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General: Plate heat exchanger for thermal store to supply C/H system

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    • CommentAuthorJeff B
    • CommentTimeNov 30th 2020
     
    I have a 400 litre vented thermal store with 3 coils – upper coil for C/H, middle coil input from wood pellet boiler, and lower coil, input from solar thermal. (The middle coil may be partially contained within the upper coil – I’m not sure).

    The C/H system works well enough but the heat transfer is not brilliant i.e. whilst the input from the boiler is approx 72C, the flow to the radiators is approx 48C at best. I suspect the surface area of the coil is just not man enough.

    Hence I am considering a suitable PHE, set up as per attached sketch. (The upper coil is labelled spare coil, which it will be if we install a PHE). I realise this set up will probably destroy the stratification within the TS but might that not be a bad thing in that the entire store will be at a uniform temperature?

    Any comments welcome.
  2.  
    Normally the CH is taken straight from the thermal store (TS) rather than via a coil or PHE for the reasons you are finding, i.e. difficult to get enough heat out quickly.

    Is there a reason you don't want to take the CH straight from the TS.
    •  
      CommentAuthordjh
    • CommentTimeNov 30th 2020
     
    Posted By: Jeff BI have a 400 litre vented thermal store with 3 coils – upper coil for C/H, middle coil input from wood pellet boiler, and lower coil, input from solar thermal. (The middle coil may be partially contained within the upper coil – I’m not sure).

    The C/H system works well enough but the heat transfer is not brilliant i.e. whilst the input from the boiler is approx 72C, the flow to the radiators is approx 48C at best. I suspect the surface area of the coil is just not man enough.

    Where do the temperature differences occur? What is the temperature of the water in the store at various heights? What are the return temperatures? This information will give you a better chance to understand where the problem(s) is occurring. You can use an infrared thermometer or an external probe if you don't have suitable readings available.

    Hence I am considering a suitable PHE, set up as per attached sketch. (The upper coil is labelled spare coil, which it will be if we install a PHE). I realise this set up will probably destroy the stratification within the TS but might that not be a bad thing in that the entire store will be at a uniform temperature?

    Any comments welcome.
    PHE schematic.pdf

    I think you have the pump flow triangles the wrong way round in your diagram?

    Is the thermal store only used to heat water for the CH? I'd have thought that meant most of the solar thermal output was wasted (because produced when not needed).

    What sort of control system will you use for the pump on the primary side of the PHE?

    I'd have thought it would be difficult to get the water coming from the primary flow output cold enough for it to be sensible to put it back into the tank near the bottom but that depends on the CH return temp.
    • CommentAuthorArtiglio
    • CommentTimeNov 30th 2020 edited
     
    Evening Jeff, we’ve swapped a few messages in the past. My mum’s pellet boiler works on heat exchangers for the central heating. She has a holiday cottage on side the side of the house , it has its own ch system , to keep the house and cottage seperated ,Systems supplied via plate exchangers which was also chosen as it keeps the boiler and buffer tank on a separate circuit as well ( idea being that in the event of a leak in system on ground floor it would’nt dump a tonne of water )Its worked perfectly from the get go.

    The plates were chosen based on the kw rating of the boilers that were replaced by the pellet system, then balanced by altering the flow rates of the pump from the buffer and the individual ch system pumps.
    • CommentAuthorJeff B
    • CommentTimeDec 1st 2020 edited
     
    PiH: Originally the C/H system was indeed taken directly from the TS. The main reason we changed to indirect was because we had a lot of problems with air locks and by going indirect meant we could have a pressurised C/H system which eliminated the air problem immediately. I asked the company who made the TS do some calculations and they concluded that the finned coil would be man enough to do the job, which it does, but takes for ever to get up to temperature!

    I am hoping that using a suitable PHE will give more surface area for heat exchange.




    Djh – the C/H system is off now until 3.30pm, so I can get some temperature data later today.

    I know there is very little heat loss between the boiler and the TS, despite the distance (about 12 metres). The boiler is set to 75C and the water reaching the TS is normally at about 70-73C when the boiler has fired up and then modulated back to a “steady-state” situation when the output is at 15% of maximum i.e. 4.1kW. (4.1kW is the minimum setting; the rated max output is 25kW). C/H flow is normally around 48C and the return low to mid 30’s.

