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Thread: I found this idea interesting on how to preheat water before it enters indirect

  1. #1
    DIY Member dwassner's Avatar
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    Default I found this idea interesting on how to preheat water before it enters indirect

    I had an energy audit done on my home and the auditor mentioned that his personal hydronic set-up has a "dummy" indirect tank that is at the end of the return of the primary loop and the water coming in from the well goes through the heat exchange (or vise versa with the boiler water going through the heat exchange). Either way he is preheating the well water before it gets to the indirect with another indirect and was claiming that it helps to keep the boiler in condensation mode because of a lower return temp. I found the idea interesting and was hoping to get some comments on it.

  2. #2
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    Condensing boiler matched with a properly sized indirect are really the ultimate in fuel efficiency and domestic hot water performance. The colder the return water - from heating system or indirect - the more the boiler will condense the by-products of combustion and recover the latent heat trapped therein. Pre-heating well water would raise the potable water entering the indirect-fired water heater thereby defeating the high differential temperature and perhaps shortening the cycle.

    At best this is an energy trade. Anything gained by one heat transfer is lost to parasitic energy cost in the next. Regardless, buying an "extra" indirect or even a holding tank to temper well water can not be a good investment. If you want to make a condensing boiler more efficient think about having a factory trained condensing boiler technician tune the boiler for optimum efficiency using a proper combustion analyser and don't forget to have him check the outdoor reset. All high efficiency condensing boilers come with an outdoor sensor and on-board microprocessor that matches boiler temperature to outdoor temperate...in reverse i.e. the colder it gets outside, the warmer the water in the radiation will be.

    Modern condensing boiler also modulate flame to maintain water temperature while space heating but also during the much more demanding domestic hot water mode. While making domestic hot water a condensing boiler will normally try to operate at 180F until the indirect is satisfied. Making it quick and keeping it hot with extra insulation is the flawless strategy that has made all other boiler/water heater combinations obsolete - given a certain class and size building be it new or old.

    If your condensing boiler with indirect is set up right you are in better shape than most of the country for space heating and DHW.

  3. #3
    In the trades Dana's Avatar
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    It's doubtful that any marginal increase in combustion efficiency done that way would ever pay the upfront cost of the return-water indirect. During any real heating load the temp of the tank would rise steadily and fairly quickly to near the return water temp. The only appreciable benefit would occur only on the first space-heating call after a substantial hot water draw. And as BadgerBoilerMN points out, it would decrease the average combustion efficiency of hot-water heating calls from the boiler. If that short term bump combustion efficiency after big hot water draws added up to even 10 therms/year I'd be surprised, but you could probably do a reasonable job of modeling it with a spreadsheet and some actual temperature measurements.

    Also don't forget that the thermal mass of the second indirect works both ways. If you take a shower at 6AM when it's 15F outside and the reset curve is delivering 140F water, the tank will saturate at about 115-120F, but then by noon when it's 30F outside and the curve is sending 125F water the tank on the return loop is heating up rather than chilling the return water. There's probably a net benefit to average combustion efficiency, but it's very small at best. If the radiation is all low-mass there may be another subtle benefit in terms of lowering the number of burn cycles too, with the thermal mass of the tank buffering the system, but in all but the rarest of cases that wouldn't translate into much increased efficiency. If it was one of those rare cases, you'd still be better off redesigning the system to make it work better overall than sinking money into a second indirect.

    What BadgerBoilerMN said, dialing in the reset curve and tweaking the boiler to spec would likely save you more than the intermittent and relatively small combustion efficiency improvement you'd get by installing a second indirect.

    For less money than an indirect you could install a drainwater heat recovery (DWHR) heat exchanger and save more than 50 therms, assuming you're a 3-4 person household that showers rather than tub-bathes. It cuts the fuel used for showering roughly in half (but would not measurably improve the efficiency of bath draws or short-draws.) In these days of very low-priced gas it could take a decade or more to pay it off, but at least there IS a measurable payoff, whereas with the second indirect it's unlikely to pay for itself within the anticipated life-cycle of the indirect. Pre-heating the water to the main indirect with DWHR would never raise the temperature of the water in the tank to the point where it cut into the raw combustion efficiency the way preheating with a second indirect would.

  4. #4
    DIY Junior Member Smithtb's Avatar
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    Ok, i'm no engineer, but i have to weigh in here.

