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Community and Q&A

Indirect Water Heater Vs. Electric Resistance Preheater

NBABUCKS1 | Posted in General Questions on

Quick question regarding overall efficiency.

My incoming domestic water is around 40 degrees F and my boiler is unable to heat it up to a temperature that is warm enough for showers. I was thinking of two options.

1. Installing a electric resistance 30 gallon preheater. This would preheat the incoming water to around 60 degrees or so before it was fed into the boiler to bring it upto temp for showers.

2. Install an indirect hot water heater tank.

Both of these options would be in my insulated crawl space but not in a conditioned room in my crawl space. Does my idea make any sense or a really bad way of looking at it?

My current rates

kWh = .15
Fuel Oil: $4.15 gallon


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  1. GBA Editor
    Martin Holladay | | #1

    What type of boiler do you have?

    How big is your indirect tank?

    What type of heat exchanger do you have?

    Why can't your boiler keep up? If you have a flat-plate heat exchanger, I'm guessing that your heat exchanger is getting clogged with minerals and needs to be flushed and cleaned.

  2. NBABUCKS1 | | #2

    I have the quietside QXM-8

    Unfortunately I don't know the answer to all those questions.

    I do not have an indirect tank yet, I am trying to figure out what is the most economical choice.

    Per the literature regarding heat exchanger:

    Both the Heating Heat exchanger and the DHW
    Heat Exchangers are made from 444 Stainless Steel.
    The DHW HX is located inside the main Heat Exchanger

    More reading yields a plate heat exchanger. Any idea how I could clean out the mineral deposits?

  3. Expert Member
    Dana Dorsett | | #3

    Descaling embedded tankless hot water coils only works if the scale-up is on the potable side (often is, usually accompanied by a bigger pressure drop/lower flow due to the scaling.) If it's all grunged-up on the boiler-water side of the coil you might get some performance improvement out of a clean-up, but it's usually toast. If it's plumbed with isolating ball valves and ports to make descaling easy, go for it, but keep expectations low.

    But in fact, even on day-1 the hot water performance of embedded coils are usually fairly pathetic. And the lower you set the low-temp limit on the boiler, the crummier the heat transfer rates are. Most residential boilers with tankless coils need at least a 160F low limit to handle a shower flow, and if the burner size is under 120,000BTU/hr even then it won't keep up if the space heating thermostat is calling for heat.

    Plumbing an electric tank in series works better if you plumb it as the boiler pre-heating the water to the tank rather than the tank pre-heating to the boiler. Electric HW heaters are far better insulated than boilers, and will have much lower standby loss even at high storage temps. If you set it up with the tank set to 120F, even if the boiler is delivering a tepid 95F at shower flows, that's still more than half the heat coming from the boiler, which is what you want during the heating season. During lower flow rates the boiler feeds 120-140F water to the tank and the electric elements only fire up when the flow is faster than the boiler's coil can keep up with. T

    In that configuration you can then turn the boiler off in summer and avoid the (very substantial) standby losses of the boiler. The thermal mass of the ~70F boiler at 70F will do some pre-heating for the water, but your incoming water temps will also be much warmer.

    Plumbed that way you can also lower the low-limit on the boiler to something like 140F (consult the manufacturer before going lower than 140F) even during the heating season, which lowers it's standby losses, increasing net efficiency. The thermal mass of the hot water heater buffers the instantaneous high loads such as showers or tub-fills, but the boiler is still providing most of the heat to the incoming water even then.

    An indirect would be more efficient during the heating season, but would do no better than about 50% efficiency in summer, even if you were cold-starting the boiler and using heat-purging boiler controls. A 0.9 EF electric tank at 15 cents/kwh gives delivers 3070BTU of net water heating for 15 cents, or:

    ~20,466 BTU/$.

    An oil boiler running 50% efficiency in water-heating-only mode with $4.15 oil delivers ~69,000BTU for $4.15 is

    16,625 BTU/$.

    That means heating water with the boiler in summer is 20-25% more expensive than just a straight electric tank solution, so just turn it off. But during the heating season the boiler's combined space-heating/water-heating efficiency will be more like 75% or better, and the economics tip the other way, and it's better to get as much of the heat out of the boiler as you can, rather than off the power grid.