    Unlike “normal” systems our boiler cannot be controlled by the C/H timer or the room stat (there is no means of doing that) so the boiler and the C/H are on separate timers. The boiler is fired up about half an hour in advance of the C/H coming on which gives it time to top up the TS and be well up to temperature when it does come on. Similarly the C/H is timed to go off about half an hour before the boiler so that the latter has time to top up the TS again before shutting down for the night.

    I have put a pipe stat on the flow pipe from the boiler so that the circulation pump which pumps the water from the boiler to the TS cannot start until the boiler has attained 70C. This avoids the previous problem whereby the pump circulated hot water back from the TS to the boiler, and the boiler acted like a radiator!

    Re: flow triangles. Apologies for the sketch, it was done in a hurry, so please ignore the flow triangles! In fact maybe you could advise on the flow directions as I haven’t considered that yet. Which way should the water from the TS be pumped around the PHE? Should the pump take the water from the top of the TS and pump back down via the PHE to the bottom, or vice-versa? Intuitively I would go for the former I think.

    Yes, the TS is only used for C/H. The original scheme assumed that there might be some input from solar during the spring and autumn months to enhance the C/H but that turned out to be a pipe dream (no pun intended!). The TS now acts as a heat dump during the summer months.

    Re: control for PHE. I’m thinking that the pump in the PHE circuit will be triggered at the same time as the C/H pump.

    Re your last sentence – I’m sorry, I don’t know what you mean. Can you re-phrase it please?



    Artiglio – yes indeed we have – hope all is well with you. Thanks for your input. I have been in touch with one of the (many!) PHE suppliers on the Internet and one of the well established ones is doing the size calculation for me based on the info I have provided them with.

    As there is not a huge cost differential between the various sizes of PHE’s I presume there is no harm in going for an overkill rather than have one that is “just right” for the job? Are there any downsides to having an overly large unit?
    • CommentAuthorJeff B
    • CommentTimeDec 1st 2020
     
    I'm presuming that the PHE pump will inevitably disrupt stratification in the TS. I'm taking that to be a positive rather than a negative as the whole of the contents will equilibriate.
    • CommentAuthorowlman
    • CommentTimeDec 1st 2020 edited
     
    <blockquote><cite>Posted By: Jeff B</cite>
    PiH: Originally the C/H system was indeed taken directly from the TS. The main reason we changed to indirect was because we had a lot of problems with air locks and by going indirect meant we could have a pressurised C/H system which eliminated the air problem immediately. I asked the company who made the TS do some calculations and they concluded that the finned coil would be man enough to do the job, which it does, but takes for ever to get up to temperature!



    Wouldn't it have been easier to pressurise the TS? You could then have done what PiH suggests and taken your CH flow direct? A decent air eliminator would have taken care of any residual air in the system, which may have come about because of the original, presumably open vented TS.
    • CommentAuthorJeff B
    • CommentTimeDec 1st 2020 edited
     
    <blockquote><cite>Posted By: owlman</cite><blockquote><cite>Posted By: Jeff B</cite>
    PiH: Originally the C/H system was indeed taken directly from the TS. The main reason we changed to indirect was because we had a lot of problems with air locks and by going indirect meant we could have a pressurised C/H system which eliminated the air problem immediately. I asked the company who made the TS do some calculations and they concluded that the finned coil would be man enough to do the job, which it does, but takes for ever to get up to temperature!

    Wouldn't it have been easier to pressurise the TS? You could then have done what PiH suggests and taken your CH flow direct? A decent air eliminator would have taken care of any residual air in the system, which may have come about because of the original, presumably open vented TS.</blockquote>

    No, not possible to pressurise the TS sadly. It is basically a thin copper tank with a header tank on the top.
    • CommentAuthorJeff B
    • CommentTimeDec 1st 2020
     
    djh - here are some temperature recordings with the boiler at a steady 12.6kW output:

    Boiler flow: 72C Boiler return: 44C (both readings taken at the boiler itself)
    Boiler flow: 71C Boiler return: 46C (both readings taken at the TS)

    C/H flow: 50C C/H return: 33C ((both readings taken at the TS)

    Probe temperature at mid point in TS (more or less adjacent to C/H flow outlet): 50C
    Probe temperature at 1/3 rd point in TS (more or less adjacent to boiler return connection): 49C
    Probe temperature at low point in TS (more or less adjacent to solar thermal connection): 33C

    Max radiator temperature: 43C
    • CommentAuthorrevor
    • CommentTimeDec 1st 2020
     
