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.




    • CommentAuthornbishara
    • CommentTimeOct 11th 2021
     
    Quick question…I thought pipes to oversized radiators had to be bigger, but recent quote for 8.5 kw daikin air source said 15mm. Partner says it’s daikin so must be ok…am I missing something or getting the wrong end of the stick?!:)

    Also refrigerated insulated pipes on outside of house…to the tank…bit surprised by this?
    Thanks!
    • CommentAuthorphiledge
    • CommentTimeOct 11th 2021
     
    Final runs to all our rads are in 10mm so 15mm will supply significantly bigger rads than our 'standard' non low temp rads.
  1.  
    The amount of heat that a radiator can give out is a function of its size and the temperature difference between the temp. of the radiator and the surrounding air and the speed of that air. The closer the rad temp. is to the air the less heat will be transferred. (to the point where air and rad. temp. are the same then no heat is transferred) So in order to get a given quantity of heat out of a radiator the closer the rad. temp. to the air temp. the bigger the rad. needs to be. (If you increase the air flow over a rad. (fan assist) them more heat can be transferred to the air). Radiators are usually calculated with a 20 deg. temp. difference between input and output.

    Water contains heat (energy) at a fixed rate of 4200 joules / kg regardless of the temp. So water with a start temp of 80 deg. falling to 60 deg. will loose 4200 x 20 = 84,000 joules transferred from rad. to air. However water going from 40 deg. to 20 deg. will have the same decrease in energy. but the ability of the rad. to transfer that energy (heat) to the air is reduced because of the reduced rad. to air temp. difference.

    So bigger rads. are needed because of the reduced rad. to air temp. difference but because the energy contained in the water for a 20deg. temp difference is the same the same pipe size is sufficient. (Increasing the speed of the water is ignored here because a faster speed would give more energy available to the rad. but would not increase (by much) the ability of the rad. to use that energy).
  2.  
    For low temperature radiators, the deltaT between flow and return temperatures (eg 20⁰C that Peter refers to ) is usually reduced, eg to 5degC, otherwise the radiators would need to be very large. You then need more flow of CH water at that lower deltaT, to deliver the same heat capacity, so you need bigger pipes and circ pump. It's a trade off between spending money on bigger pipes versus spending money on bigger radiators, so it should depend on circumstances such as what pipes and rads you already have fitted and how much wall space is available.

    The installer should ultimately take responsibility for that trade off, they seem to skip over this bit at quotation stage then change it during installation, so doesn't hurt to ask about it now.

    In our case some rooms need bigger radiators so will be plumbed in 15mm to each radiator, joining together in 22mm. Other rooms already have adequate radiators on 10mm pipes and we won't change them.

    The heat pump also has a preferred deltaT which might be 5degC. If this doesn't match the deltaT chosen for the radiators, then you need a buffer header and an extra pump, with more losses, so that might swing you towards having a low deltaT and high flow in the radiators.



    The pipes on the outside wall (from the outdoor unit to the indoor unit) are warm and need to be insulated. They might carry the heat pump working fluid (eg R32) which the industry always seems to refer to as 'refrigerant' though it isn't refrigerated in the case of a heat pump. That needs special pipe, which they seem to call 'refrigerated insulated pipe' and costs a packet.
  3.  
    Posted By: WillInAberdeenFor low temperature radiators, the deltaT between flow and return temperatures (eg 20⁰C that Peter refers to ) is usually reduced, eg to 5degC, otherwise the radiators would need to be very large. You then need more flow of CH water at that lower deltaT, to deliver the same heat capacity, so you need bigger pipes and circ pump. It's a trade off between spending money on bigger pipes versus spending money on bigger radiators,

    Correct but if the pipe sizes are not running on the limit (and they usually aren't) it may be enough to increase the pump speed or fit a bigger pump. However more water speed increases the noise level from the pipes.
  4.  
    That's right, that's how we will reuse some 10mm pipes but with increased flowrate, so we don't need to increase the radiators size.(eg to a heated towel rail).

    Other places, we are using the same pipes, but replacing the radiators with double-fin ones, to keep the deltaT high, so that the flow doesn't increase too much for the pipes.

    In general, if you halve the deltaT, then you need 2x the flow rate, which means 4x the pipe pressure loss, so 8x the pump power and noise.

    But if you double the pipe diameter, you reduce the pressure loss and also the pump power, by a whopping factor of 32. So a seemingly small increase (10mm -> 15mm) has a big effect on the flows, but is much more disruptive to install.
    • CommentAuthorJeff B
    • CommentTimeOct 12th 2021
     
    Where there is a drop down flow and return 10mm microbore manifold system e.g. in a bungalow, would it be beneficial to increase the flow by replacing the 10mm pipes with 15mm just where they are visible and therefore easily accessible? i.e. in the trunking coming vertically down the walls and then horizontally to the radiators.
  5.  
    Posted By: Jeff BWhere there is a drop down flow and return 10mm microbore manifold system e.g. in a bungalow, would it be beneficial to increase the flow by replacing the 10mm pipes with 15mm just where they are visible and therefore easily accessible? i.e. in the trunking coming vertically down the walls and then horizontally to the radiators.

