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    • CommentAuthorJeff B
    • CommentTimeFeb 24th 2017
     
    I have a 25kW wood pellet boiler that provides DHW and runs the C/H system. The C/H system is open vented i.e. has the traditional F&E tank in the attic. A 400 litre thermal store forms part of the C/H system. IMO the system was badly designed in as much as the 400L. thermal store is a waste of space! I understand the concept of having a TS acting as a buffer tank so that the wood pellet boiler is not constantly cycling on/off; however 400L is far too small for this purpose and one should be looking at 2000L or so. I have neither the space nor the cash for a 2000L TS!

    My question is: should I just get rid of the TS altogether and would it really have any detrimental effect on the boiler?

    You are probably thinking – why get rid of the TS? Well several reasons:

    1. I would like to change to a pressurised system so I can get rid of the F&E tank and all the issues that come with having that. The TS cannot be pressurised.
    2. It costs a small fortune in cleanser and corrosion inhibitor because of the large volume of the TS (it is a direct set-up BTW), which is probably more than the volume of the rest of the system put together.
    3. I cannot pressure flush the system because of the TS.
    4. There is currently a problem with air/gas in the C/H system, so hopefully moving to a pressurised system would help with that too (after giving the system a good clean out).

    Any thoughts most welcome.

    Thanks.
    • CommentAuthorArtiglio
    • CommentTimeFeb 24th 2017
     
    My boiler 40kw has a 1000l buffer, this has an expansion vessel rather than an FE tank, this is integrated with 2 heating systems ('house and annexe) via plate heat exchangers. Whislt this is not technically the best way of doing it, it has the benefit of effectively meaning there are effectively 3 wet circuits , each of which can be maintained or bled independent of the others.
    There were early issues on cold days if there was hot water and heating demand called for at same time from both heating circuits, with programme times staggered by 10 mins, by the time the demand is peaking the boiler has fired up and running to meet the demand.
    In summer the boiler will fire once a day. We could have gone for a larger buffer, but the standing losses from such a store were considered be likely to outweigh any possible increased efficiency/convenience. The store is in an outbuilding and so the losses were of no benefit to the heat demand.
  1.  
    The TS, at 400lts won't act as a conventional TS as usually discussed here i.e. a heat store to provide when the wood burner is out, but it should be enough to act as a buffer to stop the boiler short cycling. I would expect that the boiler would short cycle much more if the TS was removed, especially when it is lightly loaded. IMO it should be plumbed in so that the boiler heats the TS and the CH and DHW run off the TS - is that how it is plumbed? (along with the usual 3 way valves to control the CH return/flow temps) DHW must be indirect somewhere - how is that managed? Inhibitor should not be a big expense, once it has been put in, how often are you changing the water or topping up the inhibitor? Air/gas usually collects in a 'favourite' place in a system - and that's where you put the air bleed valve (AAV) Is the air a new problem and did anything change to cause this.
    • CommentAuthorringi
    • CommentTimeFeb 24th 2017
     
    How is the boiler burner controlled?

    Is there a thermostat at the top of the TS to turn off the boiler and at the bottom to turn it on?

    If the boiler is controlled on its own internal thermostat, can you set the T it is turn on at to be higher then the T it is turn off at?

    I assume you have a pump from the boiler into the top of the TS, then the bottom of the store connected to the boiler's return, with a separate pump from the top of the TS to your heating, return into the bottom of the thermal store.

    I am expecting that your TS is not plumbed correctly or there is a control issue as 400lts should be more then enough to act as a buffer to stop short cycling.
    • CommentAuthorJeff B
    • CommentTimeFeb 24th 2017
     
    Posted By: Peter_in_HungaryThe TS, at 400lts won't act as a conventional TS as usually discussed here i.e. a heat store to provide when the wood burner is out, but it should be enough to act as a buffer to stop the boiler short cycling. I would expect that the boiler would short cycle much more if the TS was removed, especially when it is lightly loaded. IMO it should be plumbed in so that the boiler heats the TS and the CH and DHW run off the TS - is that how it is plumbed? (along with the usual 3 way valves to control the CH return/flow temps) DHW must be indirect somewhere - how is that managed? Inhibitor should not be a big expense, once it has been put in, how often are you changing the water or topping up the inhibitor? Air/gas usually collects in a 'favourite' place in a system - and that's where you put the air bleed valve (AAV) Is the air a new problem and did anything change to cause this.


