Home  5  Books  5  GBEzine  5  News  5  HelpDesk  5  Register  5  GreenBuilding.co.uk
Not signed in (Sign In)

Categories



Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

Buy individually or both books together. Delivery is free!


powered by Surfing Waves




Vanilla 1.0.3 is a product of Lussumo. More Information: Documentation, Community Support.

Welcome to new Forum Visitors
Join the forum now and benefit from discussions with thousands of other green building fans and discounts on Green Building Press publications: Apply now.




  1.  
    Is it possible to get a 4-way mixing valve for DHW? We have 2 coils for our thermal store. At present the DWH has a mixing valve that mixes the hot and cold to get the set temperature. My thinking is that it would be more efficient with an ASHP to mix the warm and hot to get DHW. Is there such a product?
  2.  
    Like this - here they seem to have joined 2 x 3-way mixer valves together to achieve it.
      DHW 4-Way Mixer Valve.jpeg
    •  
      CommentAuthordjh
    • CommentTimeOct 24th 2021
     
    That looks a bit strange. What drives flow through the top coil?
  3.  
    The cold water pressure.

    The cold water flows up through the bottom coil. The first thermostatic valve decides if it's already hot enough for comfort, or if some/all of it also needs to be diverted through the top coil to be heated some more. The second thermostatic valve decides if it is now too hot for safety, and blends it down with cold water. (Edit: are they drawn the correct way round?)

    You might want the two valves to have slightly different target temperatures (comfort and safety) rather than combine them in a single valve.

    The whole setup needs 3 inlets and 2 outlets, unlike the 4-way mixer shown on the central heating.


    Indeed the ASHP will be most efficient if the DHW is heated as much as possible by the lower-grade heat in the bottom of the tank, rather than taking the higher-grade heat from the top of the tank and then trashing it by blending with cold. If heat is drawn only from the bottom of the tank it avoids destratifying the top of the tank. But once the bottom of the tank is too cool, you still want to be able to access the heat from the top section.
  4.  
    Looking at it again, I'm not sure it can be quite right:

    Surely the cold should only ever mix with the bottom loop output (to cool it down in the rare instance that the bottom loop output is hotter than needed for DWH), so the blue should mix with the orange.

    The next mix should then be the combined output of the blue v orange mixer v the output of the top coil (red). However, I agree with @djh, I'm not convinced that the valve will work. Maybe I'm wrong?
  5.  
    I think this is how it should probably be arranged, but happy to be corrected!
      4-way mixer small.jpg
  6.  
    Here's the ESBE arrangement though.
      Esbe.png
    • CommentAuthorGreenPaddy
    • CommentTimeOct 25th 2021
     
    The first schematic I believe to be incorrectly drawn, as it implies 2 outputs on the first valve (or a reverse flow from an unpowered coil), which I think lead to DJH's query.

    I prefer the ESBE arrangement, as it guarantees a limited output temperature, due to the cold blend at the final output (note: the inputs are arranged slightly differently on the TMV, in case anyone hadn't noticed, but amounts to the same thing).

    On your hand sketch, I think you could in theory have hotter water from the first TMV than your max set final output.

    'I'm sure it's not just for financial reasons, but I wonder what the payback would be for the £100 TMV and labour, versus any efficiency savings?
  7.  
    At present, we only have 1 mixer, which I think must very inefficient. I need to get another, but wondering what arrangement would be best. I think my arrangement would be more efficient than the ESBE one where the bottom section is really hot, but my arrangement might risk scalding as the mixer valves are not terribly effective where there isn't a good differential between the two input temperatures.
    • CommentAuthorGreenPaddy
    • CommentTimeOct 26th 2021
     
    Not sure I follow your logic here? When the base is really hot, you have to cold blend any way, so having 2 TMV's adds little. When the bottom is cooler, you won't be adding any cold blend to the base.

    As I said above, and you noted yourself, if the base is hotter than your max output, you may not be able to sufficiently cool the final output temp. I'd suggest that contravenes building regs, where a controlled max DHW output temp must be in place. A potentially hot blend for cooling doesn't do that. Apart from regs, it makes safety sense.
    • CommentAuthorEd Davies
    • CommentTimeOct 26th 2021
     
    Posted By: GreenPaddyAs I said above, and you noted yourself, if the base is hotter than your max output, you may not be able to sufficiently cool the final output temp.
    Doesn't that depend on the set point of the lower mixing valve? If that's set fairly low, 30°C say, then ComeOnPilgrim's scheme ought to work OK, I think.
  8.  
    In the Esbe setup, the first TMV needs to be set at the "comfort" setting to get the DHW just warm enough to use, say 40-45degC,

    The second TMV is the "safety" setting, say 48degC, so it doesn't get too hot.