    If the pipework was marginal so that a high speed was needed to get the desired quantity of heat to the radiator then going up to a 15mm pipe for the down pipes would probably negate some of the noise associated with high velocity water in pipes. It might also remove the need to turn up the speed on the pump quite so much or for a more powerful pump because whilst the restrictions of the 10mm pipe in the loft will still be there the 15mm pipe will have less resistance and it is the cumulative resistance that counts. Pipe resistance is a function of diameter, length, bends and fittings.

    Resistance creates pressure drop so a high pressure is needed with higher resistance. To get a given quantity of water down a pipe a certain flow rate will be needed, increase the pipe dia. then a lower flow rate can be used. Calculations can be made to get the best fit but nobody (that I know of) does this for central heating.

    A calculator for pressure drop can be found here
    http://www.pressure-drop.com/Online-Calculator/
    Have fun!
    • CommentAuthorJeff B
    • CommentTimeOct 13th 2021 edited
     
    <blockquote><cite>Posted By: Peter_in_Hungary</cite><blockquote><cite>Posted By: Jeff B</cite>Where there is a drop down flow and return 10mm microbore manifold system e.g. in a bungalow, would it be beneficial to increase the flow by replacing the 10mm pipes with 15mm just where they are visible and therefore easily accessible? i.e. in the trunking coming vertically down the walls and then horizontally to the radiators.</blockquote>
    If the pipework was marginal so that a high speed was needed to get the desired quantity of heat to the radiator then going up to a 15mm pipe for the down pipes would probably negate some of the noise associated with high velocity water in pipes. It might also remove the need to turn up the speed on the pump quite so much or for a more powerful pump because whilst the restrictions of the 10mm pipe in the loft will still be there the 15mm pipe will have less resistance and it is the cumulative resistance that counts. Pipe resistance is a function of diameter, length, bends and fittings.

    Resistance creates pressure drop so a high pressure is needed with higher resistance. To get a given quantity of water down a pipe a certain flow rate will be needed, increase the pipe dia. then a lower flow rate can be used. Calculations can be made to get the best fit but nobody (that I know of) does this for central heating.

    A calculator for pressure drop can be found here
    http://www.pressure-drop.com/Online-Calculator/
    Have fun!</blockquote>

    Thanks for that. My question was an academic one really as I have no intention of replacing any pipework at the moment. All the CH pipework is 10mm microbore and the majority of it is in the attic. It would be a big job to replace all the pipework up there starting back at the manifolds, but just replacing the vertical drops not quite so onerous. It might be worth it if this was on the critical path of any decision to install an ASHP in the future where having even just some of the 10mm replaced with 15mm could tip the balance in favour.
    • CommentAuthorphiledge
    • CommentTimeOct 14th 2021 edited
     
    Posted By: Jeff B
    Thanks for that. My question was an academic one really as I have no intention of replacing any pipework at the moment. All the CH pipework is 10mm microbore and the majority of it is in the attic. It would be a big job to replace all the pipework up there starting back at the manifolds, but just replacing the vertical drops not quite so onerous. It might be worth it if this was on the critical path of any decision to install an ASHP in the future where having even just some of the 10mm replaced with 15mm could tip the balance in favour.


    The answer to your original question is yes. Increasing sections of the pipework size will reduce the resistance and increase flow for the same pump ouput. Having long radius bends formed on a bender rather than the more usual abrupt elbows will also help with flow and reduce noise. Anything that gets the pipes straighter and larger diameter helps.

    If your 10mm pipe is concealed in trunking just be concious that 15mm pipe needs bigger trunking and is far harder to manipulate to follow the contours of wonky trunking/walls.
    • CommentAuthornbishara
    • CommentTime7 days ago
     
    Thanks, that’s reassuring! I’m still surprised about the outside pipe though – as I understand it, it goes from the outside unit upstairs to the water tank… Is that usual?! I’m hoping that things have changed (it’s a daikin by the way) because last time I looked at Heat Pumps, it wasn’t a good idea to be heating your hot water with them but I’m assured that it’s fine now…?

    Thanks,

    Tania
    • CommentAuthorwholaa
    • CommentTime7 days ago
     
    I guess with a heat pump, there is warm water moving through the pipes a lot of the time, does this make thicker pipes more efficient in an additional way as they will lose less heat?
Add your comments

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

© Green Building Press