Using an existing ASHP hot water supply to heat a pool

[peekay]

Hi all. 

 

We have an 12kW air source heat pump with a 400l tank for our domestic hot water. Temperature set to 55°C.

 

We have just bought a 3.5m diameter, by 1m deep above ground swimming pool that we will put up for the kids to play in over the next few weeks once the warmer weather returns. This will likely stay up until autumn. 

 

We are looking at buying a small either 3.5kW or 5kW pool heat pump to warm keep the water warm to around 25-28deg  (example: https://www.tradeinn.com/swiminn/en/poolex-nano-action-a5r-2020-5kw-2.5-m--h-inverter-heat-pump/139816348 )

 

 

My question:

 

Rather than filling the pool with the cold water supply, and having the small pool heater work to bring the water up to temperature over a few weeks, would there be any benefit in filling the pool with water from our hot water tank 400l at a time until the pool is at the required temperature and then letting the small pool heater take over?

 

I've read that I should expect it to cost about £10 a week to maintain the pool temperature, does anyone in the South East have any thoughts on whether this is a realistic budget? And is there any benefit in trying to insulate the pool in addition to the standard thin polythene pool cover? We have some old foam garage floor tiles that we were planning on putting under the pool for protection/insulation but would welcome any other tips about setting up or running a similar pool.

 

 

Thanks

 

[JohnMo]

Gone are the days it got filled and you just on with it, if the sun warmed it great.

 

If your heating you may need to treat it also, as it will soon start to smell. Plus be pretty unhealthy, as you will be optimal temperature for legionella and other bugs 

 

 

[peekay]

  On 15/04/2025 at 07:30, JohnMo said:

 

 

If your heating you may need to treat it also, as it will soon start to smell. Plus be pretty unhealthy, as you will be optimal temperature for legionella and other bugs 

 

 

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Thanks, I've already ordered various pool chemicals.

[SteamyTea]

That size is basically a small spa bath.

Get a separate ASHP for it and keep it covered.

 

If you get a nice rash on your body, change the water.

 

https://my.clevelandclinic.org/health/diseases/23358-hot-tub-folliculitis

[saveasteading]

Insulation. The easy bit is below. Most heat goes off the top in simple heat transfer but also in evaporation .

Is small enough that you can lay a bubble wrap on the surface and a tarpaulin over the top.  You could add extra layers. 

It keeps the dirt out too and birds/insects/children.

You could also build a timber surround and fill it. That could link with access/jumping platform/seating.

 

£10  per week might be about right with all that.  Less in summer.

[peekay]

  On 15/04/2025 at 08:14, saveasteading said:

Insulation. The easy bit is below. Most heat goed off the top in simple heat transfer but also in evaporation .

Is dmall enough that you can lay a bubble wrap on the surface and a rarpalin over the top.  You could add extra layers. 

It keeps the dirt out too and birds/insects/children.

You could also build a timber surround and fill it. That could link with access/jumping platform/seating.

 

£10  per week might be about right with all that.  Less in summer.

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I've got lots of PIR board offcuts from the build, so might try and fix it together in to a pool sized panel to create a 'lid' for the pool to use in addition to the thin tarpaulin type one. Will keep it out of the water though, as I'm not sure what effect the pool chemicals will have on the PIR board. Thanks.

 

Still no thoughts on whether using my domestic ASHP hot water supply to do the initial pool fill is a good idea or not though.

 

The pool and small ASHP pool heater have now arrived, so will be starting the fill over the weekend.

 

 

 

[saveasteading]

  On 17/04/2025 at 10:30, peekay said:

what effect the pool chemicals will have on the PIR board. 

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Very high chlorine level. So likely to be damaging. I would prefer a foam or bubble wrap from a tub or pool supplier. That will be slightly insulating and totally stop evaporation. Otherwise or as well, a removable cover bearing on the walls, with as much pir as you like.

