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unusual set up: indirect as preheat for small combi?

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I'm new at this, and am not sure if my idea makes sense so looking for some feedback.
Boiler would be a Rinnai E50 and indirect a Heat-Flo 60g, three small zones plus the indirect as a 4th zone.

I'm thinking of routing all my zone returns to flow through the indirect (manifold to indirect input) before returning to boiler. There would also be a tee in this line so that the indirect is a zone as well. Check valves would prevent backflow. Temp setting for the indirect storage would be roughly 65-70F.

The basic philosophy is to one, preheat incoming cold water (water comes in at anywhere from around 34-45F) to increase the DHW output of the combi (set to Max temp output from combi then through mixing valve), and two, to scrub heat from the return flow to maintain high condensing efficiency. I think it should also help to minimize short cycling.

(The stated min flow rate for the indirect HX is 14gpm. I'm not sure yet what the combined flow rate of all zones will be, but if necessary I could install a bypass controlled by a PRV, though 14gpm seems like plenty of capacity.)

This also essentially makes any standby heat loss from the storage negligible since heat loss in winter is to a heated space, and very little heat loss will occur in summer since storage temp will be very close to average ambient.

Btw, the inspiration here is cost opportunity... I've found very good deals on the equipment, so not looking to compare to a system at retail pricing, just want to ask if anyone sees problems or inefficiencies.

Thanks!

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 23,675
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    Interesting change in heat balance -- but overall you will neither gain nor lose anything. Any heat gained by running the returns through the indirect will have to be made up, sooner or later, by running the boiler to heat the indirect...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • greenmountain
    greenmountain Member Posts: 15
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    Interesting change in heat balance -- but overall you will neither gain nor lose anything. Any heat gained by running the returns through the indirect will have to be made up, sooner or later, by running the boiler to heat the indirect...


    not suggesting there's some magical heat gain here.... all the heat in the indirect always comes from the boiler regardless, whether heated as a zone call or by residual heat in return lines.

    the primary goal is to increase the relatively paltry DHW production of the combi by preheating the incoming water, and secondarily to serve as a sort of unconventional "buffer" helping maintain condensing temps on the return water and possibly reducing short cycling as well.



  • mattmia2
    mattmia2 Member Posts: 9,981
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    I'd be more concerned than usual about growing legionella in that setup.

    Are you using existing emitters? Sizing the emitters to run at low temps and using a mod con and properly sized indirect would make more sense. There is a good chance any initial savings you see on the discounted cost of the combi will be eaten up in increased repair costs and shorter life of the combi and the costs of the controls you need to make it do this.
  • Alan (California Radiant) Forbes
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    I used to use lot of those Rinnai E50c boilers because I did a lot of small house jobs and they were ideal, but I thought they discontinued them. That said, their user interface was not friendly and counterintuitive.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
    SuperTech
  • greenmountain
    greenmountain Member Posts: 15
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    where is main concern for legioella? assuming the indirect storage?
    Supposedly it grows best between 77-113F... maybe if it's set 60-65F it'd be a little safer?
    there will be a UV treatment prior to entering the indirect, but I suppose there's always a possibility of contamination...

    Ideally I'd set the output temp of the combi to 140F to kill any bacteria, but that does not appear to be an option on this combi...



  • greenmountain
    greenmountain Member Posts: 15
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    savings are pretty dramatic... looking at a total cost for combi and indirect around $1200 (new -old stock).

    why would there be shortened equipment life with this set-up compared to a more traditional arrangement?
    Even if this setup didn't prevent short cycling, I wouldn't think it would cause it, or is there another source of additional wear and tear?

    I suppose if risk for Legionella is high, that alone makes it an unsound system.
  • Alan (California Radiant) Forbes
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    Isn't the E50 a combi boiler?
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • greenmountain
    greenmountain Member Posts: 15
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    ok, scratch that last comment... found something suggesting that the output temp can be set to 140F (dip switch number 6), but wondering how much it matters with legionella.
    Looks like it takes about 30 minutes at 140F to insure killing of bacteria... Don't most on demand heaters only put out water @ 120 - 125F anyway? How is there not a danger of legionella there too? less storage time? what about cold water lines in very warm climates?
  • greenmountain
    greenmountain Member Posts: 15
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    Isn't the E50 a combi boiler?

    yes
  • Jamie Hall
    Jamie Hall Member Posts: 23,675
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    ok, scratch that last comment... found something suggesting that the output temp can be set to 140F (dip switch number 6), but wondering how much it matters with legionella.
    Looks like it takes about 30 minutes at 140F to insure killing of bacteria... Don't most on demand heaters only put out water @ 120 - 125F anyway? How is there not a danger of legionella there too? less storage time? what about cold water lines in very warm climates?

