DHW with air source heat pump

[Jamesessex]

Very grateful for any advice. 
 

it’s a new build property with a vaillant air source heat pump. 
 

the domestic hot water is set to a temperature of 60 degrees for which the air source heat pump manages around 45 degrees and then an immersion heater kicks in to get this to around 60. 
 

My question is how frequently should the immersion be used to obtain the 60. 
 

is this best daily or less frequent. 
 

45 degrees is more than adequate for showers etc but appreciate the bacteria issue of not heating to 60. 
 

I’m wondering if it is acceptable to heat the water to 45 degrees 6 days a week and then have the immersion on for day 7. 
 

Many thanks 

[joe90]

Welcome, I do not heat to 60’ at all with my ASHP, this legionella thing has been debated here and I don’t believe there has been one case of residential infection in this country. I heat my DHW to 48’ and find that more than adequate fir all uses. My DHW tank is a bit larger as I will not blend water for use as much. 

[SteamyTea]

19 minutes ago, Jamesessex said:

question is how frequently should the immersion be used to obtain the 60. 

Never, unless you run the risk of running out of hot water.

I am assuming that you have one of those UVC that heats incoming mains water via a coil in the cylinder.

Do you worry about waterborne desease in your drinking water?

 

The whole legonella thing is a different problem, more to do with airconditioning, not DHW.

The H&E Executive have muddied the waters, uf you excuse the pun, by insisting that taps in large buildings get tested. I suspect this was a pacifier to a vocal minority, who had no idea what they are talking about.

Edited December 3, 2020 by SteamyTea

[ProDave]

Yes another here who heats the DHW to 48 degrees with the heat pump.  the only time it gets heated hotter is when there is surplus solar PV to dump into the immersion heater.

 

The conclusion of the debate was if it is mains treated water into an unvented tank, it will be free of bacteria, because it is sealed in an UVC nothing can get in, and there is enough residue of chlorine to keep it clean, so no legionairs is possible.

 

Different matter if it is borehole or other private water and a vented water tank.

[SteamyTea]

2 minutes ago, ProDave said:

Different matter if it is borehole or other private water and a vented water tank.

and then a large fan to whip up the surface into microscopic particles, of just the right size and temperature, that can then me sprayed over a small, but vunerable, group of people.

[ProDave]

44 minutes ago, SteamyTea said:

and then a large fan to whip up the surface into microscopic particles, of just the right size and temperature, that can then me sprayed over a small, but vunerable, group of people.

I am not sure I understand that as the fan of an ASHP is nowhere near the water tank so how will it stir up particles?

[SteamyTea]

23 minutes ago, ProDave said:

I am not sure I understand that as the fan of an ASHP is nowhere near the water tank so how will it stir up particles?

Old cooling systems used to have a weir of cold water, this was cooled by a fan.  It is these systems that caused Legionella, not modern ASHPs, or UVC.

Though I am willing to start a rumour that unless you refuse to shower in water that has come from a UVC that has not been purged weekly, and kept above 65°, permanently, but only after extra chlorination, and only a rainstorm showerhead in use, you will get legionella.  Oh, and the electricity to run it all will give you cancer (an unspecified one, but one must be caused by it).

And not drying your bottom properly gives you piles.

Showers, best avoided then.

 

Not that I care, I am reheating rice, and we all know that is deadly.

 

Edited December 3, 2020 by SteamyTea

[PeterW]

The water temperature issue may not be one of any nasties in the water itself, but some showers do not work well with water that is below 48°C and 50°C for some of the digital mixers. 
 

@Jamesessex when is the heat pump / immersion timed to come on ..? And what is your heating ..? UFH or Radiators ..?

[TonyT]

I would say the risk in a domestic situation is negligible with town ( supplied by water authority and containing chlorine) water

 

as long as the cold water incoming is under 20deg c you should be fine.

 

as long as dead legs of pipe work aren’t installed and left with water you will be fine.

 

as long as the cold water storage of any is turned over regularly you will be fine.

 

download free HSE L8 guidance for further info

[Jamesessex]

3 hours ago, PeterW said:

The water temperature issue may not be one of any nasties in the water itself, but some showers do not work well with water that is below 48°C and 50°C for some of the digital mixers. 
 

@Jamesessex when is the heat pump / immersion timed to come on ..? And what is your heating ..? UFH or Radiators ..?

Hi, the heat pump comes on at 2am until 4am and then the immersion heater does 4am to 6am 

[ProDave]

13 minutes ago, Jamesessex said:

Hi, the heat pump comes on at 2am until 4am and then the immersion heater does 4am to 6am 

And what re heats the tank in the day if you get low on HW?

[Jamesessex]

11 minutes ago, ProDave said:

And what re heats the tank in the day if you get low on HW?

Ah yeah the tank is heated again by the heat pump for an hour in the afternoon for the small amount of hot water needed in the evening. Most of the hot water is used in the morning only after the overnight heating 

[PeterW]

@Jamesessex just as an aside, the immersion is on for too long to get you from 45-60°C assuming this is a 210 litre tank. It will take an hour and 10 mins to get to 60°C which means from 5am to 7am it is cooling down and you’re not getting the full benefit of E7 overnight tariff. 
 

I would change the timing on the ASHP to 3:30 5:30am then the immersion from 5:30am to 7am. It may not save you much but it will be a start. When does the heating kick in ..?

 

[TonyT]

12 hours ago, PeterW said:

@Jamesessex just as an aside, the immersion is on for too long to get you from 45-60°C assuming this is a 210 litre tank. It will take an hour and 10 mins to get to 60°C which means from 5am to 7am it is cooling down and you’re not getting the full benefit of E7 overnight tariff. 
 

I would change the timing on the ASHP to 3:30 5:30am then the immersion from 5:30am to 7am. It may not save you much but it will be a start. When does the heating kick in ..?

 

 

doesn’t matter if it’s programmed to be on too long the immersion thermostat will cut the power to the immersion element when it reaches temperature regardless of the timeclock being ‘on’

 

[dpmiller]

7 minutes ago, TonyT said:

 

doesn’t matter if it’s programmed to be on too long the immersion thermostat will cut the power to the immersion element when it reaches temperature regardless of the timeclock being ‘on’

 

 

/presuming the immersion stat's correctly set and actually working...

[PeterW]

8 minutes ago, TonyT said:

 

doesn’t matter if it’s programmed to be on too long the immersion thermostat will cut the power to the immersion element when it reaches temperature regardless of the timeclock being ‘on’

 

That isn’t the point I’m trying to make - it will continue to reheat the water - due to losses - over that last hour until the timeclock stops the heating period. You need to be looking to optimise the heating cycle so it hits its peak temperature just before the last point in the time period, and only for as long as necessary. It saves energy and reduces system losses. 

[SteamyTea]

I have 're-timed' my E7 period to the last 3 hours.  It has the added benefit of using the lower carbon period as well.