    What is the potential output of the boiler? What size pipework is there feeding the TS from the boiler how much distance is there between the 2. Have you tried speeding up the pump? how many rads have you and what is the total heat output of the rads. What size pipework feeding the rads. Do you find the boiler cycles a lot i'e on/off.
    •  
      CommentAuthordjh
    • CommentTimeDec 1st 2020
     
    Posted By: revorWhat is the potential output of the boiler?
    Posted By: Jeff B(4.1kW is the minimum setting; the rated max output is 25kW)

    Posted By: revorhow much distance is there between the 2
    Posted By: Jeff Bthe distance (about 12 metres)
    •  
      CommentAuthordjh
    • CommentTimeDec 1st 2020
     
    You seem to be getting a reasonable drop both around the boiler loop and the central heating loop. But you seem to lose quite a lot between the TS and the radiators. Do the pipes run somewhere that the lost heat is useful?

    Presumably the boiler is not a condensing unit?

    The size of PHE will determine how much control you need over the primary speed. Best consult an expert.

    I think your diagram shows the flows in the right directions, except both pumps are reversed.

    I'm not now convinced about what I said about where to put the PHE return back into the TS. Maybe it is better to have it low down to stir the tank.
    • CommentAuthorrevor
    • CommentTimeDec 1st 2020
     
    12 metres is quite a way between the 2 what is pipe diameter? Not only is the temperature important but the amount of flow through the system is more so. As rule of thumb 35 mm pipe carries 2 x 28 mm which carries 2x 22 which carries 2x 15mm amount of heat at a given flow rate. Many examples of poor performance in heating systems is down to poorly specified pipe size. Without doing the sums 25 kw boiler particularly with your run needs 28 mm pipe at least. I plumbed my 27 kw boiler to the TS 2.5 metres away in 28mm it was borderline between 22 and 28 so went bigger. If you can get your hands on one of these booklets you can check out what you have got against it.

    https://www.cibse.org/knowledge/knowledge-items/detail?id=a0q20000008I7odAAC
  14.  
    If I'm understanding your description...

    - the water in the TS as measured is 50oC where you feed the C/H.

    What temperature are you expecting the rads to be at, if you feed them 50oC water? The problem would seem to be the heat input (control) from the boiler, rather than the transferring heat of out to the rads.

    - you are feeding the C/H from the mid point in the TS.

    Is that the upper connection point for the C/H coil, or the lower? (ie. which way is the flow through the coil)

    Feels to me like you are adding more and more layers to this set up, without really getting to grips with what it's supposed to be doing for you. It's just a buffer tank, to store the heat from the pellets, so the pellet boiler gets a good run, rather than cycling.

    I also wonder about the controls of the pellet boiler. What temperature sensor is telling it to start and stop. If it runs long enough, the TS will get hot, like 70oC, and your rads will be fed with say 65oC water. There's something afoot here, and my money is on it not being the C/H coil.

    I would consider linking the boiler flow to the return side of the C/H coil, with a control valve. The valve is open when the boiler is running, and closed when the boiler is off. That way you'll get much hotter water going to the rads when the boiler is running, as it'll come direct from the boiler.

    Then when the boiler has reached it's switch off point (what ever it is that gives it it's shut off signal), then the link valve shuts, and the C/H pump continues to draw from the TS. That arrangement also has a benefit of feeding the return from the rads through the lower boiler coil and back to the boiler, which will make it cooler than the current return to the boiler, which your boiler will like, as I imagine it's a condensing boiler, being a pellet boiler so likely not that old.

    I've fitted PHX to TS's for generating DHW. It's added complexity, if you want it to be efficient, which is much more simply done by an internal coil.
    • CommentAuthorJeff B
    • CommentTimeDec 1st 2020
     
    <blockquote><cite>Posted By: revor</cite>What is the potential output of the boiler? What size pipework is there feeding the TS from the boiler how much distance is there between the 2. Have you tried speeding up the pump? how many rads have you and what is the total heat output of the rads. What size pipework feeding the rads. Do you find the boiler cycles a lot i'e on/off.</blockquote>

    Max output = 24kW

    Pipework diameter = 28mm

    Distance between boiler and TS = 12m approx

    Pump speed - which pump are you referring to, the C/H pump or the boiler-to-TS pump?