    Originally the TS did serve both the DHW and central heating but there was competition when the C/H was on and there was demand for DHW at the same time e.g. first thing in the morning with showering etc. We ended up with luke-warm water at the taps! Hence we then installed an unvented 180L cylinder to provide DHW (indirect) separately. This has a solar coil as well so no need for the boiler in the summer months now.

    I don't have to top up the system at all and we have a couple of automatic air vents at the high points where the gas/air collects. The air issue has been around for quite a while but as we have double the recommended amount of Fernox F1 in the system I haven't been worried about corrosion. Indeed the system water is crystal clear. However I have recently become aware that the gas may not be air (or at least not all of it) as it will burn will a blue flame when opening a radiator bleed valve for example. This is worrying and I've sent samples to Fernox for testing. I'm currently awaiting their report before deciding what to do next.
    • CommentAuthorJeff B
    • CommentTimeFeb 24th 2017 edited
     
    Posted By: ringiHow is the boiler burner controlled?

    Is there a thermostat at the top of the TS to turn off the boiler and at the bottom to turn it on?

    If the boiler is controlled on its own internal thermostat, can you set the T it is turn on at to be higher then the T it is turn off at?

    I assume you have a pump from the boiler into the top of the TS, then the bottom of the store connected to the boiler's return, with a separate pump from the top of the TS to your heating, return into the bottom of the thermal store.

    I am expecting that your TS is not plumbed correctly or there is a control issue as 400lts should be more then enough to act as a buffer to stop short cycling.



    Is there a thermostat at the top of the TS to turn off the boiler and at the bottom to turn it on?
    There is a thermostat but it opens/closes a motorised valve on the boiler flow rather than turn the boiler on/off. The boiler cannot be remotely controlled.

    If the boiler is controlled on its own internal thermostat, can you set the T it is turn on at to be higher then the T it is turn off at?
    I don't know. It's set at 65C. However often when the boiler has gone off, the internal temperature can reach 80C and then it drops to about 55C before it comes on again but I guess that's just the normal hysteresis effect?

    I assume you have a pump from the boiler into the top of the TS, then the bottom of the store connected to the boiler's return, with a separate pump from the top of the TS to your heating, return into the bottom of the thermal store.
    Yes that's right.

    Short cycling is not an issue at the moment. My question is will it become an issue if I disconnect the TS? My boiler can modulate from 25kW down to 5 kW so I'm wondering if it would simply tick-over at 5 kW at times of low heat demand rather than turn off & on repeatedly.
  2.  
    I had no problem power flushing my system I just connected it up either side of the TS so it didn't go through the TS. As PiH says 400 litres is plenty for a buffer but nowhere near enough for a TS, and without it I would have thought you will get loads of short cycling
    • CommentAuthorJeff B
    • CommentTimeFeb 25th 2017
     
    Posted By: GotanewlifeI had no problem power flushing my system I just connected it up either side of the TS so it didn't go through the TS. As PiH says 400 litres is plenty for a buffer but nowhere near enough for a TS, and without it I would have thought you will get loads of short cycling


    Gotanewlife - ah, that's interesting re the power flushing. Do you have manual valves so that you can isolate the TS? Also how do you flush the TS itself then?
    • CommentAuthorJonG
    • CommentTimeFeb 25th 2017
     
    537 litres by mathematical calculation assuming a 20 degree differential between boiler flow temp and heating system return is the smallest buffer for a 30 min run time which is what we normally design to when using pellet.

