Boiler pump for hot tub.

Discussion in 'Plumbers' Talk' started by Steve10123, Jun 16, 2022.

  1. Steve10123

    Steve10123 Member

    6F2497E5-32F8-4ED8-A42E-9D86A7EED3C1.jpeg Hi all. I have bought a cheap boiler to run my hot tub as others have on the interweb and have a question that you amazing plumbers will probably know the answer to straight away.

    I have the setup as pictured below and I am about to run some tests to get the best output all round.

    I am looking for the best balance of flow vs efficiency vs speed to heat.

    I originally booked the boiler up to test with the garden hose and power was quite good out of the boiler. Gas regulation was low and temperature setting half way to get 40 degree water.
    I fitted the central heating pump and it seems a lot less power OUT of the boiler, and can't get 40 degrees anymore. It sits around 60 with gas regulator on its lowest and temperature dial (which just essentially slows the water flow through it) also to lowest. I thought the pump would have pushed more than it does.

    I need faster flow OUT for another reason, and that's to push the freshly heated water around the hot tub and mix with the cold.
    I am going to run a series of tests using the CH pump on all three speed settings. Once through the boiler and once bypassing it. And then do the same using 10mm return pipe in the hope that it will come out at a higher pressure to create my mixing effect. I will know the answer shorty to this, however I don't have any 22mm pipe and don't really want to buy a length just for a test, but if I replace the supply and return lines with 22mm, will it increase the flow or not?

    it doesn't appear that the hosepipe or 15mm is restricting the flow INTO the boiler as the garden tap before pushed much more water through, so don't think that is restricting it.

    any ideas or knowledge on the physics of this would greatly be appreciated.

    Thank you in advance.
     
  2. Nexial

    Nexial Active Member

    Alarm bells are ringing in my head for a few reasons,
    Domestic central heating pumps are made of cast iron, not designed to be used in "aerated" systems as they will rust... and even faster with chlorinated water too.
    This of course depends on the exact pump.

    You may also want to check the heat exchanger to make sure that this is compatible with the chemicals that you will be using in the hot tub, or you may end up with a catastrophic failure in 6 months

    https://www.copely.com/tools/flow-rate-calculator/

    Basic flow rate calculator, 10mm pipe at 5 bar will give you less than 10 litres of water per min, compared with 22mm which will give over 100 litres per min..... so yes there is a bit of difference depending on the size pipe you use.
     
  3. nigel willson

    nigel willson Screwfix Select

    Not to mention lashing up pump wiring near a hot tub full of water
     
  4. jonathanc

    jonathanc Guest

    I really think that whole lash up affair is very unwise. Water and electricity don’t mix well and sitting in a home made human slow cooker sounds dangerous to me. Sorry but At the very least I would use those hot tubs as designed
     
  5. GRH

    GRH Active Member

    '... home made human slow cooker ...' wish I'd thought of that, never mind I'll just have to steal it. :)
     
  6. Steve10123

    Steve10123 Member

    Hi all. Thanks for your comments. The plug socket there is only temporary whilst I run tests. It will eventually run out of that housing and into a IP rate socket. However the junction boxes in my house boiler room are screwed to the floor directly under and in front of my 160l tank

    I've ordered a could of different style pumps as when I tested last night I was getting the equivalent of 3.33lpm which is very poor.
     
  7. techie

    techie Screwfix Select

    If the tub is an inflatable, get a proper one with a heat pump
     
  8. adgjl

    adgjl Screwfix Select

    Where is the exhaust of the boiler? Is it filling the gap between the surface of the water and the top of the hot tub with poisonous fumes?
     
  9. Steve10123

    Steve10123 Member

    I do have the electrical pump heater unit that comes with it, but currently in summer it's going to cost £80 per month to run. I have seen other people do a similar LPG boiler set up and it has almost quartered their costs.
     
  10. Steve10123

    Steve10123 Member

    The exhaust is on the top of the boiler, the housing that I made for it will be fitted on the external wall of the house away from the hot tub
     

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