Optimizing Flow Rate for Above Ground Pool Heater Installation

[DaveC426913]

TL;DR Summary

How do I determine what my flow rate is?

I am having a heater installed for my 12ft round above ground puddle pool. The heater is a K-Star K-5 (5kW) heater.

The manual says the heater's max flow rate is 8GPM and I'll need to install a diverter with a valve to bypass it until the flow is <8GPM.

How do I determine what my current flow rate is and how do I know when I've diverted enough to set it below 8?

I think an inline real-time flow-meter might be overkill, unless it's simple and inexpensive, but I'm not sure. I think I could just get a one-time reading and set my diverter based on that.(BTW, I have 1.5" hosing.)

 

[Dullard]

I'm not sure why the heater has a maximum flow value specified, unless it's a back-handed way to limit case pressure / dP. I would probably use a 5-gallon bucket and a stopwatch to get in the ballpark (start with the 'bypass' wide open).

 

[gmax137]

@Dullard beat me to it. If the heater isn't plumbed in yet, can you do it like this? Then Close valve "out" and open "drain," then throttle "byp" until it takes 38 seconds to fill the 5 gallon bucket (catching the flow out of "drain."

 

[russ_watters]

I assume this is being added to an existing pool filter system? Do you have a performance spec for that?

 

[hutchphd]

Can you capture the flow somehow? This is not rocket science precision so 8gal/min is a quart every two seconds. Can you get an estimate somehow? A slower flow will just make the effluent water a little hotter. At one liter/2sec, a 5 kW heater will raise the effluent temperature by $2.4 ^o C$ . So there is no scalding danger for slower flows.

 

[DaveC426913]

I would probably use a 5-gallon bucket and a stopwatch to get in the ballpark (start with the 'bypass' wide open).

Oh. This is pretty low-tech.

I have not plumbed the heater in yet; it is currently only pump/filter. Can I simply measure the outflow back into the pool? That should be easy enough.

The outflow has a limiter than turns it into a jet (like sticking your thumb over a garden hose) to facilitate circulation in the pool itself.

Does this lower the flow rate in the whole system? If so, I should ensure I measure the flow after this.Actually, so will the heater plumbing.

It sounds like:
- I really need to plumb the heater and bypass in before taking measurements, since that will dramatically affect the rate.
- I can just leave the bypass open and measure the outflow (after all connections), then close the bypass until I ...

... wait a minute. The goal is to control what's going through the heater. Measuring the outflow into the pool doesn't tell me how much is going through the heater.

I'm not sure how I can measure that without the whole system being set up and sealed. Detaching any part of it to catch outflow will (significantly) change the rate of flow. That implies an inline flow-meter.

Or am I missing something still?

I assume this is being added to an existing pool filter system? Do you have a performance spec for that?

It is but that won't give my any numbers. I can tell you the size of the pump, but that'll tell you nothing about the effect of filter and plumbing on flow rate.

 

[DaveC426913]

A slower flow will just make the effluent water a little hotter. At one liter/2sec, a 5 kW heater will raise the effluent temperature by $2.4 ^o C$ . So there is no scalding danger for slower flows.

Interesting. I had not thought of that as a rationale for limiting flow. I'd assumed it was a pressure thing.

Temperature limiting is a "soft" limit (i.e. whatever I'm comfortable with), compared to pressure-limiting which I would not want to flout.

 

[gmax137]

I'm not sure how I can measure that without the whole system being set up and sealed. Detaching any part of it to catch outflow will (significantly) change the rate of flow.

Did you see the diagram in my previous post? It is not perfect but it is surely close enough. Plus you want to plumb the heater in with valves so you can take it out when it breaks down. So the only addition is the drain valve, also probably a good idea to have anyway.

 

[Dullard]

If the total flow returns to the pool via the same jet, you have a situation (as you note) where the bucket is useless. You need a flow meter. If you're handy (and cheap), you can build one. a 0.7" orifice will produce approx 12" wc (drop) at 8 GPM. A piece of clear tubing connected up/dnstream of the orifice will serve as the pressure gauge. Exactly how you implement that depends on what you have available.

