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Discuss Buffer Tanks - 2 or 4 port connections in the Renewable Energy area at

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  1. Worcester

    Worcester Plumber

    OK, A nice technical one for the Bank Holiday Weekend :)

    Buffer tanks - 2 port connection or 4 ?

    Which is best 2 or 4 port, in What situation and why?

    What factors e.g. does it depend upon the heating load types (e.g. underfloor / simple zone controlled rads)

    So under what circumstances would a 2 port connection be best and when would 4 port connections be best.

    Heat Pumps or Biomass Boilers?

    Adding to that in each scenario which is the best way to send the call for heat to the Heat Source (HP or Biomass Boiler?

    Finally when to use a loss loss header in preference to a buffer tank?

    Example 1 Biomass boiler, long time to come on, like to run for a long time, long time for boiler to cool down - i.e. a large hysteresis, as such boiler manages buffer tank based on top and bottom temperatures and hysteresis on both. Heat load considers buffer as an inexhaustible supply of heat. Call for heat from rads / underfloor merely turns on circulation pump, so 90% of installations recommended as 4 port, though I have seen a few controlled this way though plumbed as 2 port, so with a proper back end management system means when boiler fired up that heat goes to heating circuits first.

    Example 2 Ground Source Heat pump (non inverter) supplying mix of underfloor heating and rads, as all heat demands could be shut off manufacturers recommendation has been install buffer with a 2 port connection so that when heat load is less than production HP will heat up buffer tank and also meets the minimum flow requirements so doesn't get High pressure faults. - Call for heat from heating system

    Example 3 Air Source Heat Pump (Inverter controlled) supplying mix of undefloor heating, fan coil and radiant panels, where load could vary from 600W to 10KW,
    a) Supplier 1 recommended low loss header with call for heat from heating system
    b) Supplier 2 recommends 100 Litre 4 port buffer tank with call for heat from heating system.
    c) Supplier 3 recommends 100 Litre 4 port buffer tank with call for heat to HP from buffer tank with single sensor and 6°C hysteresis.

    So question is Why the different recommendations re heat pumps, whether its GS or AS or Biomass - What's good and bad about each option?

    Discussed it with various manufacturers and they simply maintain their position with no explanation as to why. :(
  2. Nostrum

    Nostrum Plumber GSR

    No idea why manufacturers recommendations vary as they are all very similar products.

    LLH option for air source would depend on total water content of system I would imagine to prevent short cycling.

    Froling recommend 2 port connection for buffer tank so that heating doesn't have to wait for buffer to be charged, I've always piped them in a 4 port configuration as always controlled them as you mentioned, boiler to buffer temp, radiators controls circulation pump only.

    I can't really see how it makes any odds unless you intend switching the heat source off before buffer is satisfied to help reduce heat loss through buffer? If you could control the flow to buffer depending on flow rate as opposed to temperature, it would make more sense.
    Last edited: May 2, 2015

    IDCHAPPY Trusted Plumber GSR

    I wish i was clevererer :(
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  4. Worcester

    Worcester Plumber

    I don't see how it does that (prevent short cycling) I can see how it prevents the low flow and hence HP faults on the heat Pumps.

    That's the only reason I can see for that as well.

    I suppose there is one other scenario which is where the boiler is undersized for short term peak demand, so the boiler and buffer tank are providing heat directly, that may work better in a 2 port configuration...

    So which is better for overall efficiency and hence fuel economy?
  5. Nostrum

    Nostrum Plumber GSR

    It's to do with minimum water content I believe.

    The hysteresis set by the controller will also probably effect the required minimum water volume in order for the heat pump to run for a set amount of time. If the hysteresis is small then a small volume of water would take very little time to return at the set temperature. Although the controllers generally have anti short cycling settings as well. This is my interpretation of it anyway.

    As for the biomass buffers and under sizing, Your looking at a much larger volume of water required then though, an accumulator, unless your set point is considerably greater than your flow temperature.

    I was talking to the warmflow rep the other day who says their pellet boiler doesn't need a buffer in most circumstances. I've always wondered why the buffer sizes are so large for biomass, other than to cover as you say, the slow start up times etc. Most gas boilers have a similar turn down ratio.

