1. Visiting from the US? Why not try DIYnot.US instead? Click here to continue to DIYnot.US.
    Dismiss Notice

Proposed new system

Discussion in 'Plumbing and Central Heating' started by Barley, 8 Apr 2013.

  1. Barley

    Barley

    Joined:
    4 May 2011
    Messages:
    21
    Thanks Received:
    0
    Location:
    Nottingham
    Country:
    United Kingdom
    I am replacing my old boiler and rehashing the whole system. I would appreciate any helpful suggestions
    I will install a 300l vented Heat Store, and a 40kW Worcester 40Cdi with shunt pump between,
    The CH will be fed by Thermal mixer valve with manual selection. Vented system to avoid pressurising the rad circuit.
    The DHW is supplied by an internal coil at mains pressure
    The system has 3 zones plus a dump zone for a swimming pool when used. Each zone will have a motorised valve, and each controlled by it own programmer [time & temp]
    In total there are 25 rads & rails. 2 baths & showers, 7 hot taps
    I am thinking to use an auto/modulating pump.
    How do I select the pump? The whole currently runs from a 15-60 on speed 3, this seems fine but the system is only roughly balanced,
    Calcs suggest heat losses of 32kw plus DHW and plus the 45,000l pool.
    Any comments please.
     
  2. Sponsored Links
  3. Onetap

    Onetap

    Joined:
    19 Jun 2006
    Messages:
    1,845
    Thanks Received:
    191
    Location:
    Essex
    Country:
    United Kingdom
    Don't ask for design input from an internet forum.

    Dump the thermal store.
     
  4. Barley

    Barley

    Joined:
    4 May 2011
    Messages:
    21
    Thanks Received:
    0
    Location:
    Nottingham
    Country:
    United Kingdom
    WHY ?
     
  5. Onetap

    Onetap

    Joined:
    19 Jun 2006
    Messages:
    1,845
    Thanks Received:
    191
    Location:
    Essex
    Country:
    United Kingdom
    It is a condensing modulating (I think) boiler, that works more efficiently at lower return temperatures. Current recommendations are 70/50 degC F/R at design conditions ( < 0 degC outside). You can operate the system more efficiently and more confortably, for most of the heating season (> 0degC outside) with weather compensation.

    This conflicts with the thermal store requirements in which you need a high temperature differential to generate DHW at 60degC ish from the stored heat; typically Tflow > 80 degC. Thermal stores with internal pipe coils were thought up pre-1987 when unvented water heaters were not allowed. They're obsolete, get an unvented cylinder if you want high pressures. You don't need a thermal store if there is no requirement to store heat. Thermal stores suffer from a lot of scaling in hard water areas, due to the high temperatures and additional limescale precipitation.

    Further, most boiler manufacturers offer controls that will do weather compensation (variable flow temperature) by modulating the burner; a 3-port motorized mixing valve is not required. This system is better than the 3-port thermostatic proposed. The heating shuts down and the boiler runs on high flow temperature when there's a DHW demand, tolerable with modern high-efficiency cylinders.

    A sealed system will suffer less corrosion than open vented.

    You're spending a lot of money on a system, it would be worthwhile to pay for competent design, rather than hurl yourself headlong into the many pit-falls.
     
  6. dcawkwell

    dcawkwell

    Joined:
    2 Nov 2010
    Messages:
    3,422
    Thanks Received:
    516
    Location:
    Humberside
    Country:
    United Kingdom
    Seal system.

    2 x 24kW boilers and a low loss header.

    Separate pumps and zone valves off the low loss header for each zone.

    300 litre unvented cylinder.

    Swimming pool head exchanger required is as much as I know
    about heating a swimming pool.
     
  7. SimonH2

    SimonH2

    Joined:
    4 Nov 2010
    Messages:
    5,843
    Thanks Received:
    594
    Location:
    Cumbria
    Country:
    United Kingdom
    So the OP has a recommendation to ditch the thermal store on the basis that it'll interfere with efficient (condensing) boiler operation, and another suggestion to use a low loss header which is virtually guaranteed to do the same :rolleyes:

    The obsession over high store temps is misplaced. A 300l store is likely to have a significantly sized coil (but need to check the specs) and running at such high temps isn't needed. Besides, even if you did want to run at 80&#730;, it's perfectly possible to supply that while still keeping the boiler condensing - it will just mean reducing the boiler flow rate a bit.

    A modulating pump works very well in such setups. What you propose is much the same as I put in the flat - but mine was smaller and didn't have any zone valves. Ditch the room stat(s), put TRVs on all rads and let the TRVs together with the modulating pump take care of everything. Makes for a nice quiet system.
     
