when do we need a second pump for CH?

Discussion in 'Plumbers' Talk' started by rk_diy, Jan 23, 2020.

  1. rk_diy

    rk_diy Member

    New pipe works and combi will be installed when renovating the 5 bed single storey extended bungalow its a bit long house and the boiler is on the rear side rather then central location of the house.
    With conventional design, the total pipe run with branching is around 45+45m for flow and return pipes for the total 9-10 radiators.
    Another design option is to put a manifold (with or without pump?) about 7m away from combi in a central location and then individual return/supply for each radiators. In this setup the total return/supply pipe length almost same as 47+47m but each radiator is about 3 to 7m away from the manifold.

    Whats your experience and opinion when we compare two design options? Which works better?
    Do we need a second pump to boost the flow?
  2. sam spade

    sam spade Active Member

    1. What is the heat loss of the house?
    2. How many kW of radiators do you have?
    3. Boiler details?
    4. You need to calculate the flow & pressure loss of the index rad for both options.
  3. TheMorg

    TheMorg Active Member

    It will be fine. A bigger concern will be the potentially long pipe runs for the hot water.
  4. rk_diy

    rk_diy Member

    hopefully the plumber/heating engineer will calculate all but better to have some idea on design stage.
    its a detached house with solid walls. probably we will install Viallant ecoTec 938 or 838 and the existing raditors in total are about 12Kw.
    The decision at this stage is to use the manifold or not. If we use the manifold just for distribution now, its much easier to add a manifold pump if requires in the future but not many plumbers have experience with manifold systems for radiators.Its a bit try and see experience :)
    Kitchen and showers are not that far but you are right, and this is why storage boiler is one of the options.
  5. The Teach

    The Teach Screwfix Select

    When is the work going to be done ? Maybe get the design done now,including quotes.

  6. rk_diy

    rk_diy Member

    I am getting the quotes as its now time to do the plumbing but if I ask them the manifold option for radiators usually the reply was "if you want we can do it but its your choice" :)
  7. MGW

    MGW Screwfix Select

    I wish my house did not have two pumps, two pumps means two relays, single pump motorised valve micro switch does the job. In my house two pumps so the granny flat is independent of the main house, originally no motorised valves clearly some one thought if they select which pump runs they can select which area is heated, however if one pump selected before motorised valve were fitted the other area was still heated with the flow in reverse direction.

    I had expected the plumber to remove one pump when motorised valves were fitted, but he didn't so I was left to wire them up, thermostat turns on valve and valve turns on pump and relay and the relays turn on central heating boiler.
    rk_diy likes this.
  8. sam spade

    sam spade Active Member

    Let's hope the installer does calculate the heat loss and not assume that the 12kw of rads is correct.

    The 838/938 will give plenty of hot water, provided the incoming flow-rate and pressure are high enough to warrant a 38kW boiler. But they will be massively oversized for the heating (assuming 12kW is correct). Don't forget that heating requirement is calculated based on the worst case, i.e the coldest day expected, so 95% of the time the actual requirement will be much less than this. The boiler therefore needs to modulate (reduce output) as far below the design requirement as possible. If it doesn't, the boiler will spend most of it's life running in on/off mode. The minimum CH output of the 838 and 938 is 7.1kW. This means the boiler will be running in on/off mode when the outside temperature is over 8C (assuming -1C outside, 21C inside and 12kW requirement).

    If you want to maximise the savings from having a condensing boiler it needs to run with the return temperature below 55C. Vaillant boilers are designed to work with a 20C differential so this means a flow of 75. At these temperatures (75C flow, 55C return) the output of a radiator is only 85% of the nominal output. So 12kW becomes 10.2kW. At 70C/50 it's only 73% or 8.8kW. This needs to be taken into consideration or you will not have sufficient heating in the house.