    Radiators: Single - 6; double - 7; towel rails - 2. Total kW approx: 18.5kW

    Pipework to radiators: manifold 10mm system. From TS to manifolds 22mm.

    Boiler cycling - no, the boiler does not cycle a lot since I set the minimum output to 4.1kW. It happily ticks over at this setting. During cold weather, as it is currently, it doesn't appear to go off at all but I don't monitor it all the time of course.
    • CommentAuthorJeff B
    • CommentTimeDec 1st 2020
     
    <blockquote><cite>Posted By: djh</cite>You seem to be getting a reasonable drop both around the boiler loop and the central heating loop. But you seem to lose quite a lot between the TS and the radiators. Do the pipes run somewhere that the lost heat is useful?

    Presumably the boiler is not a condensing unit?

    The size of PHE will determine how much control you need over the primary speed. Best consult an expert.

    I think your diagram shows the flows in the right directions, except both pumps are reversed.

    I'm not now convinced about what I said about where to put the PHE return back into the TS. Maybe it is better to have it low down to stir the tank.</blockquote>

    The C/H pipes do run through the attic and drop-down to the radiators on the ground floor and through the dwarf walls into the first floor rooms (it's a dormer bungalow) but they are well insulated. They are not brilliantly insulated in the TS room (a job I am getting around to!) but heat lost here is within the envelope so not really wasted heat.

    The wood pellet boiler is not a condensing boiler as far as I know.

    PHE: I have had a design and spec sent to me from the technical help department of a well established UK PHE manufacturer based on the info I sent them. The cost of the PHE they recommend is eye-wateringly expensive (to me anyway)!
    • CommentAuthorJeff B
    • CommentTimeDec 1st 2020
     
    Revor - the pipe diameter from the boiler to the TS is 28mm. The pipes are well insulated - as you can see there is very little drop in temperature (about 1 or 2C) between the water leaving the boiler and arriving at the TS.
    • CommentAuthorJeff B
    • CommentTimeDec 2nd 2020 edited
     
    Green Paddy – thanks for your input. Here are the answers to your questions:

    - you are feeding the C/H from the mid point in the TS.
    Yes.

    Is that the upper connection point for the C/H coil, or the lower? (ie. which way is the flow through the coil)
    The upper.

    I also wonder about the controls of the pellet boiler. What temperature sensor is telling it to start and stop?
    The internal sensor within the boiler. It is set at 75C.

    I would consider linking the boiler flow to the return side of the C/H coil, with a control valve.
    Do you mean split the boiler flow then – part to the boiler coil and part to the C/H coil?

    ------------------------------------------------------------------------------------------------------

    I realise I am not going to get much heat out of the radiators when feeding them water at 50C. Most of the time this is ok but on very cold days it takes a long time for the living room to get up to temperature (21C). I’m hoping that with the greater surface area available for heat exchange with a large PHE this will improve. A 21 plate gasket PHE with a heat transfer area of 0.9 sq.m. has been recommended. I presume having an even larger PHE would not be detrimental for the relatively small extra cost involved?

    As I explained in my post above, the boiler is hardly cycling at all - it runs continuously at 4kW output as it rarely achieves the target 75C (usually steady at about 72/73C).

    I don’t really see the PHE as added complexity but simply changing from an (inadequate) “internal” finned coil heat exchanger to an external plate heat exchanger. It certainly will be added cost!
    • CommentAuthordereke
    • CommentTimeDec 2nd 2020
     
    @JeffB excuse me if I am being daft - but if you are struggling to get enough heat out of the rads at 50C wouldn't you be best off replacing them higher output rads? This might also put you in a better place for switching to a heatpump when your pellet boiler reaches end of life.
  20.  
    Jeff B - you said that the CH was direct from the TS but was changed to indirect to solve air problems in the CH. Would a cheaper option be to investigate the reasons for the air problems and then revert to CH direct from the TS? (or put a AAV in the CH where the air accumulates).

    I have found in the past where there is an expansion tank on top of the main tank with little pipe distance between the 2 tanks that air has been drawn into the main tank pipe work via the connection between the 2 tanks. The solution has been to put a loop of pipe in to the expansion pipe, down to the bottom of the main tank then back to the expansion tank giving a longer column of water between the 2 tanks which has solved the problem.
    • CommentAuthorphiledge
    • CommentTimeDec 2nd 2020
     
    Jeffs original sketch showed a "boiler/ASHP" so maybe any system changes should be focussed on working with a HP??
    • CommentAuthorJeff B
    • CommentTimeDec 2nd 2020 edited
     
    Posted By: dereke@JeffB excuse me if I am being daft - but if you are struggling to get enough heat out of the rads at 50C wouldn't you be best off replacing them higher output rads? This might also put you in a better place for switching to a heatpump when your pellet boiler reaches end of life.


