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# Actual effiency of indirect water heating with modcon

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Member Posts: 1,405
I have not seen any numbers on the actual % efficiency of producing DHW with a Vito 200/Vitocell 300 indirect combo.

Since the DHW supply output is at 75C (167F) the boiler efficiency should range from 97% to 90% depending on load (should be mostly 94% or above) but how efficient is the transfer of heat within the indirect? If it's something like .8 then the overall efficiency would only be around 75%.

• Member Posts: 420
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... where do you lose energy?

A BTU is a BTU... whether it gets transferred to the indirect or slings back to the boiler, it's only lost if it leaves the "system", right? You put your finger on the most important question, i.e. what is the efficiency of the boiler while it is running with certain return water temperatures.

The recovery rates that Viessmann publishes for its Vitocells should allow you to calculate how many BTU's were used to heat a given amount of hot water in the tank. With a known flow rate, that energy turns into a ΔT the tank HX should be experiencing.

You can then start playing what-if scenarios which will depend on the desired tank temperature, the desired supply temperature, etc. For customers who do not need constant high-fire water production, I'd consider programming the Vitotronic to maintain a 20°F supply ΔT over the tank setpoint, or even less. That'll pretty much guarantee condensation most of the time.
"Let me control you"

Lost in SOHO NYC and Balmy Whites Valley PA
• Member Posts: 6,928
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The heat transferred inside the indirect is 100% efficient. The only loss is transmission loss via the piping connecting the boiler and indirect. It's the boiler temperature and boiler output level that determine the efficiency of the system.

The Vitodens 200 heats an indirect in two different ways (determined by coding):

1) Supply temp will go up to 172F (the absolute high limit).

2) The supply temp will go no higher than 36F above the the desired tank temperature.

In either case the burner will modulate at the max temp. If the boiler is firing at 100% and the max cannot be reached, that means the indirect is completely absorbing the output of the boiler--again, the only "loss" is heat lost from the piping connecting the boiler and indirect.

The Vitodens 200 boiler efficiency is about 85.5% with 167F supply @ 100% burner output and "standard" 140F return temp. This combination (max output at max temperature) will only rarely--if ever--occur. If the boiler is firing at max output for any length of time, the temperature will be lower than max; if the boiler temp is maxed for any length of time, boiler output will lower than maximum. Compared to the minimum efficiency of the Vitodens 200 at max temp @ full output, reducing either the temperature or the output level rather quickly increases efficiency.

The heat exchangers in Viessmann indirects are almost legendary--large surface area and very low head loss. Even their smallest indirect can instantly absorb the full output of the 8-32 Vitodens in most conditions. Larger indirect models (with larger HX coils) allow instant absorbtion of full output at lower and lower supply temperatures.

Viessmann provides exceptional literature regarding their indirects. Study carefully, compare to the particular model of the Vitodens 200 in use and you'll be able to determine which DHW production supply temp setting (to the max or limited to 36F above DHW temp) will be most suitable.

In general the "to the max" setting will minimize the time required to refresh the DHW tank. The "limited to 36F above" setting will maximize efficiency at the possible expense of slowed DHW recovery. How much (if any) DHW will be slowed will depend on the particular combination of boiler and indirect models.

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dhw

The 36°F limiting function can be set lower. I have heard of some setting them as little as 5°F above the tank temp with a Vitocell 300. With the large heat exchanger surface, it takes much less temperature difference to transfer the output of the boiler.

The lower the boiler temp, the more efficient it will be. However, depending on sizing and how the hot water is used, optimum DHW production may provide more satisfactory operation.

As Mike pointed out, the Vitocell technical data manuals have every bit of information you could need. I have never seen a manual for an indirect that is as detailed. Generally, you are lucky to get a first hour rating, and maybe even a pressure drop curve for the heat exchanger coil.
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interesting

I was under the impression that there was always a loss through a heat exchanger but it makes sense that if the BTU didn't go to the DHW then it had to go back to the return otherwise where would it go?

The tank is set at 140F so it looks like the internal high limit is the limiting factor. Keeping the tank at a lower temp may be more efficient but you'd have to worry about bacteria like Legionaire's disease and having less capacity. A device to limit the DHW output temp to 120F is mandatory here, so we don't have to worry about scalding.
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Forgot to mention that the differential can be adjusted--sorry. If I ever get my 120-gallon Vitocell 300 installed to the Vitodens 6-24, I'm going to find out just how low I can go and still have 100% boiler output to the indirect. By the charts, it looks like 130F supply for a 120F tank will be generous.

Won't install the indirect until I've utterly given up on solar DHW/space heating integration.
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Don't forget that the Vitodens 200 has a built-in "Anti-Legionella" function that you can set via coding. No need to constantly keep the indirect at such a high temp unless needed to meet peak demand.
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Dress warmly while standing out in the cold

Heat exchangers may operate with no visible losses to the outside such as those we see in stack losses, nonetheless heat exchangers suck. The ideal heat exchanger only exists on the glossy pages of textbooks.