[JohnMo]

  On 17/04/2025 at 10:30, peekay said:

Still no thoughts on whether using my domestic ASHP hot water supply to do the initial pool fill is a good idea or not though

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Do what ever you feel, your water. Getting there water from hot tap to pool may need thinking about. No issues filling with cylinder water, just need a 50/50 mix with cold or there about.

[Iceverge]

Can you temporarily tee off a loop from the central heating and run a coil through the pool water? 

 

If you have solar PV you could chuck a cheap resistance water heater into it and just use it instead of exporting. 

 

Cost of running will be impossible to calculate without knowing your 

exact setup and pool. 

 

If it's an inflatable pool then you will have a pretty low heat loss so long as you insulated the floor and get a good lid. 

 

If you can get your hands on some bubble wrap it would possibly be a practical way to wrap a non insulated pool and you could cut a floating lid from the same. 

[Iceverge]

Depending on how much work you want you could always just dump say half the water every day and refill with hot cylinder water to hear it up again. 

 

Simple enough calculation to be done depending on how much you are paying for water Vs how much you are paying for electricity. 

 

 Perhaps not the best use of water but that's your call to make. 

[saveasteading]

There must be a product which is a coil encased in a metal tube. An immersion heater with an enclosure to keep it off the material.

[Dillsue]

  On 17/04/2025 at 11:54, Iceverge said:

 Perhaps not the best use of water but that's your call to make. 

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Up until the last few days we'd had 1 day of rain in the last 2 months.....witness the wildfires we've been seeing in the media. The nearest reservoir to us is only half full as we come out of winter.

 

Might be worth being a bit conservative with something we really don't want to run short of??

[JohnMo]

  On 17/04/2025 at 15:09, Dillsue said:

Up until the last few days we'd had 1 day of rain in the last 2 months.....witness the wildfires we've been seeing in the media. The nearest reservoir to us is only half full as we come out of winter.

 

Might be worth being a bit conservative with something we really don't want to run short of??

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Sounds like hosepipe bans in spring instead of summer!

[SteamyTea]

  On 17/04/2025 at 12:48, saveasteading said:

There must be a product which is a coil encased in a metal tube. An immersion heater with an enclosure to keep it off the material.

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A Willis heater. Used them all the time for spa baths we made over 30 years ago.

Could have a heat exchanger and a Willis in series.

 

 

A bit about evaporation losses from water surfaces here.

https://www.thermexcel.com/english/program/pool.htm

Bubble wrap is as good as anything.

But make sure no one can get in the pool when it is on.

 

 

 

 

Edited April 17 by SteamyTea

[saveasteading]

  On 17/04/2025 at 15:41, SteamyTea said:

A Willis heater.

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That is an inline heater isn't it? so could be used for warming water during filling.

 

I meant a heater on the end of a cable, plugged in one end and splashed in the other.

That way the water stays put and gradually gets thicker.

 

The Romans would have heated a sword and plunged it in the water.

[SteamyTea]

  On 17/04/2025 at 15:56, saveasteading said:

  On 17/04/2025 at 15:41, SteamyTea said:

A Willis heater.

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That is an inline heater isn't it

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Yes.  Nothing fancy at all.

Could be used for inline heating, but at around 2.8 kW, would need a relatively slow flow rate (I could work it out, but can't be bothered).

  On 17/04/2025 at 15:56, saveasteading said:

I meant a heater on the end of a cable, plugged in one end and splashed in the other.

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Safety would be my concern.

  On 17/04/2025 at 15:56, saveasteading said:

gradually gets thicker

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Not sure what you mean.  Water will get thinner as it heats, though limescale may make the heating element thicker.

 

  On 17/04/2025 at 15:56, saveasteading said:

The Romans would have heated a sword and plunged it in the water.

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I assume you mean Ancient Romans.

Maybe this fellow can answer that, ask the right person on here you may get an answer.

