Possible pump change, what are the calculations for correct purchase selection?

[Richard_S]

Hi all,

it's that time of year again...

I have 80-90 cubic metres of water in my pool and currently run a Hayward Max Flo XL VS pump 1.5HP (approx 8yrs old), with a 50mm inlet/outlet. This is attached to a sand filter which has a spec of 5000 litres - 12,000 litres per hr at a max flow rate of 50 cubic metres/hr/m2. (I believe the conversion is 25-43 GPM). General piperwork I believe to be 50mm/2 inch (I cannot check at the mo).

The issue I am currently having is that I need to run the pump at around 2600rpm for the pool pilot digital SWG to say that there is enough flow. The pump also sounds a little unwell currently, being much more noisy than last year.

Whilst I could have the pump serviced, for various reasons this might prove extremely difficult. It might be easier to buy a new Hayward Variable speed pump.

I was looking at the Hayward TriStar VSTD SP3216 (1.5HP) as a replacement, as I am not entirely convinced that the Max Flo is not under specified for the pool. I note however that this has a 63mm inlet/outlet and whilst I am sure that I can find adaptors to fit this to the old pipework, is this pump too powerful for the system?

So in a very roundabout way, how do I work out what is the correct sized pump for the pool, which will give sufficient flow to the SWG without having to run it a full beans? The ultimate goal is to run the pump at the lowest speed for 24hrs/day with the odd burst to skim the pool. It needs to be as un-attended as possible as I am not local to the pump.

Hopefully that makes vague sense????

 

[ajw22]

A 1.5 HP is plenty for your size pool and equipment.

What is the PSI on your sand filter?

When was the last time your filter had a deep cleaning?

Do you have an MPV on the sand filter with a RECIRCULATE setting?

Sand Filter Use and Care - Further Reading

www.troublefreepool.com

 

[Richard_S]

A 1.5 HP is plenty for your size pool and equipment.

is that for the original Max Flo (1.5HP) or a new TriStar (1.5HP) - or both?

What is the PSI on your sand filter?

Not entirely sure - just have a red/orange/green gauge at the top and it obviously changes depending on how fast I run the Max Flo. Most of the time it is just below the green, so not actually registering. I backwash if it ever hits the middle of the green.

When was the last time your filter had a deep cleaning?

I haven't actually ever done a deep clean as per your link, just completely changed the sand about 6yrs ago

Do you have an MPV on the sand filter with a RECIRCULATE setting?

MPV - I guess you are talking about the valve on the sand filter? I do have a re-circulate option, along with backwash, filter, to waste, rinse etc.

 

[ajw22]

Put your MPV on recirculate and see how the flow changes. That will tell you if your filter is causing your poor flow.

1.5 HP is plenty from both pumps. You don’t use all of it when running a VS pump at less then max RPM anyway. So the HP has little meaning.

I don’t know how you know when to backwash your filter without a functioning PSI gauge. Pick a pump speed where it gives a good reading. That can be max RPM. Note the clean filter pressure and that is your reference point for best clean filter performance and flow. Clean your filter when the PSI increases by 25%. Without that reference point you have no way to judge your filter and pump performance.

 

[Richard_S]

Yes, I use 3000rpm as my clean speed/filter pressure reference point - sorry wasn't clear. Would the TriStar move more water at the same nominal rpm as the Max Flo? I am assuming that the bigger pump, with bigger inlet and outlet would up the water pressure on the existing pipework and at the same time increase flow? I wouldn't have to up the size in pipework, assuming that the system could take the extra pressure?

Been a while since I did fluid dynamics so perhaps I have this all wrong?

 

[ajw22]

It's not that simple since a higher HP pump uses a different impeller to move more water.

Changing the pipe size will change the pressure of the flow, not the volume of the water.

 

[Richard_S]

OK, so assuming the Pool Pilot is saying the flow is too low for Chlorine production, then would changing to the TriStar deal with that - everything else being equal!!

 

[ajw22]

You sure you don't have a bad flow sensor in the Pool Pilot?

Any 1.5 HP pump should give sufficient flow for a SWG flow sensor.

What RPM are you running the VS pump at when you get low flow?

I think changing pumps is not addressing the root cause problem which is likely outside of the pump.

 

[Richard_S]

Anything below 2200 RPM gives low flow on the pool pilot so kinda defeats the object of running the pump for longer at slower speeds (and saving electricity). I think as you say, I need to do some research on how the pool pilot senses flow. Will look into the Tri Sensor and see if that isn't the problem - it's never been cleaned or changed and is approx 7yrs old, so that may well be the issue. Thanks for all the help btw, great to be able to bounce ideas off you. I think as some mechanic once said to me, replace the cheap stuff first and work you way up....Which of course is blindingly obvious, but sometimes does need to be said!

 

[ajw22]

Did you try running your filter in RECIRCULATE mode to take it our of the flow and see if you can run your SWG at a lower RPM?