Plumbing - proper balancing/bypass

[Spedley]

I installed a heat exchanger for my small swimming pool, however I have to balancing incorrect. Whilst it does work it is very sensitive.

A - 1.5" input from 400W pool pump through normal central heating pump valve, restricted flow into 28mm copper (couldn't get full bore from screwfix).
B - 28mm tee with 22mm down to heat exchanger
C - full bore (ish) ball valve
D - tee back up from heat exchanger
E - pump valve back to pool

I get about 80 L/m through to the pool which is about perfect, however small adjustments on valve C equates to heat exchanger being totally off or fully one. The heat exchanger is rated 90KW max but my boiler is currently running at 40KW and I'd like to adjust the flow to close to ideal to maintain a 65'C return temp and keep the flow rate to the pool as high as possible.

What is the 'proper' way to plumb this in?

[EDIT: corrected typo 80L/s to 80L/m)

 

[Terrywookfit]

What size boiler is feeding it
What is heat exch rating.
What are primary and secondary flow rates and temps.
Why is bypass C open.
Heat exch should be downstream of filtration.

 

[Nozzle]

Terrywookfit:
I should expect bypass C is open in order to "blend" hot outgoing water with colder incoming water to control the output temperature.

Spedley:
I think there might be too little restriction in your bypass and so opening the ball valve just that little bit further means it's much easier for the water to bypass the heat-ex and therefore does. In my opinion you need a different type of valve - get yourself a globe valve or similar, designed explictly for throttling. (something with multiple turns and a lead screw, but not a gate valve). Also, have you ensured the inlet and outlet ports on primary and secondary sides are counter-flow?

Nozzle

 

[Terrywookfit]

Terrywookfit:
I should expect bypass C is open in order to "blend" hot outgoing water with colder incoming water to control the output temperature.
Nozzle

With a flow of 80 l/s and 40 kw boiler......I would not have thought it needed blending.

 

[Nozzle]

My calcs show 40kW with secondary flow @ 80l/s will give a temperature rise 8.4C

What is the flow rate on the primary side?

 

[Spedley]

The blend is to keep the pressure down and the flow rate up. The heat exhanger has a lot of resistance and if all the water goes through it the flow rate drops to about 50L/m and the pressure puts too much strain on the pool plumbing.
Also, the boiler return temperature drops to about 30'C and will cause condensation and rot in the boiler.

I did consider putting a valve on the heat exchanger to slow the flow which should give the main valve at C more control but I think there is probably a 'proper' solution.

 

[Spedley]

My calcs show 40kW with secondary flow @ 80l/s will give a temperature rise 8.4C

What is the flow rate on the primary side?

That sounds about right, I think it was about 7.4 ish degrees but with errors in flow rate and temperature readings.

Primary Side? The flow through the whole system in 80L/s, the flow through the heat exchanger is unknown.

 

[Terrywookfit]

My calcs show 40kW with secondary flow @ 80l/s will give a temperature rise 8.4C

What is the flow rate on the primary side?

Are you sure about your calc.??

80 litres /sec = 288000 litres per hr or 63350 gallons.

 

[Terrywookfit]

The blend is to keep the pressure down and the flow rate up. The heat exhanger has a lot of resistance and if all the water goes through it the flow rate drops to about 50L/m and the pressure puts too much strain on the pool plumbing.
Also, the boiler return temperature drops to about 30'C and will cause condensation and rot in the boiler.

I did consider putting a valve on the heat exchanger to slow the flow which should give the main valve at C more control but I think there is probably a 'proper' solution.

It would be much less restrictive fitted downstream of your filtration.

Are you sure the pump is not 80 l/pm. 400w seems very small for 80 l/s... About same as my Koi pond !

 

[Spedley]

Are you sure about your calc.??

80 litres /sec = 288000 litres per hr or 63350 gallons.

Oops, per minute!

The heat exchanger is downstream of the filter.
Pool->pump->filter (3 port valve)->A in diagram ->E in diagram -> pool

 

[Crezzer]

Oh my god!! Your putting pool water through the copper and brass fittings? How long do you think they will last? Head loss through that lot would be incredible?

 

[Crezzer]

I would of got a bowman to keep it easy.

 

[Spedley]

Oh my god!! Your putting pool water through the copper and brass fittings? How long do you think they will last? Head loss through that lot would be incredible?

I put copper in the pool to prevent algae anyway. Copper fittings were used up until the 70's and last many decades. Head loss shouldn't be too bad through a small portion of 28mm (26.3mm internal) copper, it isn't massively lower than 32mm (1.25") that the pump is supplied with.
I agree it isn't ideal but for a small family pool (12'x3' circle) it is a whole lot cheaper than using proper pool equipment, believe me, I've tried! Anybody want a 135,000BTU pool heat exchanger?

 

[Spedley]

I would of got a bowman to keep it easy.

The cheapest bowman I could find new was about the £300 and non were available on ebay at the time. I initially used a 135,000BTU Certikin heat exchanger but could only get 60% the flow through it that it required to perform to rated spec. I want to run at 50Kw or 175,000BTU and to achieve this with a bowman at 60% flow rate would be the best part of £500. It would be cheaper to but a new filter and 1KW pump but then it would cost more to run and be excessive for a 9,000L pool.

Total cost so far (excluding Certikin which I'll put back on ebay) is £120 and will happily do 50KW with reasonable flow rates.

 

[Crezzer]

The wall thickness of modern copper pipe is a lot thinner than in the 70s, doesn't it go through 22mm?