  8. dcawkwell

    dcawkwell

    Joined:
    2 Nov 2010
    Messages:
    3,422
    Thanks Received:
    516
    Location:
    Humberside
    Country:
    United Kingdom
    Low loss headers are essential for multzone systems and allow the distribution of flow to the zones. It will have no effect on the efficiency of the boiler. The boiler will only fire when there is demand from one or more of the zones.
     
  9. SimonH2

    SimonH2

    Joined:
    4 Nov 2010
    Messages:
    5,843
    Thanks Received:
    594
    Location:
    Cumbria
    Country:
    United Kingdom
    When I read that, I thought I must have misunderstood what a LLH is, so I looked it up, and lo, it is what I thought.

    Surely, unless the zone flows add up to "reasonable proportion of boiler flow", then the LLH is a large bypass, and will raise the return temp to the boiler. Unless you are working on very low temperatures, this means you'll stop the boiler condensing.

    In effect, a thermal store is a very large LLH, large enough to store enough heat to avoid having to keep the boiler running when the return temp starts to rise.
     
  10. Sponsored Links
  11. dcawkwell

    dcawkwell

    Joined:
    2 Nov 2010
    Messages:
    3,422
    Thanks Received:
    516
    Location:
    Humberside
    Country:
    United Kingdom
    No the low loss header allows distribution of the flow. So the flow is grabbed by the zone pumps taken away and returned to the header at lower temperature. The return temperature on the low loss header should be similar to the return temperature if it was just a normal system.
    In the case of a thermal store you are heating the thermal store to a temperature and controlling this with a separate thermostat. The means keeping quite a high return temperature to keep the store at a reasonable temperature.
     
  12. SimonH2

    SimonH2

    Joined:
    4 Nov 2010
    Messages:
    5,843
    Thanks Received:
    594
    Location:
    Cumbria
    Country:
    United Kingdom
    Only if those flows add up to a significant part of the primary flow. If the loops have variable flow - as they will with TRVs - then under part load you will have only a fraction of the flow round the loops as you have round the headers. Under those conditions, the headers act like huge bypasses.
    So if the loops are only taking 50% of the primary flow, then your delta-T at the boiler is half that across the rads, and at 25% the delta-T at the boiler is only 1/4 that of the rads.

    In a normal domestic environment, without LLHs, the flow through the boiler will normally reduce somewhat when the heating is on part load, which reduces the effect - actually eliminating it until the flow drops to the point where an autmoatic bypass starts opening.

    Have you ever used a properly designed/setup store ? Your description doesn't fit. If properly designed, the boiler return will mostly be at or below that of the rad loop return temp - and in the flat, I have a TMV to prevent condensing in the old non-condensing boiler. Note that the rad loop return temp can be significantly lower with a store given the lack of a bypass which is normally needed to ensure a minimum flow rate for the boiler.

    That can be different with a heat bank, but that is one area where I consider a heat bank to be significantly inferior to a thermal store.
     
  13. Onetap

    Onetap

    Joined:
    19 Jun 2006
    Messages:
    1,845
    Thanks Received:
    191
    Location:
    Essex
    Country:
    United Kingdom
    Not quite; it'd be better if you read what I have written and stop putting stupid emoticons on your replies.

    The OP has probably long gone, but as I understand it, he had a requirement for space heating, typically a continuous 70 degC (or usually less, see what I have written) during the heating season and an intermittent requirement to generate DHW at 60 degC or so.

    My recommendations are above. The logic is simple, the system is more efficient if the flow temperature is restricted to the minimum necessary.

    Perhaps you could explain why he needs a thermal store, when he has no requirement to store heat (other than for DHW), requiring a blender to reduce the temperature of the stored hot water for space heating and a another blender to reduce the temperature of generated DHW?

    'Professional' (as against amateur) used in the context of sports has a different meaning to 'professional' used in the context of work.
     
  14. SimonH2

    SimonH2

    Joined:
    4 Nov 2010
    Messages:
    5,843
    Thanks Received:
    594
    Location:
    Cumbria
    Country:
    United Kingdom
    Been back and read what you wrote, I don't believe I have anything to change in my opinion as to what the meaning of your postings has been.
    Which was not your first post, if you read what you have written.
    Actually I haven't said that he does need a thermal store, what I have suggested is that advice from someone who gives the impression that they can see no situation where a thermal store is best (or even that it's is a reasonable option) may not be the best advice. True it's not in the same leagues that the "ditch <whatever>, fit a combi" brigade, but your first post was "Dump the thermal store." with no qualification.