Agreed, a heat exchanger does not carelessly spill actual BTU bits of heat you can scrape off the floor, but, what these suckers do is waste the opportunities to transfer heat - in ways that seem scandalously lazy. The BTUs go by and no one bends down to pick them up; the exchanger will plainly respond to you that it is not in its job description to do so. There.

And we can't do much of anything about these work rules.

Martin seems to have found a marketed number of 80% efficiency for a domestic hot water tank heat exchanger efficiency - a bizarre number considering no actual flue losses are going on, meanwhile, I do not believe that 80% number is ever achievable in a low temperature heating set up. The actual thermal efficiency of a domestic hot water tank on a cool boiler probably hovers around... ready... 2%... where almost all valuable heat transferring opportunities are lost beyond our grasping reach.

Again, BTUs are not specifically lost at the bottom of the domestic tank, it's their potential harvest that is - it's not the same thing as stack losses. On the other hand, the more you let these opportunities go without touching them, the more you have to run your burner which is pouring out these valuable opportunities for your pleasure. Hiring an excessive number of middle men heat exchanger to put between the fire and your final use will leave you with... nothing. Nothing but total waste in a boiler running full time for a shut tap of hot water.

That's theory. The practical example is easily conceived. We're in the plumbing business, so, let's connect your domestic hot water supply to two dozen domestic tanks, all in a row one after the other, with a low temperature boiler at their head, feeding hot stuff into the first domestic coil, thus warming up the first tank. Then whatever comes out of this first tank, pipe it into the input coil of the second tank, now the second tank starts getting hot. Thirdly, pass this hotish stored water into the next coil for heating the next tank. Do this until you're bored; just because we can. That would be a nifty install picture.

Then, fire up the boiler, step into the shower, open the hot water tap, and hopefully you haven't undressed yourself because it will be cold. Where's the heat going? Nowhere! It's not following through the way too long chain of heat exchangers. The potential for warm and sudsy foam just does not exist in such a set up and everything the burner is peddling is lost to the birds. Everything. (Low temperature system designs teeter dangerously along this vertiginous cliff, but that's another topic)

There is math that exists to figure all this out, not very complicated but it is not the simple discounts we take off on sales prices at the department store. Always be very careful when playing with percentages.

So, twelve domestic tanks in a row is bad. Obvious. Is one tank good then? Well, no, with just one tank exchanger you will loose lots of heat transfer opportunities -- but there are other benefits to what domestic tanks do: buffering being a big bonus and storage without flue losses being another big one. Those two items make the tank highly desirable even though the tank's job description comes with weird no-bend-down-to-the-floor rules.

Use them but use them wisely. Note also, that a steam heated domestic tank comes with greater thermal efficiency, quite a bit greater depending on what you're comparing. These same bizarre considerations are also what make the big differences in Diesel engine efficiency over the placid gasoline dos. A measure of authenticity of sorts.

Martin, you're scratching your head on the right spot. Thanks for asking the good question and I hope my explanation came out clear enough.

Best regards
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Do indirects and storage tanks still lose about half a degree an hour? That is the last info I have. I would think you would have to figure the standby losses when calculating hot water efficiency because the burner has to run once in a while to reheat the water tank. Maybe I'm thinking of a different type of efficiency than was being asked about.
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Not sure that I understand your analogy Christian.

With those "two dozen indirects in series", you'd certainly get hot water--fairly rapidly as long as the final tank in the series was the first served by the boiler. (I'm neglecting matters of head loss here.)

From a cold start (boiler and all indirects cold) the return temp to the condensing/modulating boiler will be exactly equal to the temp of the water in the tanks. ANY mod-con would zoom to 100% output and the first heat exchanger served (the last tank in the series) would see the hottest possible water and will transfer the same amount of heat to the water in the tank as if it were the ONLY indirect.

Granted the supply temp to tanks further down in the series would rather rapidly drop to the point of almost no heat transfer, but as temperature of the water in each tank rises, less heat transfer occur and it will be available to those further down in the series. You would--eventually--heat the water in all of the tanks. You would however have DHW as soon as the water in the last tank (first one served) was heated--and again that would happen in essentially the same time required to heat it were it the only tank used.

Isn't a "perfect" heat exchange process one where the there is no differential temperature? e.g. the exchanger and the medium are at the exact same temperature regardless of how much energy is added to the system? Rather similar to a "black body" which simultaneously receives and transmits 100% of all radiant energy that strikes it?

I didn't mean to imply that the exhance process between a mod-con and an indirect is "perfect"--just that a btu is not in any way "degraded" because of the exchange process. A temperature differential is certainly required but given a generously sized heat exchanger (such as in the Viessmann tanks), it's quite possible to exchange LOTS of BTUs at a surprisingly low differential temperature.
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Depends on the surface area of the tank as a function of water volume, the level of insulation and the difference in temp between the water in the tank and the surrounding air.

1/2 degree F per hour does however seem to be a fairly common (and safe) rule of thumb for nicely insulated indirects at "typical" temps (inside and outside the tank).