    The buffer should allow the boiler to run on after a demand to effectively burn off any remaining fuel on the grate and transfer the energy from this burn out to a useful receptacle vis a vis the buffer. If the buffer is too small the risk is that the ember burn out is interrupted because the return temp rises too high, the boiler cuts out and un-burnt fuel is left on the grate, which then usually ends up in the ash box as opposed to your heating system in the form of heat energy.

    The modulating nature of pellet boilers does allow for a smaller buffer, but if it is too small and cannot the absorb the energy from a low level demand on a spring day for example, the boiler will cycle again due to a combination of rising buffer temperature and return temperature.

    The buffer should allow for some compensation for the next firing cycle by providing energy to the secondaries before the boiler is fired. Ideally the boiler is usually less than the peak load, and the buffer sized to cover the min run time and make up the difference between the boiler output and the peak load, so that combined they can cover the requirements, alternatively a smaller buffer, boiler smaller than peak demand and a back up source can also work.

    Alas the UK hasnt followed NE design.

    The cycling isnt always easy to spot, but generally if the boiler monitors and records enough data it can be seen in the relationship between draught fan run hours and number of ignitions, anything less than 0.5 hour run time per ignition and it is short cycling.
    • CommentAuthorJonG
    • CommentTimeFeb 25th 2017
     
    In terms of your gas issue, if it is combustible it may very well be a byproduct of corrosion in the system somewhere, but overdosing with inhibitor is not necessarily the answer.

    Have a look at the German regs VDI, which provided fill water is soft enough and ph values can be achieved no inhibitor is required and in general terms can actually be detrimental.

    It's tough with an OV system, but plates can minimise the amount that is OV.
    •  
      CommentAuthordjh
    • CommentTimeFeb 25th 2017
     
    Posted By: JonGAlas the UK hasnt followed NE design.

    What does NE mean in this context?
    • CommentAuthorJeff B
    • CommentTimeFeb 25th 2017 edited
     
    Posted By: JonGIn terms of your gas issue, if it is combustible it may very well be a byproduct of corrosion in the system somewhere, but overdosing with inhibitor is not necessarily the answer.

    Have a look at the German regs VDI, which provided fill water is soft enough and ph values can be achieved no inhibitor is required and in general terms can actually be detrimental.

    It's tough with an OV system, but plates can minimise the amount that is OV.


    JonG - thanks for your responses. Can you clarify what you mean by NE and also "plates can minimise the amount that is OV" please?
    • CommentAuthorJonG
    • CommentTimeFeb 26th 2017
     
    NE is Northern European who have far more experience of using biomass than we do.

    Plates are plate heat exchangers which can be used between open vent and sealed systems, so that as much as possible can be kept sealed and better protected from corrosion.
    • CommentAuthorJeff B
    • CommentTimeFeb 26th 2017 edited
     
    Posted By: JonGNE is Northern European who have far more experience of using biomass than we do.

    Plates are plate heat exchangers which can be used between open vent and sealed systems, so that as much as possible can be kept sealed and better protected from corrosion.


    Thanks JonG. I seem to remember looking into plate heat exchangers a few years back but IIRC we needed such a large beast to provide the heat output we required that we dropped the idea (too expensive).

    But now you've got me thinking again! My thermal store has an unused coil in it. This could be used to circulate the boiler flow (instead of direct heating as it is now) and the two tappings currently used for the boiler flow and return could be used to couple up to a PHE to feed the C/H system. Would that work do you think? That way the thermal store would simply be a reservoir of hot water and I could pressurise the whole system, getting rid of the F&E set up altogether?
    • CommentAuthorJeff B
    • CommentTimeFeb 26th 2017
     
    Posted By: Peter_in_HungaryThe TS, at 400lts won't act as a conventional TS as usually discussed here i.e. a heat store to provide when the wood burner is out, but it should be enough to act as a buffer to stop the boiler short cycling. I would expect that the boiler would short cycle much more if the TS was removed, especially when it is lightly loaded. IMO it should be plumbed in so that the boiler heats the TS and the CH and DHW run off the TS - is that how it is plumbed? (along with the usual 3 way valves to control the CH return/flow temps) DHW must be indirect somewhere - how is that managed? Inhibitor should not be a big expense, once it has been put in, how often are you changing the water or topping up the inhibitor? Air/gas usually collects in a 'favourite' place in a system - and that's where you put the air bleed valve (AAV) Is the air a new problem and did anything change to cause this.