    I do not advocate any one option as "best". There are several that would meet his needs - no one of them is best, just different tradeoffs of a variety of aspects.

    If I were designing a system for the sort of size/requirements of the OP, I would indeed consider a thermal store. I would probably not however install a blending valve on the CH system, and I would not run the store hotter than is needed for the CH (with some provisos). As far as the CH is concerned, I'd consider the store as a large low-loss header (as dcawkwell suggested) but without the efficiency impairment that comes from the very low volume of water in the LLHs. That does, of course, require that the DHW coil be appropriately designed for the load ...
    As to the DHW, well the only difference is that a thermal store has an explicit mixer, the reheat coil on a vented or unvented cylinder effectively has a mixer internally that no-one sees or considers. You're heating water around the coil, and this will mix with the cooler water in the cylinder. As I see it, rather a 6 of one, half a dozen of the other situation. If the store temp is low enough, then in principal it's OK to run the DHW without a TMV - but that would depend on what temp the CH is running at.

    Now I'll make a statement that I will point out (before you do) that I don't have facts to back up - but I'll give the logic behind it.
    Logically, I'd expect thermal efficiency of a condensing boiler to be more heavily affected by return temp than flow temp - and I'll give my reasoning. AIUI, the basic arrangement of a condensing boiler is a pre-heat exchanger that uses the cooler return flow to condense the water out of the exhaust, followed by the main heat exchanger to transfer heat from the flame to the water - though I can see ways in which careful design could combine the two. It follows that the colder the return flow, the cooler you can get the exhaust, and the more heat you can extract for a given fuel input. Considering the different temperature gradients, the effect isn't going to be nearly as big as switching between condensing and non-condensing modes - but I find it hard to believe that (everything else being equal), achieving a lower exhaust temperature is going to lower the boiler efficiency.

    What I can say, from observation, is that the return temps from a thermal store can be "quite low" - and decrease with reducing CH heat demand (or increased DHW demand).
    Really ? Check a dictionary then.
     
  15. Onetap

    Onetap

    Joined:
    19 Jun 2006
    Messages:
    1,845
    Thanks Received:
    191
    Location:
    Essex
    Country:
    United Kingdom
    The first post was short, i was busy, my reasons have been explained above.

    How does he benefit from having a thermal store? What is the advantage of heating the water required for space heating above the required flow temperature?

    What is the advantage of generating DHW by (over) heating the primary water and generating the DHW on demand via heat exchanger? Surely it is simpler and more efficient to to heat and store the DHW at the 60 degC recommended to avoid legionella?

    The main justification for thermal stores was when it was as a means of getting around the prohibition on unvented DHW storage cylinders.

    I had a thermal store until fairly recently. They have their uses, but none that apply to the OP SFAIK.
    Yes, really.
    http://en.wikipedia.org/wiki/Professional

    You're having an argument about engineering with a professional engineer.
     
  16. Barley

    Barley

    Joined:
    4 May 2011
    Messages:
    21
    Thanks Received:
    0
    Location:
    Nottingham
    Country:
    United Kingdom
    As the ‘original poster’ [OP ?] I’ve not long gone as Onetap fears, I am following all replies with interest. And thanks for all the input.

    Well yes, I can see why he suggests that the Thermal Store is not necessary, so let me confirm my objectives.
    A] CH at 40C – 70C as needed
    B] DHW at about 50C
    C] Pool, taking as needed when available or timed for off-periods

    Currently I have a 51kW plus a 30kW boiler which take about 8cu/mtrs if operating at the same time. This comes off a standard 6.4cu/mtr meter. It does still work however.
    Brit Gas tell me that I need a new supply pipe and a commercial meter – this is unacceptable.
    As a point, the rads max output exceeds the heat losses by about 50%

    The objective of the Thermal Store is to build up a buffer store of hot water at a temp from 60 -80C as proven needed in practice.

    The CH will draw via TMV a temp 40 – 65C as needed. Possibly this would be modulated by weather compensation controlling the TMV. The return temp could be expected to be 30 – 50C which I am told suits the condensing boiler.

    The DHW can draw at 50C from the HX in the store. [Not compromised as it would be without the heat store, and needing to draw from the boiler temp.]

    The Pool would be able to take any excess heat available, when not needed by other zones. [In summer this means ‘most of the time]

    Does this explain my preference for the heat store?

