Heating Soaking Tub - Hot Water Tank or Inline heater?

[mike2016]

I'm planning to install a 260 litre hot water tank with an integrated heat pump. I need to top up the heat in a soaking tub (210 litres worth) every day and have the option of an inline water heater from a spa company (easier to install and get my head around but more expensive to run) or to use the secondary loop from the hot water tank and a plate heat exchanger and presumably two pumps (unless there's one with two chambers?). 

 

Which is the best option and what would I need to make use of the heat pump method? The second option sounds complicated to me and more expensive to setup but if it worked would be cheaper to run. Anyone done anything similar / what parts did you use / would recommend? Thanks. 

[dpmiller]

swimming pool heatpump maybe?

[mike2016]

Seen those! I've A2A for space heating & cooling and a separate heat pump hot water tank so a third heat pump is an option but more holes in the wall!!! 

[JohnMo]

23 hours ago, mike2016 said:

planning to install a 260 litre hot water tank with an integrated heat pump.

Trouble with the heat pump it tends to be pretty small, so just heating the cylinder can be a long job - many hours. So not sure it would cope doing the equivalent of another cylinder. So that may be a non-starter.

 

Why not just use a basic heat pump to do everything, use the hot tub thing as a huge buffer for UFH. Then a big plate heat exchanger between the hot tub and UFH. Heat pump gets super long (efficient) run times.

 

It would be cheap enough to install, pretty cheap to run.

 

Would be a system very flexible heat pump size.

 

Example of what available with a short look 

https://www.ebay.co.uk/itm/196989855620?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=8MWPRtN6SYW&sssrc=4429486&ssuid=8KgwDlAbTJS&var=&widget_ver=artemis&media=COPY

 

[JohnMo]

2 hours ago, mike2016 said:

need to top up the heat in a soaking tub (210 litres worth) every day

Assume this will be chlorinated or something, so it's not a smelly pool full of nasties after a couple of days. Will make a good breeding ground for stuff like legionnaires etc...

[SteamyTea]

17 minutes ago, JohnMo said:

use the hot tub thing as a huge buffer for UFH

That is a (expletive deleted)ing brilliant idea.

[mike2016]

5 hours ago, JohnMo said:

Assume this will be chlorinated or something, so it's not a smelly pool full of nasties after a couple of days. Will make a good breeding ground for stuff like legionnaires etc...

It's filled once per week, you shower before you get in. Used in Japan extensively - called an Ofuro. 

 

6 hours ago, JohnMo said:

Trouble with the heat pump it tends to be pretty small, so just heating the cylinder can be a long job - many hours. So not sure it would cope doing the equivalent of another cylinder. So that may be a non-starter.

 

Why not just a basic heat pump to do everything, use the hot tub thing as a huge buffer for UFH. Then a big plate heat exchanger between the hot tub and UFH. Heat pump gets super long (efficient) run times.

 

It would be cheap enough to install, pretty cheap to run.

 

Would be a system very flexible heat pump size.

 

Example of what available with a short look 

https://www.ebay.co.uk/itm/196989855620?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=8MWPRtN6SYW&sssrc=4429486&ssuid=8KgwDlAbTJS&var=&widget_ver=artemis&media=COPY

 

 

There's no underfloor heating. The hot tank would be 55-60 and I'd be drawing off this once per day to top up the heat in the bath. I'll have PV and Battery so I can time shift in line water heater electricity usage to reduce cost. Not sure about a third heat pump, but maybe down the road......

[Nickfromwales]

14 hours ago, mike2016 said:

I need to top up the heat in a soaking tub (210 litres worth) every day

Top up or heat back up from ambient (cold)?
 

Big difference, so how would heat be preserved to make you say top it up daily vs heat it daily?

 

What’s the target temp of the then heated tub water? 

[Nickfromwales]

Heat direct with a small pool ASHP is a no brainer here, get a little portable outdoor unit and plug n play?

 

or simpler again is:

 

for change of £200, but direct electricity. 

[mike2016]

I'm assuming it will drop down to 21oC and I need to get it back up to 38oC. I'll build a cover out of rigid insulation that might help a bit......

 

[Nickfromwales]

12 minutes ago, mike2016 said:

I'm assuming it will drop down to 21oC and I need to get it back up to 38oC. I'll build a cover out of rigid insulation that might help a bit......

 

210L of water isn’t much, assuming this is indoor?

 

Is the tub insulated?

 

I think the option to heat from the EAHP UVC via heat exchangers would drive the HP in the cylinder very hard indeed, and perhaps have an adverse effect on its longevity, for one, but also when you factor in the inefficiency of the PHE converting heat then it adds up against it further. 
 

Running the tub as a buffer would mean having it heated all the time you are running UFH, so you’d need to insulate it and deal with evaporation losses as humidity too, if kept above the ambient room temps. Likely you’d need dehumidification then too. 

[mike2016]

I'll have MVHR and hope keeping the tub covered will handle the humidity, don't want the MVHR in boost all the time!! I've no underfloor heating only A2A. Will probably just run an inline water heater as you say looping it into the hot tank would add pressure to the heat pump there. I can always loop it in later by keeping all the pipework etc in the utility room. Don't really want an additional outdoor unit & heat pump at this stage but could add later if required. 

[Alan Ambrose]

As @Nickfromwales says, attention to the insulation both during and after use will work wonders. You should be able to do a simple heat loss calc and therefore heat pump sizing fairly easily. Ditto humidity.