Such loss (standby loss) however has nothing to do with the efficiency of the boiler, the heat exchange in the tank or the transmission loss between them.
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back to original topic

I've read that the typical stand alone gas water heater tank has an efficency rating of around 60%. What I was attempting to do with my original question is to come up with an approximate efficiency rating for an indirect tank.

Leaving theoritical heat transfer physics and data sheets aside, if a household was paying \$100/month for DHW with a standard 60 gallon gas water heater tank, how much would we expect their bill to be if they switched to using an indirect DHW Vitodens 200 / Vitocell 300? That's the real question I'm trying to answer.
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\$45 - \$65 per month compared to \$100.

You cannot ignore the data sheets. Size the Vitocel indirect volume correctly to the peak DHW load as modified by the available input from the particular Vitodens 200 model and you can rather easily achieve 90%+ seasonal DHW efficiency.

Yes, I know these percentages don't add up, but standby loss of a gas/oil fired water heater compared to an indirect are extreme.

• Member Posts: 147
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I think I was begining to answer your question in a round about way with standby losses. A bock direct fired 30-40 gallon water heater has a standby loss rating of about 1.2 degrees per hour. I'm assuming this is at 120 degrees although I didn't see the temperature they used.

http://www.bockwaterheaters.com/technical/engineering.html

If the bock water heater has an energy factor rating of about 65% and has a steady state efficiency of about 80%, that leaves about 15% loss unaccounted for which I think most (if not all) could be counted as standby loss.

If an indirect has a standby loss of .5 dph compared with 1.2 dph, we get a 6.25% loss from the tank.

Maybe I'm all wet but I think you can take the efficiency of the appliance and subtract about 6.25% plus other negligible factors such as losses from the boiler water btu's escaping via the venting after a call for hot water ends. (1-2%?)

Edit

The info I got the 1.2 degrees per hour loss was from Table 31 on page 13 from the last pdf on the page of the link I gave.
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Blowing my own horn and measuring its losses

I modified the earlier post to make it clearer my dozens of indirect tanks feed off of each other, no. 1 taking its heat from the boiler. No. 2 finding its supply of input heat from the domestic water storage of tank no.1. The boiler water going no further than the coil in tank no.1. Tank no. 24 is where you hope to see a timely rise in temperature for you hot water needs.

Given lots of infinite time, the set up will fully warm up, you'll get one tank load of hot water in no. 24, but the final BTU output of the whole thing is nothing - very simply because we've diluted the delta T to nothing useful anymore, yet the boiler will still be pushing heat out into a constricted pipe. Oh sure, the aquastat will cycle like crazy, perhaps saving some fuel on an effort that is otherwise one giant waste. You may be limiting the energy demands (if you're lucky) but the efficiency of this is close to zero.

The ideal exchanger has infinitely large surfaces, while at the same time it has zero contents and zero mass.

Mathematical oddities such as these exist: for instance the long slender funnel-like trumpets angels use for annunciation purposes have that property. Those we see in churches are of course approximations, but the mathematical tube shape of the trumpet can be tapered and extended to infinity. Angels live in the heavens. Now, just because we can, we can easily compute both the surface of this trumpet and also the volume that is contained within.

What happens next is nothing short of miraculous. We can pour a measured amount of water inside the tube of the trumpet, it will overflow, it isn't even large... while at the same time we can never ever buy enough paint to cover it. This geometric oddity has a finite volume while it has an infinite surface.

And this has not much to do with boilers anymore. Heat exchanger could be shaped like trumpets, they'd nonetheless still be the source of all our inefficiencies, use them carefully.

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Measuring the cost of stack losses to be saved

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As mentioned a bunch of times already, it is the standing losses we avoid with the use of indirect tanks that are the energy bonus we're after - so be it if system thermal efficiencies and performance are changed.

Stack losses can be big, they can be small. In relation to the overall hot water usage, they may be relatively large, they may be relatively small. For instance, a standard hot water tank feeding just one lavatory sink that is used very infrequently would have us horrified at the standing losses - here an electric tank is best. The full spectrum allows for the tankless heater, the tankless plus indirect tank, and the hot water heater each filling their niche with optimum performance.

How much savings to be had? This is most simple to measure and it has not much to do with the percentages found in the operating efficiencies of the new and old set up (operating efficiencies for domestic water production are anything anywhere in residential hot water usage)

To measure the cost of the stack losses on any domestic hot water heating scheme, perform this test over the span of a vacation.

Make sure no one will use any hot water nor any cooking range gas for the time of the test. At the start of the test carefully record the reading on the gas meter. When you come back, record again the reading on the gas meter. This is the amount of gas that was consumed in keeping the birds happy through stack losses mostly. Prorate the amount for a month and that's what you'll save by going without stack losses.

If you get \$5 to \$10 in simmering costs, you decide if it is significant in relation to your \$100 usage bill.

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More stuff

Domestic hot water schemes get old real quick. They all get limed up in about 10 to 20 years at which point your operating efficiencies are meaningless. If your current set up is old, you'll see large reductions in summer gas usage no matter what equipment you buy next. Periodic clean out save money.

I hope this is more of an answer you wanted.
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