    DHW is now managed separately in an unvented solar cylinder upstairs - indirect. (Previously it was plumbed as you suggest but we ended up with tepid water).The boiler flow is directed to both the DHW cylinder and the TS; when the DHW cylinder is satisfied, a motorised valve closes the boiler flow to that and it carries on to the TS alone until that in turn is satisfied.
  3.  
    Posted By: Jeff BBut now you've got me thinking again! My thermal store has an unused coil in it. This could be used to circulate the boiler flow (instead of direct heating as it is now) and the two tappings currently used for the boiler flow and return could be used to couple up to a PHE to feed the C/H system. Would that work do you think? That way the thermal store would simply be a reservoir of hot water and I could pressurise the whole system, getting rid of the F&E set up altogether?

    It is generally not a good idea to use an indirect coil to put heat from a boiler to a TS - there is just not enough capacity of heat transfer to cope with the output of the boiler and serious short cycling or over heating of the boiler would be the likely result. Before you further contemplate this move check the rating of the coil and match this to the output of the boiler to see how they match.
    Posted By: Jeff BThe boiler flow is directed to both the DHW cylinder and the TS; when the DHW cylinder is satisfied, a motorised valve closes the boiler flow to that and it carries on to the TS alone until that in turn is satisfied.

    Sounds good - I presume the indirect coil could not handle the full output of the boiler so a parallel feed is needed. Does the boiler short cycle lots if the heating is not on (i.e. temp satisfied in the TS) but the boiler is used for DHW

    BTW I am not a fan of plate heat exchangers when used with a TS as the PHX needs a high flow rate to get the rated heat output and has return temps. both of which conspire to wreck the stratification that is so important to get usability and efficiency out of the TS
    • CommentAuthorArtiglio
    • CommentTimeFeb 26th 2017
     
    The plate exchangers in my system were bought off of ebay using little more than educated guess work, tgether they were under £150, the pumps on the domestic heating side are normal central heating pumps (reused those in the old lpg systems)
    There is a problem of the boiler sometimes being fired up when the buffer receives a slug of cold water from the 25m unfderground pipe running from boiler shed to house.This now rarely happens after altering demand timings and boiler settings But statification in the buffer is not as crucial as it would be in a thermal store.
    The use of plate exchangers has meant the old heating circuits have no way of contaminating the new boiler circuit. A leak in either heating circuit will not result in the 1000lt buffer draining into the house.
    The system works well and is easy to keep an eye on, I've no doubt outright efficiency could be improved somewhat and would look at weather compensation as my first choice if changes are made.
    • CommentAuthorJeff B
    • CommentTimeFeb 26th 2017
     
    Posted By: Peter_in_Hungary
    Posted By: Jeff BBut now you've got me thinking again! My thermal store has an unused coil in it. This could be used to circulate the boiler flow (instead of direct heating as it is now) and the two tappings currently used for the boiler flow and return could be used to couple up to a PHE to feed the C/H system. Would that work do you think? That way the thermal store would simply be a reservoir of hot water and I could pressurise the whole system, getting rid of the F&E set up altogether?

    It is generally not a good idea to use an indirect coil to put heat from a boiler to a TS - there is just not enough capacity of heat transfer to cope with the output of the boiler and serious short cycling or over heating of the boiler would be the likely result. Before you further contemplate this move check the rating of the coil and match this to the output of the boiler to see how they match.
    Posted By: Jeff BThe boiler flow is directed to both the DHW cylinder and the TS; when the DHW cylinder is satisfied, a motorised valve closes the boiler flow to that and it carries on to the TS alone until that in turn is satisfied.