    The indirect, should you choose to use it, should be held at 140 F. I'm not certain, from your comments, what you were planning go hold it at, but that's what it should be held at.

    The Legionella (and other bacteria -- but that gets all the press) problem is with stored hot water. If you aren't storing it, such as with an on-demand, Legionella at least isn't a problem. Just what may be in your water already... and presumably you have severe doubts about the quality of your water, or you wouldn't be considering UV -- which is expensive and, unless given regular, careful maintenance (the glass tubes must be cleaned weekly) utterly useless for virus, and almost so for bacteria.

    The combi has "relatively paltry" domestic hot water production? But that's what the indirect is for... to store hot water so you have enough of it when you want it. And that's why it makes more sense to go with a mod/con properly sized for your heating load, rather than a combi.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • greenmountain
    greenmountain Member Posts: 15
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    ok, scratch that last comment... found something suggesting that the output temp can be set to 140F (dip switch number 6), but wondering how much it matters with legionella.
    Looks like it takes about 30 minutes at 140F to insure killing of bacteria... Don't most on demand heaters only put out water @ 120 - 125F anyway? How is there not a danger of legionella there too? less storage time? what about cold water lines in very warm climates?

    The indirect, should you choose to use it, should be held at 140 F. I'm not certain, from your comments, what you were planning go hold it at, but that's what it should be held at.

    The Legionella (and other bacteria -- but that gets all the press) problem is with stored hot water. If you aren't storing it, such as with an on-demand, Legionella at least isn't a problem. Just what may be in your water already... and presumably you have severe doubts about the quality of your water, or you wouldn't be considering UV -- which is expensive and, unless given regular, careful maintenance (the glass tubes must be cleaned weekly) utterly useless for virus, and almost so for bacteria.

    The combi has "relatively paltry" domestic hot water production? But that's what the indirect is for... to store hot water so you have enough of it when you want it. And that's why it makes more sense to go with a mod/con properly sized for your heating load, rather than a combi.


    Was thinking of setting indirect temps at approx 60-70F, as a pre-heat for combi DHW input.
    It's in OP.

    UV Tx is because it's surface/spring water (shallow well). tested pos for bacteria but nothing dangerous/fecal. people have been using the water a long time with no sickness of any sort I've ever heard of... but still use UV as a precaution. My well is what it is... Either the UV is working as it should or there's not much of anything to worry about in the water...

    With an indirect, after use is shut off, heated water remains in the lines and (esp with any type of insulation present) seems like the temps in those lines could remain well within the danger range for bacterial growth (77-113F), no?

    This combi is a mod/con that is properly sized for my load, but DHW is rated for 75FDT @ only 2.1gpm.
    Prefer to get 2.1gpm of 140F instead of sometimes potentially even under 100F w/o any preheat.

    I'm pretty seriously looking at this particular combi b/c I can get it for dirt cheap (saving thousands...)
    Yes, I could just set up the indirect as a zone and hold at 140F, but that would be way overkill for my DHW storage needs, and I'd be losing efficiency in summer by holding all that water at such a high temp. The boiler is also undersized for utilizing that indirect as designed, though I don't fully understand what the btu requirements for the indirect are based on. Seems like adding heat is adding heat... bigger boilers just do it faster, right?
  • mattmia2
    mattmia2 Member Posts: 9,981
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    With an indirect, after use is shut off, heated water remains in the lines and (esp with any type of insulation present) seems like the temps in those lines could remain well within the danger range for bacterial growth (77-113F), no?

    It may remain at that temp but there isn't much of it and it gets replaced every time the hot water is used. In a storage tank it can sit there for long periods and mix with fresh water to contaminate that depending on how much gets used daily.