 

https://www.youtube.com/watch?v=wmax3ntBS0Q

[saveasteading]

  On 17/04/2025 at 17:23, SteamyTea said:

gradually gets thicker

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As users discard skin, oils and wee. It gets killed by the chlorine but stays there.

[SteamyTea]

  On 17/04/2025 at 17:41, saveasteading said:

As users discard skin, oils and wee. It gets killed by the chlorine but stays there

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The worst job I have ever done was cleaning out the spa bath DE filter at the Sheffield YMCA, I have no idea what those young men did in it.

[Michael_S]

WE added some valves to the ASHP water circuit teeing off a separate circuit that we stuck through a plate heat exchanger (from a gas boiler, £20 off ebay) and then had the pool filter circulation pump go through this as well - so the heat exchanger is basically run as a 'zone'.  Add appropriate timers to the heat pump and the filter pump and jobs a good un.

[Nickfromwales]

A lot of partial water changes will be necessary to keep this anywhere near healthy and stable tbh.

 

If you’ve a heat pump > UVC then just dumping that into the ‘pool’ would be the easiest and reasonably efficient way of just knocking the chill off the water. 
 

Having this heated is going to be a big uplift in cost to maintain, so prob best to set your expectations accordingly. 
 

  On 17/04/2025 at 11:54, Iceverge said:

Depending on how much work you want you could always just dump say half the water every day and refill with hot cylinder water to hear it up again. 

 

Simple enough calculation to be done depending on how much you are paying for water Vs how much you are paying for electricity. 

 

Perhaps not the best use of water but that's your call to make. 

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As 👆 but with the caveat that we see value in doing what puts a smile on our kids faces.

 

If any of this is offset by off peak electricity or micro-generation then I could live with it tbh.
 

If it was mine, I’d have a blended valve with a timer and solenoid for filling, with a dedicated outside tap and a hose to the pool. Then I’d partially empty it each evening when in use and then maybe overheat heat it from midnight - 05:00.
 

That would be in conjunction with a good cover on and a good quality local pump and filter being on and regularly cleaned.

 

Otherwise this will quickly become a vat of unhealthy (invisibly so) goop which my kids would be told to avoid.

 

I fitted a 19’ x 5’ above ground heated pool for a client, tiered garden which we reduced at the end so the pool was 4’ below ground and wrapped by a deck, was a nice thing to have if you could afford it. I put up a dedicated ‘shed’ for the boiler and pump house and sand / cartridge filter etc etc and the cost was thousands to get this all right.
 

Unfortunately you’re either dedicating your life and time to this with a big investment in infrastructure, or just emptying and filling with potable (already mildly chlorinated) water prior to use, on demand, and sucking up the cost to provide the heat and the water.

 

Any heated, maintained pool is a money pit and is hugely time / attention hungry. Hot tub owners are one of 2 people; lazy smelly buggers who don’t care and keep adding more chemicals, or immaculate and particular folk who keep the unit spotless, clean, and healthy, and pay the price and have the time.

 

Oh, and with kids, I’d NEVER entertain putting an electric heater into the pool….. The only place for a certified inline electric heater is on the return of the pumped filtered loop, but then you’d just bite the bullet and use a cheap pool ASHP so the running costs were 1:3 / 1:4 which isn’t too bad. Again, used pre 05:00 to do the bulk of the heavy lifting to further reduce costs.

 

@SteamyTea how long would the temp need to be at 45°C (or more?) to do a legionella purge for the 3.5m x 1m pool volume? 

[SteamyTea]

  On 18/04/2025 at 09:52, Nickfromwales said:

how long would the temp need to be at 45°C (or more?) to do a legionella purge for the 3.5m x 1m pool volume? 

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Legionella is not the problem, that needs atomised water to get down the windpipe and into the lungs of immune system compromised people.

Ghonheria would be a bigger problem after the immaculate and particular folk have had their swingers party.