting the dilution hood properly at the top edge of the damper. Most I've seen (a very small sample indeed) were inserted into the stack, above the dilution hood. I've only seen one mounted below the dilution hood.

    For most showering households retrofitting drainwater heat recovery heat exchanger has a larger apparent net efficiency improvement than moving to a tankless (even a condensing tankless), usually at a lower upfront cost. It doesn't cut the standby loss one bit, but it dramatically cuts the amount of heat that literally runs down the drain, which is a much bigger number than the standby loss. OTOH if the recovered heat rate makes it possible for your teenager to take 40 minute showers it may add to the total fuel use. ;-)
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