    Sounds good - I presume the indirect coil could not handle the full output of the boiler so a parallel feed is needed. Does the boiler short cycle lots if the heating is not on (i.e. temp satisfied in the TS) but the boiler is used for DHW

    BTW I am not a fan of plate heat exchangers when used with a TS as the PHX needs a high flow rate to get the rated heat output and has return temps. both of which conspire to wreck the stratification that is so important to get usability and efficiency out of the TS


    Peter – thanks for your response. Our wood pellet boiler does not have the means to be started remotely, so I set the timer on it to come on half an hour before the timer on the C/H kicks in at 7.30am (on until 10.00am). The system is then off all day until 4.00pm (then on until 10.00pm); again the boiler is timed to come on half an hour before the C/H system. There are only the two of us here and so our DHW demand is relatively small, hence little or no cycling of the boiler. I really need to check that aspect more though, as I really don't have a handle on it. Also we keep the DHW tank contents at 65 – 70C and blend it at the mixer taps with cold, so the consumption of hot water is further reduced.

    Re the PHE idea. In reality our TS is no longer used as a thermal store per se – it is essentially just a buffer tank, so I presume maintaining stratification would not be a big issue? It has a solar coil but during the summer it is just used as a heat dump because we don’t need the C/H then. By using a PHE I could separate the boiler side of the circuit from the C/H system, keep the former on a vented basis and the latter could be a closed, pressurised system?
    • CommentAuthorJeff B
    • CommentTimeFeb 26th 2017 edited
     
    Posted By: ArtiglioThe plate exchangers in my system were bought off of ebay using little more than educated guess work, tgether they were under £150, the pumps on the domestic heating side are normal central heating pumps (reused those in the old lpg systems)
    There is a problem of the boiler sometimes being fired up when the buffer receives a slug of cold water from the 25m unfderground pipe running from boiler shed to house.This now rarely happens after altering demand timings and boiler settings But statification in the buffer is not as crucial as it would be in a thermal store.
    The use of plate exchangers has meant the old heating circuits have no way of contaminating the new boiler circuit. A leak in either heating circuit will not result in the 1000lt buffer draining into the house.
    The system works well and is easy to keep an eye on, I've no doubt outright efficiency could be improved somewhat and would look at weather compensation as my first choice if changes are made.


    Artiglio - thanks for your response. I like it! The idea of separating the boiler circuit from the C/H circuit is very appealing as I could then pressurise the latter whilst keeping the boiler side of things, including the TS, at atmospheric (i.e. vented). (My TS cannot be pressurised, unlike a normal buffer tank).

    My TS is also a long way from the boiler but the pipework is inside the house. We put in a pipe thermostat and a 3 way motorised valve and created a loop. Only when the flow from the boiler reaches about 50C will the MV open and allow flow into the TS.
    • CommentAuthorArtiglio
    • CommentTimeFeb 27th 2017
     
    Jeff B,

    Apologies I did'nt explain properly earlier, the boiler and buffer etc are situated in a boiler room about 25m from the house, (across the farmyard) where as the "distribution panel" for the heating systems is in a utility room in the house. It's the district main pipe from the buffer to the panel that returns a "slug of cold water" to the buffer.
    The boiler responds solely to the temperatures in the buffer, coming on when the buffer temp drops to 58 degrees and switching off at 78 degrees, this allows the heat from the pellets already burning to transfer to the buffer which invaraibly brings it up to the design temp of 80. The upper temp could be increased but the standing losses from the buffer increase markedly if you do so ,and as we've pretty much stoped any short cycling by staggering the heating/dhw programming there is no need to do so.
    The boiler electronics are such that the circulation pump between buffer and boiler will not come on until the boiler temp is 2 degrees above the buffer temperature or 60degrees whichever is the higher.
    Hopefully that explains it a little better.
    • CommentAuthorJeff B
    • CommentTimeFeb 27th 2017
     