    For an analysis of the winter/summer net water heating efficiencies of a number of different indirect vs. embedded coil situations see:

    Look at table 2, page 7 (p.12 in .pdf pagination). Only system 3 and system 5 break out of the 50% range by any margin. System #3 is an Energy Kinetics System 2000, which is a highly engineered combination, not a retrofit indirect. If you're a hydronic design genius you can get somewhat close to that with an indirect using heat-purging economizer controls like the Intellicon 3250 HW+ on a pretty-good cold-start oil boiler, but in reality most would be doing well to hit 50%, even with the better controls.

  4. Expert Member
    Dana Dorsett | | #4

    According to the supplemental sheet ( the QXM8 120 should be able to deliver 3.4gpm @ 105F with 40F incoming water, which is a pretty-good shower flow (2x the output of low-flow heads).

    The QXM8 150 should be good for 4.4gpm which is an old-school gusher of a shower.

    The QXM8s have sophisticated heat purging controls, but not a lot of stored thermal mass (5 gallons of water), still, even the smaller 120K burner should be able to keep up with a single shower flow. A 2.5gpm flow with a 65F temperature rise is 81,250 BTU/hr, and an 85% efficiency 120K burner is delivering 102,000 BTU/hr.

    So, if the controls are correctly sensing the DHW load and inhibiting space heating calls and the thing is still not keeping up with a shower, it means you either have scaling on the heat exchanger or a higher flow shower. Descaling the potable side involves pumping a dilute acid solution through the HX for an hour or so with a small pump and a 5 gallon bucket (white vinegar at 4/1 dilution is commonly used, but there are also commercial purpose-formulated solutions- google it.) This is usually only an issue in hard-water areas, and it might be wise to install a water softener ahead of the boiler if the water is hard enough to scale it into poor DHW performance in less than a year.

    The summertime water heating efficiency of this boiler is an open question, since it is better insulated and comes with smarter controls than old-school cast iron. But with only 5 gallons of thermal mass to play with it won't be anywhere near the 85% steady state efficiency. It might beat 60% though, which would make it something of a wash, relative to heating with resistance electricity in summer.

  5. user-968917 | | #5

    I would also consider (if it is feasible and fits your plumbing stack) installing a drain water heat recovery unit. It "should" raise your incoming water temperature considerably and "should" allow for a substantially better output from your boiler. Of course there will be a lag before steady state is achieved and of course this only works for showers and simultaneous flows...

    Don't want to hijack this thread, but....Dana: my expertise is 99% forced air and I'm working with a client who wants to replace an old 1950's boiler (and original cast iron rads) with a HE boiler in a 1911 2 1/2 story house with a design heat loss of approx 100,000 BTUs. He's considering indirect DHW. I've advised on making all other envelope improvements first to allow for lower water temperatures and smaller unit, discussed adding hydronics to accessible floors to further decrease required water temperatures, and preached the importance of outdoor reset to maximize or achieve condensing efficiencies, but have little experience on indirect DHW.

    What is your opinion on condensing natural gas boilers and indirect DHW in a very cold climate?

    Low Mass or High Mass?

    Best type of DHW indirect tank?

    What kind of DHW efficiencies would you expect in an optimally selected and installed/set-up system?

    FYI, in my area natural gas is currently $0.23 a cubic metre and electricity $0.07 kWh, making resistance tank DHW attractive....



    PS: Martin, this would be a great topic for future articles. I know you've made reference to indirect in your previous articles on DHW choices, including a mention of some dismal data on very low indirect DHW efficiencies (perhaps this was for non condensing oil?). I've tried doing online research on this topic and opinions appear to be all over the place...Consider this to be an official article request!

  6. GBA Editor
    Martin Holladay | | #6

    Thanks for the suggestion. I'm working on an upcoming article right now for Fine Homebuilding on water heater payback -- I hope to address indirect water heaters in the article.

    This isn't an issue of interest to many Americans, however, since so few Americans have boilers. It's easy to forget that fact if you live in northern New England. When you leave this area, you discover that furnaces rule.