    As for the need to have a second pump, provided the correct sized pipes are used, based on flow rate and pressure loss in each section. ther shouldn't be any need. The index circuit needs to be calculated.
    MGW and rk_diy like this.
  9. rk_diy

    rk_diy Member

    Thanks Sam, thats very useful information. Sounds like you are an experienced heating engineer :) are you still in the trade?
    regarding to 12Kw radiators, it might be wrong because of the adding up old radiators estimated BTUs and converting, because without any calculation I was told, we might need a boiler over 35kW+ for a 4-5 bedroom detached bungolow anyway.

    using the manifold without its own pump and mixing set is straight forward (just for distribution of flow/return indivudually for each radiator) but if we add a pump set with its own mixing valve then things gets a bit confusing. as the UFH manifolds usually have working temp range upto 65 degree but for radiators we need around 70-75 and i am not sure if the mixer valves supports that range. I think its better to use the manifold just for distribution (and flow control if required) but if a pump required then add an inline pump rather then using manifold mixing pump sets.
  10. sam spade

    sam spade Active Member

    Why UFH manifolds? I thought you were using radiators.
  11. rk_diy

    rk_diy Member

    Yes radiators Central heating system but there is a "star plumbing" design option by using distribution manifolds:

    These are the same manifolds used for zoned UFH systems usually without pump but also pump set can be added to the manifolds to boost the flow.
    The main idea is to have point to point connection and individual radiator control via manifold ports/actuators and room thermostats.
  12. The Teach

    The Teach Screwfix Select

    Tbh,super high powered hot water delivery gas combination boilers are absolutely brilliant :)

    they become less brilliant when there are more suitable hot water/heating products readily available on the market. why devalue a property with a gas thirsty storage combination boiler.

    Super high powered hot water gas combination boilers are an excellent replacement for a same high output boiler. Fitting one when there is alternatives will always be questionable ;) seen it all before :mad:
    sam spade likes this.
  13. sam spade

    sam spade Active Member

    I understand.

    I'm not sure how you arrived at the 45+45 and 47+47. Are these the total length of all the piping used in each option, or what?
  14. rk_diy

    rk_diy Member

    we have drafted design for each option. So 45+45 is the length of the pipes required for "ring-branching" design starting from boiler. And the 47+47 is the total length of the plastic pipes required to run flow and return from the manifold for each radiator (manifold requires about 7 m main pipes from the boiler as well)
    I will check again if that sounds not right :)
  15. sam spade

    sam spade Active Member

    The total pipe length will be useful when ordering the piping, but it's only partially significant when determining the pump requirement. This can only be determined by calculating the index circuit. The attached document explains how this is done.

    Attached Files:

    rk_diy likes this.
  16. rk_diy

    rk_diy Member

    Thanks Sam, now I need to study the document :) I was hoping to find an installer who already have experience in manifold systems for radiators but seems its quite new for them as well. hopefully an experienced and qualified plumber can sort that out once they grab the basic principal of manifold system.
  17. sam spade

    sam spade Active Member

    Radiators fed from a manifold is nothing new. It was quite common 30-40 year ago. It's often called a "microbore" system as the rads were fed by 8 mm or 10 mm pipes from the manifold. My house, built mid-80's, has such a system with the manifold on the first floor and surface run drops to the ground floor radiators. At some time, before I purchased the house, new radiators were installed with 15 mm pipe run back to the 10mm riser. I've yet to find the manifold.
  18. Jimbo

    Jimbo Screwfix Select

    Re heat loss. If you have cavities, make sure they are filled. It is the single largest improvement that can be made since the walls have the largest surface area.

    Then the loft, then the windows and doors. The latest uPVC windows achieve U value of 1.2, older windows with about 20mm spacing between the glass and shiny aluminium spacers around the edges between the glass will be about 2, older than that with much less spacing between the panes more like 3, and single-glazed 4.8.
  19. Mr I R May

    Mr I R May New Member

    Those rads furthest away from the boiler will not get very hot. 45m of pipe is a lot for a standard central heating pump.

    If you definitely can't put the boiler in a central location in the attik then it sounds like you might need a low loss header in the more central location. With the boiler feeding it from the back of the house.

    Then you can have 2 or more fully pumped central heating zones with their own 2 port valve if you wanted.

    It sounds like it might be sufficient square footage to require zoning in either case?
  20. sam spade

    sam spade Active Member

    Did you realize that this tiopic is over a year old? The job should have been finished by now and the OP takig advantage of it on this snowy Sunday

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