    Artiglio - ok, I understand. As you have undoubtedly discovered, the time from the moment the wood pellet boiler first comes on, having been off all night, until the time it produces water at the preset temperature (65C or whatever) is quite long - mine takes about 30 minutes. The problem with our original layout was that the when the boiler pump started it took the water out of the TS back to the boiler in the garage, and the latter then acted as a radiator cooling down the contents of the TS! Hence we put in the loop which minimised the amount of water returning to the boiler before the latter got up to temperature. The distance involved is about 16 metres.

    We now have a new wood pellet boiler (have had it for two years) and on this one the boiler pump does not come on until the boiler temperature reaches 60C - more or less like yours.

    Do you think the PHE method is the way I should go? I like the idea of being able to separate the boiler circuit from the C/H circuit, keeping the former as a vented layout and the latter unvented/pressurised. Also minimal changes required to the existing pipework.
    •  
      CommentAuthordjh
    • CommentTimeFeb 27th 2017
     
    One other point to keep in mind about PHE is that they have lots of small passages, so they need clean feeds of water. I'd think about filtering them, as well as isolation valves or other arrangements to flush the PHE when necessary.
    • CommentAuthorArtiglio
    • CommentTimeFeb 27th 2017
     
    Jeff, my boiler, can be run in two modes, which either maintains a very low feed rate rate and sustains a low burn or it can be allowed to go out. Its running on the continuous burn setting currently , this is little more than a glowing ember and seems to use very little in way of pellets , but enables the boiler to come to temp quickly and the boiler itself has never fallen below 50 degrees. It currently fires over the winter for about 5 hours a day and in the summer barely an hour and this may is only once a day in general. From the boiler getting a signal to fire up ( buffer has dropped below 57 degrees) to boiler/buffer pump kicking in is usually around 5-10mins.
    If the boiler is running set to burn right out (summer) then it will be about 15mins.

    As for suitability of exchangers in your system, i'm really not qualified to say, when my system was installed it was offered as an option for the reasons i've given and to me it made sense and i'm pleased with how its turned out.

    As noted above you need to make sure you're existing pipework etc is clean as the exchangers will clog easily if there is much dirt in the sytems. But as noted filters can be added to deal with this and a good flush before hand should deal with any issues. In my case the systems although relatively old (20 years) they were new throughout at installation and have been looked after ever since, so far there has been no problem at all. Plus the exchangers can be removed relatively easily if need be.

    If you whisper through an email address i'll see if i can dig out a picture or two of the set up.
    • CommentAuthorJeff B
    • CommentTimeFeb 27th 2017
     
    Posted By: djhOne other point to keep in mind about PHE is that they have lots of small passages, so they need clean feeds of water. I'd think about filtering them, as well as isolation valves or other arrangements to flush the PHE when necessary.


    djh - thanks for that. There are already two Spirotech magnetic/non-magnetic filters in the system - one in the boiler return and one in the C/H return so the system water is pretty clean. I always put in isolation valves whenever I install a new piece of kit or have the opportunity to isolate a particular part of the system when it is drained down for some reason. It always amazes me that plumbers don't do this routinely esp as full bore lever action ball valves are only about £6 each and service valves (where appropriate of course) about £1 each!
    • CommentAuthorJeff B
    • CommentTimeFeb 27th 2017 edited
     
    Artiglio - your low burn option sounds good. I don't have that facility on my boiler. I will be giving the whole system a flush sometime soon, after I get the results of the system water analysis from Fernox. That'll be the ideal time to make any mods to the system when it is all drained down.

    I'll whisper you an email address so you can send the photo of your set-up, thanks for that.

    Edited note: I've forgotten how to use the "whisper" facility! :shamed:
    • CommentAuthorJeff B
    • CommentTimeMar 4th 2017
     
    Artiglio - I have whispered you a reply.

    Jeff B.
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