  7. Expert Member
    Dana Dorsett | | #7

    I'm a huge fan of drainwater heat recovery, but not as "the solution" for a scaled up heat exchanger!

    It's only by virtue of a ~50% efficiency heat recovery unit that I get away with my micro-zoned heating system centered around a 48 gallon buffer tank at 130F, with internal HX for the domestic hot water. The modulating boiler's output never exceeds ~60-65K, even when someone's in the shower with ALL zones calling for heat, and that's with sub-40F incoming water at mid-winter. Without the drainwater heat exchanger I'd probably be in marriage counseling over it, (or have to crank up the system temp into a non-condensing range.) :-) The only time the system comes up a bit short is on mid-winter tub fills if it's filling during space heating calls, but you can literally shower all day, and never hit 70KBTU/hr on boiler output. (Trust me, my kid had TRIED! But that's where setting the time out on the bathroom's motion-sensor light switch come into play, limiting the "endless" shower to daylight hours. :-) )

    Low mass modulating condensing boilers under outdoor reset control and high mass radiation are a match made in heaven! Even right-sized radiators designed for 200F water in the 1911 version of the house won't need more than 180F after some air sealing, insulation, and storm windows, and then only at the temperature extremes. Most of the time the temp requirements will be much lower, and at about 140F output temp the return water is usually into the condensing zone for 90%+ efficiency. Even if it's divided up into a few zones (may be necessary, at least by floor) the high mass of the system water keeps it from short cycling when running at lower temp, and the output of the radiators is at or below the minimum fire output of the boiler, but going with the smallest boiler that actually meets the design heat load is still key to keeping it happy with nice long burns even at low temp.

    The hydronic systems from HELL are the ones where they micro-zone the piss out the place and replace the radiators with undersized sticks of fin-tube baseboard, then oversize the boiler by 2-3x for the design condition load AND the radiation, creating a condition where the boiler is either short-cycling itself into an early grave and lower efficiency, and can only get decent burn lengths at higher-than condensing temps when more than half the zones are simultaneously calling for heat. (That seems to be common practice in some places...) Being really AGGRESSIVE on the heat load calculations, taking into account not just the construction U-factors, but all other heat sources like plug loads, occupancy (1 sleeping human = 250BTU/hr,), and other insulating factors like drapes/shutters/curtains all count, then resist the urge to upsize it from there. Installing a boiler with 100K output for an actual heat load that works out to 65K is more of a problem to be designed around than a solution for anything.

    Using a mod-con boiler and an indirect is about as efficient as condensing tankless heaters if you set it up right, and can be bit MORE efficient. Controlling the indirect as the "priority" zone (suppressing other zone calls when the indirect is calling), sizing the tank for the biggest tub you have to fill, and keeping it at a sub-130F storage temp is usually the right approach, so that the boiler is at a condensing temp for most of (if not all of) the recovery burn. Run as a priority zone the boiler runs full-out when heating hot water, which cuts into combustion efficiency a bit, and as the return temps from the heat exchanger inside the indirect exceed ~125F the combustion efficiency drops below 90%, but it's still north of 80%, even at the end of a burn. Unlike a condensing tankless heater, it will NEVER short-cycle on a call for heat from the tank, due to the high thermal mass of the water volume in the tank. If you insulate all of the near-boiler and near-tank plumbing the standby losses of an indirect are usually similar to those of an electric HW heater, which is pretty small. In practice, with all of the plumbing insulated to limit standby losses a mod-con + indirect will run between 75-85% efficiency, depending on amount of hot water used. Without insulating the plumbing it'll be in the 60s. or even high-50s. (System 11 in that Brookhaven National Laboratory test came in at 58.7% for the summertime simulation: But it'll do better during the heating season, since the bulk of the distribution & system losses apply to the space heating side.)

    This isn't quite as good in hot-water-only mode as a condensing tank HW heater like the Vertex or Polaris, but it will beat most condensing tankless HW heaters in most applications, due to the lack of short-cycling losses. In real-world use a 0.98EF tankless runs in the mid-70s for efficiency due to short-cycling losses. Every ignition cycle and flue purge throws away a fixed amount of fuel & heat, whether it's a half-liter draw for hand washing, or filling a gia-normous soaking tub. The fraction of the total that's thrown away is miniscule on tub fills, but quite a significant fraction of the total on the hand-rinse.

