Hot tub boiler install

[Steveuk]

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.

 

[Basher]

You meed to maximise the volume of water you are moving through the boiler.
It's not about 'faster flow'. You can have a tiny pipe with fantastic flow speed, but you won't get much volume through it.
10mm pipe is too small for anywhere in your scheme.

Your hose pipe was probably giving 20 litres or so per minute
Maybe your pump is not giving that much?
I gather typical pumps in hot tubs can move 300 to 400 litres/minute, and that's no doubt facilitated by big diameter pipes.

Swimming pool plumbing I have been involved with used 2 inch pipe, but I gather 1.5 inch is used also. Of course all the equipment such as heater, filter, pump, all have that bore as well!

I'm not your man for the sums. Your hot tub will probably have over 1000 litres of water in it, possibly 1500? That's a lot more than a domestic heating system! My intuition, and nothing more than that, is you need bigger pipe, and a bigger pump, but not so big as to over pressure the boiler heat exchanger. Put a pressure gauge on the input to the boiler and see if you can get the input pressure up to say 2 or 3 bar with the boiler output connected via a fat pipe to the hot tub!
If your boiler connections are 22mm I would be using at least 22mm pipe everywhere.

I have my doubts about the wisdom/ safety of this setup, but I will leave that for the experts to comment!

 

[king of pipes]

The use of a heat exchanger and a separate pump to exchange the the heat from the boiler to the Hot tub water through separation would be the best option, high chlorine levels can be corrosive and damage your boiler set up ,sorry to say what you have is a bit DIY employ a professional for a day is my advice . Kop