    The objective is to first use the lowest-grade heat that is available, so (when possible) avoid mixing down high-grade heat with cold water, until the lower, cheaper section has all been used up. That might happen if the cold were mixed in before the hot?

    I'd connect the cold water into the "safety" TMV to minimise its usage and also to act more robustly.

    On a parallel thread, there was an improvement of +1.2 in CoP from using lower-grade ASHP heat, rather than making higher-grade heat and then blending it with cold water. That should easily pay for the second TMV, especially if it enables a smaller ASHP to do the job.

    I guess something similar would apply with ST, but not with PV/immersion or a solid fuel stove.

    Does beg the question of what is the purpose of each section of the TS - conventionally the top section is priority for DHW and the bottom for CH.
    • CommentAuthorGareth J
    • CommentTimeOct 26th 2021
     
    https://m.youtube.com/watch?v=PDXVIeuUD-8&feature=youtu.be

    I remember this video from the (now defunct?) TMS company.

    Seems to give the setup of control valves slightly differently although how much more effective it would be as opposed to just connecting the output of the bottom coil to the input of the top and running it as if it were one long coil, I don't know. All little marginal improvements add up I guess!
  9.  
    Posted By: Gareth Jhttps://m.youtube.com/watch?v=PDXVIeuUD-8&feature=youtu.be

    I remember this video from the (now defunct?) TMS company.

    Seems to give the setup of control valves slightly differently although how much more effective it would be as opposed to just connecting the output of the bottom coil to the input of the top and running it as if it were one long coil, I don't know. All little marginal improvements add up I guess!


    Interesting video, thanks! I'm still trying to get my head around how the setup on the LHS works. Something to ponder as I drift off to sleep tonight.
  10.  
    Posted By: WillInAberdeenOn a parallel thread, there was an improvement of +1.2 in CoP from using lower-grade ASHP heat, rather than making higher-grade heat and then blending it with cold water. That should easily pay for the second TMV, especially if it enables a smaller ASHP to do the job.


    Yes, agree. Currently, the water passes through both coils, so it will inevitably take heat from the top section, even when this is not necessary. Then it is wastefully mixed down again with cold water. If I had two mixers, I could avoid this by not mixing from the top at all if the bottom section was hot enough. Or at least taking only what was required from the top section to get up to the target temperature. I think it would significantly improve the efficiency of the ASHP as it would reduce the demand for the least efficient and hottest water.
    • CommentAuthorGreenPaddy
    • CommentTimeOct 27th 2021
     
    CoP, maybe you're looking for a solar transfer valve for the bottom coil. These allow water to pass through above a set point, but then divert it for additional heating if it is less than the required temp.

    I have used these for solar thermal, so that cooler water from the ST panel goes to the lower coil, but when above a set temp (much hotter), it diverts to pass through the upper coil, and then to the lower.

    Essentially you get one input and two outputs, so a choice of where it goes to (user or further heating). This may be what is portrayed in the first schematic, at the top of this post? Hence some of the confusion over flow direction?
  11.  
    Is that the same as a TMV, just plumbed in reverse? The thermostat moves the valve shuttle up and down to open one side port or the other, the side ports are just used as outputs rather than inputs. I assumed that's what the first pic was trying to do, but maybe got the input swapped with one of the outputs?

    The Esbe arrangement has closed-loop control which should give more accurate temperatures than the first pic which uses open-loop control for the first valve.
  12.  
    Posted By: GreenPaddyCoP, maybe you're looking for a solar transfer valve for the bottom coil. These allow water to pass through above a set point, but then divert it for additional heating if it is less than the required temp.

    I have used these for solar thermal, so that cooler water from the ST panel goes to the lower coil, but when above a set temp (much hotter), it diverts to pass through the upper coil, and then to the lower.

    Essentially you get one input and two outputs, so a choice of where it goes to (user or further heating). This may be what is portrayed in the first schematic, at the top of this post? Hence some of the confusion over flow direction?


    Hi @GP, yes, we have one of those for the solar heating. @WinA is right, it effectively does the opposite of a thermostatic valve. If < 60º it diverts to the top section only, if > 60º it diverts to the bottom section. I don't think there is one shown in the top diagram though - just a simple coil for the solar thermal. I think the diagram shows an ASHP with two divert valves going to 2 ports so that the supply can switch between UFH (middle & bottom) and DHW (top & middle).
Add your comments

    Username Password
  • Format comments as
 
   
The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   
Logout    

© Green Building Press