    All hot water systems benefit from near-heater pipe insulation (even on the cold-feed to the water heater), and on all accessible distribution plumbing. In CA utility studies something like 15-25% of all water heating energy gets abandoned in the distribution plumbing, and insulating the distribution plumbing extends the amount of time that the water in the pipes are still hot enough to be useful, rather than running the tepid water down the drain until the hotter water arrives.

    A 100KBTU/hr heat load is quite a load, even for a 2.5 story 1911 antique at an outside design temp of -25C or even lower. Are you sure it's really that big? How big is the house, in conditioned square feet, and what is your outside design temp? A friend's ~6000 square foot 1830s beast of a house has a measured heat load of about 80K at -15C, so I suppose his house would run ~100K @ -25C, but that's an exceptionally lossy house that's difficult to air seal adequately, and it has no foundation insulation at all(!). In a energy efficiency update there are many things that could cut the heat load another 15-20% that would still be cost effective in the intermediate to long term, but he also has more expensive gas & electricity than you (quite a bit more expensive, actually), and is heating it with a power & heat cogenerator, + mod-con, which takes a significant edge off the utility bills.

  8. user-968917 | | #8

    Wow, thanks for the great and detailed response, Dana.

    Our design heat loss temperature is -35 celcius/ -31 farenheit, and this house is about 1900 sq ft (not including basement.). I've re-collected all of the data, re-done a blower door test (around 9 ACH @ 50 pa) and need to complete a detailed run in HOT2000, but the design heat loss will almost certainly be around 100K (my team did almost 8,000 energy evaluations last year, so I've got lots of comparison/reference houses. One of my energy advisors also already performed an energy evaluation on this house complete w/blower door a year ago and came up with similar numbers. I am going to be as precise as possible in my calcs).

    We can make big improvements in the envelope by sealing the leaky 1/2 story (joists below knee wall essentially wide open to side attics!) and topping current ~R12 wood shavings in side and top attics to R50+. Sloped ceilings already have wood shavings, balloon framed walls and knee walls have wood shavings, windows are mostly original wood w/metal storm, and 80% of basement already insulated to R12...~ 7 foot high stone basement has 3' above grade.

    Thanks again - great info. I hope that this client is able to find a hydronics contractor in our area sophisticated and experienced enough to do this right!


  9. Expert Member
    Dana Dorsett | | #9

    Air sealing behind kneewalls is almost always a fools errand (I've played the starring role of "fool" in that movie more than once :-) ). It's usually better to go un-vented on the kneewall attic spaces and put 2-3" of closed-cell foam on the roof deck between the rafters with rock wool or blown cellulose to fatten up the R-value there (no interior side vapor barrier, or the roof deck has no path to dry.)

    Mind you, this is somewhat counter to IRC code prescriptions, but it works. For a sanity check on this approach see:

    Scrolling down to tables 3 & 4 p.11, the climate zone that most closely matches yours would be 7A, International Falls (99% outside design temp= -23F / -31C ). With composite shingle roofing even 2" of ccSPF even with an addtional R38 of fiber below it is still protective of the roof deck with normal indoor humidity levels, but if it has light metal roofing it'll take more. If there is only room to get to half the current code of R50, 2" will be sufficiently protective even with high albedo roofing, since the roof decking will be running slightly warmer at a lower total-R, but the vapor permeance of the foam layer will be the same- it will start accumulating moisture later in the season and begin drying sooner than the higher-R roof. Clearly 4" will get you there, but the drying rates are forever with foam that thick.

    Surely there are budget constraints, and this might not be the first best place to spend the money in this particular house, but the unvented kneewall attic method tends to air-seal more reliably than trying to block it at the kneewalls and joist bays.

  10. NBABUCKS1 | | #10

    I just want to say thanks for all the responses. I looked into descaling my heat exchanger. but I am not sure how I get into the boiler tank. This is something that a professional should probably do. If I don't do it right I'm w/o heat and have to pay a professional anway.


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