    It seems to me this is a pretty good way to increase the efficiency of a modcon boiler. Obviously the payoff is questionable - as with all high efficiency appliances. Low return temps are the holy grail of modcon efficciency - going from greater than 140 (non-condensing) down to temps in the lower 100's can create efficiency gains of around 10% if i'm not mistaken.

    it is likely that many modcon boilers on baseboard systems hardly ever condense unless extra baseboard has been added to the house to facilitate lower supply temps. Furthermore, when these boilers are operating at 100% fire with a target temp of 180 in prioritized dhw mode, their efficiency is lower than in space heat mode. In areas like mine, with incoming groundwater temps of around 36F, return water would be cooled substantially with a preheat tank, for quite a while. This would be especially beneficial in the morning. Cold outdoor temps would have the boiler operating at the top of the reset curve, condensing very little. With the preheat tank the boiler would be heating dhw gradually in a more efficient low fire setting, rather then in high fire dhw mode. Then later in the day if it warmed up, the boiler would not even need to fire if a system sensor is utilized.

    I have a modcon/indirect with low temp radiant, so this would not be as beneficial for me as return temps are already pretty low. What I would like to try is a dual coil solar dhw preheat tank. Solar exchanger water in the bottom coil, boiler return water in the top. In times of high solar gain, the hot dhw in the tank would heat the boiler return water, eliminating the need for the boiler to fire. When there was no solar gain, the dhw tank would cool the return water temp to the boiler. Haven't found anyone who has tried this yet.

  5. #5
    In the trades Dana's Avatar
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    Almost ALL mod-cons running baseboard heat emitter spend most of their time in condensing mode if set up correctly, even if you need 180F output to get enough heat at the 99% outside design temperature. Using outdoor-reset control strategies works just fine even with fin-tube, down to an output temp of ~115-120F with 100-110F return. Below an average water temp of ~115F the output curve of fin-tube is a bit flaky, but at 120F AWT (say, 130F out, 110F return) it's pretty solid and delivers mid-90s condensing performance.

    The amount of pre-cooling of the heating system water you'd get with DHW pre-heat tank when it's actually cold out is pretty minimal, since the heating system would run almost constantly, but the DWH draws are very intermittent. Feel free to do the math on it, but even after a full-purge of the pre-heat tank (like a tub-fill or a coupla showers) it only takes ~20KBTU to bring 40 gallons of 36F water up to a ~100F return water temp, and with any kind of heat load the burner is going to be kicking up to a higher fire (==lower efficiency, with mod-cons) to make up the high delta-T being imposed for maybe the first half-hour (at best.) As soon as the primary indirect is calling for heat it's going to kick up to highest fire to serve than zone anyway, so what you're left with is whatever cooling of the preheat tank you get in the meantime for a very marginal increase in space-heating efficiency.

    Yes, you'll get more condensing efficiency out of it than the hit you take from the higher firing rate- it's better than a wash, but unless you use a LOT of hot water it's not going to be buying you much in heating efficiency (for either the DHW or the space heating.) But a drainwater heat recovery heat exchanger WILL give you ~50% return on the water heating for showers.

  6. #6
    DIY Junior Member Smithtb's Avatar
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    Don't forget, that 20KBTU you're talking about is energy that would be normally going up the stack. I won't say free heat, but the colder return water is pulling heat off the exhaust and forming condensate, not necessairly making the burner ramp up. And if you've seen as many modcons as I have that some hack "contractor" underbid you on, only to look cross-eyed at that funny little (outdoor or system) sensor that comes with the boiler and throw it in the trash, you know that many modcons are not condensing! You're right though, they do work well in the 115-120 range when installed properly. They work much better if you can convince the customer to take it easy with the setback thermostat that some energy auditor has convinced them is the best thing since sliced bread!

    Most quality indirects only lose about 1/2 deg per hour to standby loss. Not much. Besides, all that heat goes right into the room they're sitting in, not up a vent stack.

  7. #7
    In the trades Dana's Avatar
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    It's not 20KBTU that would be going up the stack- it's 20KBTU being extracted from the return water from radiation being dumped into the DHW pre-heat tank, but re-introduced to the heating loop by running the burner at higher fire than it otherwise would as the tank comes up to return-water temp.

    Indirects rated 1/2 degree-F/hr @ 180F are losing heat at a much lower rate at 100F, but that's still not heat that would have gone up the stack- it's heat that's delivered at something like ~2-5% higher efficiency as it's coming up to temp due to the lower return temp to the burner than the heating loop might otherwise have. But getting even fully 5% higher combustion efficiency on something that's less than 25% of the total fuel use is still less than a 1% gain on overall net efficiency, and certainly doesn't make a financial argument for a second indirect for pre-heat.

    Hack contractors who run mod-cons single temp with return water above condensing temps are an embarrassment to the industry (and they exist everywhere) but they're not a majority- not even close (if local experience in MA is any guide- AK may be a very different market, as it is for many things.) Most will at least take a stab at setting up the initial reset curve, but very few systems are tweaked to the last 1% without homeowner input.

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