    As you say, if/when we ever change to an ASHP then this may become a necessity. Unfortunately my financial director (aka wife) has imposed a ban on any further significant expenditure on the heating system on the grounds that going green has already cost us a fortune! (She is correct there). Hence any further work can only involve tweaking the system, not major capital expense.

    We have had a quote for an ASHP and it was approx £8K not including the possibility of larger rads (calculations on-going), so a non-starter for the foreseeable future.
    • CommentAuthorJeff B
    • CommentTimeDec 2nd 2020
     
    Posted By: Peter_in_HungaryJeff B - you said that the CH was direct from the TS but was changed to indirect to solve air problems in the CH. Would a cheaper option be to investigate the reasons for the air problems and then revert to CH direct from the TS? (or put a AAV in the CH where the air accumulates).

    I have found in the past where there is an expansion tank on top of the main tank with little pipe distance between the 2 tanks that air has been drawn into the main tank pipe work via the connection between the 2 tanks. The solution has been to put a loop of pipe in to the expansion pipe, down to the bottom of the main tank then back to the expansion tank giving a longer column of water between the 2 tanks which has solved the problem.


    Sorry if I have misled you. Yes, there is an expansion tank directly above the TS but it was not used as such. We had a traditional F&E tank in the attic which is no longer used as we have the pressurised system. There are still several AAV's in the system and a Spirotech unit in the boiler flow.
  24.  
    Even with the expansion tank in the loft if the expansion pipe up to the f&e tank was teed off the CH pipe the suction of the flow could suck air into the system.
    • CommentAuthorrevor
    • CommentTimeDec 2nd 2020
     
    @jeffb. Looks like your pipework is man enough although never been a fan of microbore. Have you tried

    A. Speeding up the CH circ pump.


    B. Isolating as many of the rads that you can so you are feeding fewer rads to see what happens.
    • CommentAuthorJeff B
    • CommentTimeDec 2nd 2020
     
    Posted By: revor@jeffb. Looks like your pipework is man enough although never been a fan of microbore. Have you tried

    A. Speeding up the CH circ pump.


    B. Isolating as many of the rads that you can so you are feeding fewer rads to see what happens.


    A. I have tried in the past and it had no effect but I'll give it another try and see what happens. The pump is on the lowest setting which I assume maximises the heat transfer rate?

    B. This indeed is the current situation e.g. all the upstairs rads are off, but it makes no difference.

    I do not like microbore either but it's here, so that's that. In fact it could be an issue when trying to use it with an ASHP. I will not be re-piping though. I did that in our last place but will not be doing that here - far too much hassle!
    • CommentAuthorrevor
    • CommentTimeDec 2nd 2020
     
    The pump I am referring to is the pump that circulates the water through the rads loops. A certain volume of water at a given temperature will contain a certain quantity of heat increase that volume at the same temperature will take more heat to the rads and therefore heat up quicker that is off course if your boiler and coil can keep up and deliver that heat to the system. What can happen in this situation is the level of noise increases as the velocity of water increases. There are tables available (in the book I have mentioned) that give recommendation as to pipe size to deliver the quantity of heat needed and avoiding noise due to friction of the water in the pipe.
    Just another thought do you have a bleed valve on your the entry to your TS boiler coil. Could be an air lock.
  28.  
    I'm at a loss as to what the thermal store is doing for you? It's not even acting as a buffer, if the boiler is running all day.

    What is the most efficient point of operation of the boiler? I suspect it is not at the 4kW output, and also not at 75oC.

    Can you tell us the type of pellet boiler, as I'd be surprised if it is not condensing?