    I'm pretty seriously looking at this particular combi b/c I can get it for dirt cheap (saving thousands...)
    Yes, I could just set up the indirect as a zone and hold at 140F, but that would be way overkill for my DHW storage needs, and I'd be losing efficiency in summer by holding all that water at such a high temp. The boiler is also undersized for utilizing that indirect as designed, though I don't fully understand what the btu requirements for the indirect are based on. Seems like adding heat is adding heat... bigger boilers just do it faster, right?

    The numbers in the manual for boiler flow and output are what are needed to get the hot water output ratings in the manual. It will work just fine with a smaller boiler or lower flow, it will just make less hot water. You can get the same amount of hot water out of a big boiler and a small tank or a small boiler and a large tank assuming your usage is periodic.

    why would there be shortened equipment life with this set-up compared to a more traditional arrangement?
    Even if this setup didn't prevent short cycling, I wouldn't think it would cause it, or is there another source of additional wear and tear?

    A combi is more complicated than a mod-con so there are more parts to break and require repair and some of the surfaces are part of an open system that makes domestic hot water so those parts are much more prone to corrosion and scaling than the parts of a mod con which would only be exposed to closed system water. It isn't short cycling, it is just that a combi is inherently less durable.

    greenmountain
  • Jamie Hall
    Jamie Hall Member Posts: 23,675
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    Well, on the spring water... from your handle, I expect you are in Vermont. I've seen some pretty nasty springs in Vermont, but it's rare for folks to get sick from them. The immune system is a wonderful thing! I doubt very much that the UV is helping you, unless as I say you are cleaning the tubes regularly, but it won't hurt either, and if you already have it there's no harm.

    It's quite true that the heated water remains in the lines after you use it (and with an indirect held at 140, you have to have a tempering valve to bring it down to 110 or so for safety), but... that hot water has been pasteurized by being held at 140, and thus there is no hazard (note that tanks held anywhere from 70 to 110 do pose a hazard).

    Your comment on the flow and temperature from your combi is absolutely correct -- which is why I will never recommend one to anyone. A medium power combi -- or instantaneous water heater -- is capable of handling one bathroom cluster at a time, and no more. Some high power instantaneous heaters can handle more -- one with 199,000 BTUh input should manage two at a time, for instance.

    I appreciate your ingenuity and what you are trying to do. It won't save you any energy fuel -- or therefore cost, and if you can manage to keep the indirect tank down to 45 it is probably safe enough. Do NOT try to run the indirect much warmer; you are edging into the growth range for a lot of bacteria -- not just Legionella -- which while they don't make you sick in small numbers may well do so if they multiply.

    All that said, I can also say that I can't recommend the arrangement -- and, if I were still inspecting things in Vermont, which I did for years, I'd never, ever permit it.

    Oh and one other thing -- you may be able to get that combi on the cheap, but if you ever need service, will anyone look at it? Who's going to service it? Who's going to clean it? Who's going to set the combustion correctly?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • greenmountain
    greenmountain Member Posts: 15
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    Thanks for the feedback:)
    I'm actually up at 9000' in the CO Rockies.  My well water is very clean, but as I mentioned does contain some bacteria.

    This particular installation is for a low budget one person/one bath living space, so maybe doable with 2gpm @ 140F...

    In the case of any on- demand water heater, unless output temps are set out of spec, water temps at or above 140F are either never or only briefly achieved. 
    I suppose your point of less water in the lines for less time is enough to keep bacterial growth low enough to avoid issues, but overall seems like a sketchy standard and a risk that needs to be assessed differently in different environments.  

    For ex, how about a house in AZ with average cold line temps in the 70's?  Then we're pretty much counting on frequent enough flow levels throughout the entire plumbed system to keep bacterial levels low?


    In terms of who's going to fix, repair, maintain the system... well, mostly me.  I'm not a pro but I can turn a wrench.  I can find a pro to fine tune the combustion.

    Anything/everthing in a home/vehicle/body is eventually going to fail.  I'm looking at installing brand new equipment (not 50 years old... parts are still available) for a tiny fraction of the cost of only slightly newer stuff... (boiler is 2018 model, indirect made 1/21)

    If I had an unlimited budget and could buy the ideal equipment and hire the best pros for the install I wouldn't be here writing this;)

    Many thanks:)

  • Jamie Hall
    Jamie Hall Member Posts: 23,675
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    For your application the combi will work -- without the indirect at all. Keep it simple.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Alan (California Radiant) Forbes