    I think buffers are much more common on the continent and so a lot of the manufacturers don't feel they need to explain their uses to anyone. We are looking at the Heliotherm range of ground source heat pumps which have full step less modulation, yet they still require (according to the manufacturer) large buffer vessels for maximum efficiency.

    As for efficiency, I'm always wondering myself. It's a double edged sword, larger buffers will prevent as many start ups and allow longer run times at a lower temperature rise, but larger buffer will suffer from greater standing losses at the same time. The later isn't really calculated for, which seems like a bit of an oversight to me?

    I am glad your on actually, as I wanted to ask a question I'd like you to answer! I'll post a thread.
    Last edited: May 2, 2015
  6. missplumb

    missplumb Plumber

    the only reason i can see for using a 2 port tank would be with a biomass and using it as an accumulator not a buffer. We would always suggest a buffer with any biomass boiler to reduce cycling. went to look at an install done pre 2008 which had a 90kw eta hack boiler, no buffer and he was told by the installer that due to the size of the house and 2x 250ltr unvented cylinders, there is enough system for the heat to dissipate, only problem is the boiler is connected directly by either the cylinder stats or a room stat. i was there for an hour and the boiler fired at least 6 times.

    a lot of the biomass manufacturers recommend using the tank as an accumulator so the heat is drawn from there while the boiler is firing round its shunt circuit, once its upto temp and the mixer is open then the boiler takes over the heating and the tank remains uncharged until the heating demand is gone.

    We would only really fit a lowloss header if the system had various different heating systems ie. ufh, rads, air blowers etc, or the temperature and flowrates needed to be different for the different circuits. Fitted quite a few LLH as we do alot of commercial buildings.
  7. Worcester

    Worcester Plumber

    So in the mixed emitter environment would you fit a llh in conjunction with a buffer tank? - Assuming the buffer is 'remote' from the heating distribution location so you aren't able take advantage of the multiple connection points on the buffer ?
  8. missplumb

    missplumb Plumber

    Absolutely, we had an installation in a garden centre where plant room was well away from the ditribution system. Ran pumped pipework from buffer round to lowloss which then fed a hot water circuit, underfloor heating circuit, and 2 overhead fan heater circuits. All the circuits needed different flow rates so the lowloss was perfect, combined with commissioning valves and variable speed pumps.
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  9. missplumb

    missplumb Plumber

    20141128_155813.jpg this is our low loss header
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  10. Worcester

    Worcester Plumber

    Gerberit Mapress or Yorkshire Pegler Xpress? :)

    We tend to use Xpress as our local BSS keep 10x as much of that as the Mapress . 1000's of connections made and No leaks (apart from the joint we forgot to press :) ) - we like that :)
  11. missplumb

    missplumb Plumber

    We use xpress as much as we can. We have had a couple of leaks but I'm sure there would be allot more if we had soldered them all
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  12. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Why it's handy to have esp when there's water in the pipe or when you can't turn the water off and have to put a valve in
  13. missplumb

    missplumb Plumber

    i guess i missed that post - obviously stuck in the dark ages!!
    • Like Like x 1
  14. Worcester

    Worcester Plumber

    Had a interesting discussions today with the UK Froling distributor (Not the BG subsidiary) wrt buffer and low loss headers.

    One of the biggest issues on biomass boiler efficiency is to get the return temperature to the buffer / boiler as low as possible. - the boiler will always raise it if need with the return riser, it can;t lower it.

    They are strongly recommending full weather comp with mixing valves on the heating circuits (not HDW). That will dramatically lower the boiler / buffer tank return temperatures for 95% of the year, so will help the boiler to modulate properly and therefore better fuel efficiency.

    One component that would destroy that reduced return flow temperature is of course a low loss header... so they strongly recommend to design them out whenever possible. - Seems to make sense to me.

    Part of the goal therefore of getting as low a return temperatrure as possible therefore points to using a two port buffer connection for the heating circuits (take DHW of the top of the buffer tank) Interesting to note that the 'official' Froling buffer tanks only have one column of four connection points. (plus top)

  15. Nostrum

    Nostrum Plumber GSR

    Zero ?
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