    As a point regarding TRV’s, the theory is fine but their effectiveness is not frequently possible when the rads are oddly positioned or then doors are left open, as in my case. [?]
    Ps What does SFAIK mean ?
     
  17. SimonH2

    SimonH2

    Joined:
    4 Nov 2010
    Messages:
    5,843
    Thanks Received:
    594
    Location:
    Cumbria
    Country:
    United Kingdom
    It may be, it might not be. Definitely 6 of one, half a dozen of the other.
    In one, you heat a large cylinder of water (a little warmer than you need the DHW), and transfer the heat via heat exchanger. In the other, you heat the primary water hotter than you need the DHW and transfer the heat via heat exchanger. What is the return temp from the indirect coil when heating the cylinder ? I'd wager that for a lot of the time it's not that cool - does it stay cool enough to keep the boiler condensing ? There are many variables in that - the temp of the DHW cylinder round the coil, the capacity of the coil (which reduces with reducing delta-T, the required flow rate in order to keep the boiler "happy", and so on.
    I can answer those questions for a thermal store (if properly designed and implemented). The boiler return should never need to be more than the return temp from the rad loop(s) - and can be considerably lower (but note what I said earlier about heat banks being different).

    Note again, I'm NOT saying that a thermal store IS the right option for him, just that I believe it isn't automatically the wrong option either. Once you actually look at it, I find it hard to believe your preferred option of an indirect unvented DHW cylinder is more efficient.
    And that's only considering the DHW.

    Once you consider the CH then it gets even less clear. A thermal store will very effectively decouple the disparate requirements of the CH loops (variable flow rate) with that of the boiler ("quite high" minimum flow rate). If there weren't this difference in requirements, then there wouldn't be all those heating systems with bypass valves required to patch the two together. Once you have a bypass in operation, then the boiler return temp is going to be higher than the return temp from your rads.

    I'll throw in another reason why I believe that a buffer tank of some sort makes sense - as a neutral point. The OP has a very wide range of heat load. At one extreme, CH only during warm weather, at the other extreme, reheating the DHW while the CH is coping with cold weather and the swimming pool is in use.
    Designing the hydraulic circuits for 2 boilers*, 3 CH zones, DHE, and swimming pool is almost trivial when they don't interact - somewhat less trivial when they do.
    * I'd consider 2 boilers rather than one big one. Gives some redundancy for breakdowns for one thing.
    An own goal there I'm afraid. Read the very first line of the page you link to, it says "A professional is a person who is engaged in a certain activity, or occupation, for gain or compensation as means of livelihood". Yes I am fully aware of the other meaning of professional, but it is wrong to say that "'Professional' (as against amateur) used in the context of sports has a different meaning to 'professional' used in the context of work." It can have a different meaning, but it is not true to say that it does have a different meaning.
    PS - I have postnominals to my name as well, I make a point of not mentioning them here. As I'm not professionally (as in paid for it) engaged in plumbing or electrics, I don't want to risk someone thinking I might be (hence my sig).


    Out of interest, are BG telling you that you need to reduce your demand, or change the supply, "or else we'll cut you off" ? And is whoever services and maintain your boilers/system happy with the situation ?
    I suspect that BG will be concerned that if you "overload" the meter then it might not record correctly. It'll only be designed to go up to a certain flow rate, and it's not guaranteed to operate properly above that. From the safety POV, it's a case of whether you still have an adequate supply that will run the boilers properly.
    This is something worth exploring. What do you have now - just rads and an indirect DHW cylinder off the boilers ? Are you saying that you heat the DHW up to that sort of level so that you can (for example) run several baths and after mixing it down still have enough hot water to go round ? There's a tradeoff to be made here - if you can store more water, then it can be cooler. I do agree with Onetap here - there is little point heating water hotter than needed.

    I suspect there are many way you could control the TMV. Weather comp would be one of them.
    It would.
    OneTap is suggesting an unvented cylinder for heat storage - he's not suggesting a system without storage (though you will find plenty of "combi for all" people around). An unvented cylinder is very much like the vented one you probably have now - but pressurised to mains pressure† so you still get hot water at mains pressure.
    † To a point - there will be a pressure reducing valve to restrict cylinder pressure, as well as other safety valves. The regs for unvented cylinders are quite strict in that the cylinder must come with all the valves as a package and it's not allowed to "mix and match" parts. Without the safety features, an unvented cylinder can be highly dangerous, with them they should be fairly safe (AFAIK we never hear of accidents involving properly installed ones).
    Then you have another problem if your system isn't really controllable.
    So Far As I Know ?

    AFAIK, As Far As I Know
     
Loading...

Share This Page