    I think you're asking the wrong question, as regards using a PHX. Clarify the logic of your system (to yourself if no one else), and then go about simplifying the system to make it work. Taking into account your previous posts on here, I sense your frustration, but I suspect that's as a result of having a system that doesn't quite make sense, and so are forever trying to patch symptoms rather than resolving the cause. If I'm wrong, my bad :shamed:

    I've designed loads of systems with thermal stores plus log boilers, pellet boilers, GSHP, oil and gas inputs, and most combinations there of, and almost always open vented, but I just don't quite get what your system is doing. Heating a TS continuously, but only achieving 50oC in the thermal store, and drawing off half way up for CH only. Eh??
    • CommentAuthorJeff B
    • CommentTimeDec 2nd 2020
     
    Posted By: revorThe pump I am referring to is the pump that circulates the water through the rads loops. A certain volume of water at a given temperature will contain a certain quantity of heat increase that volume at the same temperature will take more heat to the rads and therefore heat up quicker that is off course if your boiler and coil can keep up and deliver that heat to the system. What can happen in this situation is the level of noise increases as the velocity of water increases. There are tables available (in the book I have mentioned) that give recommendation as to pipe size to deliver the quantity of heat needed and avoiding noise due to friction of the water in the pipe.
    Just another thought do you have a bleed valve on your the entry to your TS boiler coil. Could be an air lock.


    There is no way I would want to change the 28mm pipework from the boiler to the TS. It has a long and tortuous path from the boiler in the garage, through the attic above the garage, behind panelling in one of the upstairs bedrooms, across the top of the stairwell (discretely boxed in) and down to the TS in a walk-in cupboard downstairs, via the DHW cupboard upstairs!

    No, there is no bleed valve on the entry to the boiler coil in the TS. There is one on the C/H flow pipe where it exits the TS.
    • CommentAuthorJeff B
    • CommentTimeDec 2nd 2020
     
    Posted By: GreenPaddyI'm at a loss as to what the thermal store is doing for you? It's not even acting as a buffer, if the boiler is running all day.

    What is the most efficient point of operation of the boiler? I suspect it is not at the 4kW output, and also not at 75oC.

    Can you tell us the type of pellet boiler, as I'd be surprised if it is not condensing?

    I think you're asking the wrong question, as regards using a PHX. Clarify the logic of your system (to yourself if no one else), and then go about simplifying the system to make it work. Taking into account your previous posts on here, I sense your frustration, but I suspect that's as a result of having a system that doesn't quite make sense, and so are forever trying to patch symptoms rather than resolving the cause. If I'm wrong, my badhttp:///newforum/extensions/Vanillacons/smilies/standard/shamed.gif" alt=":shamed:" title=":shamed:" >

    I've designed loads of systems with thermal stores plus log boilers, pellet boilers, GSHP, oil and gas inputs, and most combinations there of, and almost always open vented, but I just don't quite get what your system is doing. Heating a TS continuously, but only achieving 50oC in the thermal store, and drawing off half way up for CH only. Eh??


    GreenPaddy - thanks for your input again. You must be getting as frustrated as I am. If ever you are in west Wales please feel free to come and take a look at our system!

    The wood pellet boiler is an Ekoheat 2500. I think the system is a simple design in principle but you are correct - it is very frustrating and it doesn't make sense why it doesn't work as it should! I didn't say (or at least I didn't mean to imply) that the boiler runs all day. It is only on for 3 hours in the morning (7.00am to 10.00am) and 6.5 hours in the evening (3.30pm to 10.00pm). The property is quite well insulated and draught-proofed now, so it retains the heat from 10.00am to 3.30pm and overnight. The temperature in the lounge drops about 1 or 2C in that time depending on the outside temperature of course and how windy it is. (We live on a hill in a very exposed position). I hope to add IWI to the two external walls in this room at some point which will improve the heat retention.

    During the morning period the boiler usually runs continuously, starting off at the max output of 24kW and then gradually modulating down to 4.1kW. In the evening it starts again at the max output of 24kW and gradually modulates down to 4.1kW, and will turn off once the C/H has achieved the desired room temperature and the TS is at about 70C. There is no feedback control loop here, when the boiler exceeds 75C it automatically turns itself off (I can set this temperature anywhere I like, from 40C up to max 80C).

    Even if the boiler keeps running at 4.1kW would you agree that's not unreasonable in December to heat a 4 bed detached dormer bungalow with 13 radiators (2 turned off, so 15 total)? I presume it is just topping up the TS to keep in step with the C/H requirement and of course also topping up the DHW tank (although we use very little hot water).

    My frustration is that although the boiler flow water entering the TS is at approx 70 - 71C, the max C/H flow temperature is only 50C. My reasoning/logic is that there has to be problem with heat transfer - either the boiler coil is undersized or the C/H coil is undersized or both.

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