Being lazy, (and assuming that this is an ON/OFF heater), I'd probably just measure the temperature change from the inlet to the outlet (of the heater) with the heater ON (See Hutchpd's post) and adjust accordingly. This is a 'hillbilly' thermal flowmeter. Again, much depends on what you have laying around.

 

[DaveC426913]

Did you see the diagram in my previous post? It is not perfect but it is surely close enough. Plus you want to plumb the heater in with valves so you can take it out when it breaks down. So the only addition is the drain valve, also probably a good idea to have anyway.

I did. My system (sans heater) is already set up, so not a lot of leeway. The manual's diagram is pretty much the same as your diagram. Except drain is off filter.

 

[hutchphd]

Measuring the outflow into the pool doesn't tell me how much is going through the heater.

You know if you've lived a good moral life its possible that the full pump is less than 8gal/min in which case you can't overdrive it. In any event I'll bet it is not much more. Measure the full flow and proceed from there
I have a 5kW heater on my hot tub and the little recirc pump drives it...less than a gal /minute. The water is probably 15F hotter coming out
The only downside to restricting the flow too much would be overheating and there's a lot of room there to play with as I mentioned previously. Can you lay hands on the heater exit pipe? Just make sure its nicely warm not hot. You may be overthinking this!

 

[DaveC426913]

You know if you've lived a good moral life its possible that the full pump is less than 8gal/min in which case you can't overdrive it. In any event I'll bet it is not much more. Measure the full flow and proceed from there

On the contrary, I think it'll be much higher.

The only downside to restricting the flow too much would be overheating and there's a lot of room there to play with as I mentioned previously.

I do believe it will shut itself off if it gets hotter than it likes.

Can you lay hands on the heater exit pipe? Just make sure its nicely warm not hot. You may be overthinking this!

Yes. The manual said that flow should not exceed 8gpm but it didn't say why.

If it's a temperature issue, that's certainly something I can check first hand without instruments or disassembly or extra plumbing.

Thanks!

 

[DaveC426913]

This is my setup:

 

[hutchphd]

The temperature issue only is a problem for too slow flow.
So does the pump run continuously? 8gal/min would be ~500gal/hr so it will cycle the pool in 6 hr. Surely it needn't be much bigger than that?

 

[Tom.G]

I have two comments on the plumbing:
1) A union downstream of the Shutoff Valve may be useful
2) If the Violet line, ('To Pool') enters below water level, a shutoff valve there would be useful (probably with unions)

Cheers,
Tom

 

[DaveC426913]

I'm still not clear why there would be a max flow - even (indeed, especially) if it's a temperature thing.

Whenever I ask pool-savvy people, they seem to posit that, if there's too much flow, somehow the water is not there long enough to get heated - as if heat is a passenger failing to board a moving train.

Of course, this can't be true. Faster flow can only help the heat transfer from element to water.The reason I'm harping on this is because, ideally, I would set the system up for maximum heating, so that I can raise the temp as quickly as possible on a day people want to go in.

 

[hutchphd]

The max flow is not a temperature thing. Sorry if I was unclear. Probably pressure or turbulence but I don't know.

 

[DaveC426913]

The max flow is not a temperature thing. Sorry if I was unclear. Probably pressure or turbulence but I don't know.

Thanks for clarifying. That jives with my thoughts too.

 

[DaveC426913]

Wow! That came together fast!
From pulling the trigger to up-and-running the whole project took 5 days. (When you're doing it for your granddaughter, no expense or effort is spared.)

Getting an electrician right away sure helped. But I also think I got the last pool heater in North America. People are going nuts preparing for a Covid Summer.

Costs:
Electrician (repurpose dryer circuit, run line to back): $1200
Heater: $660
Fittings: $500 (!)
- 30ft PVC: $100
- 12ft flexi: $50
- 3-way valve: $90
- shutoff valve: $15
- a whack of unions: ~$100
- elbows: $50

That less than the cost of a trip to Cuba, so it's paying for itself.

 

[DaveC426913]

OK, now for the payoff:

patience ... patience ...

 

[Tom.G]

In that photo of the installed heater, There should be at least a drip shield (hood) over the power outlet.

There are also configurations available with a clear plastic front enclosure that is hinged at the top and the cord(s) exit at the bottom.

Both good to reduce corrosion, nuisance tripping of GFI, etc.

Cheers,
Tom