Confused about PV feeding an immersion

[gc100]

Hi,

 

I just happened to mention to the ASHP company that I would like to use PV when there is enough/excess capacity to heat the hot water via an immersion. They had planned to use the Ecodan pre-plumed hot water tank (300lt), and they don't seem to think this is possible.

 

What have people done here?? Have you drilled/tapped a secondary immersion to the tank, or do you just use the power from the PV to run the ASHP/immersion as and when it needs? 

 

I'm confused as how this works. The plumber has sent some info on a different system (NIBE S line), but from a quick scan through the documentation, these support solar coils (is thermal heating, rather than PV).

Thanks

Edited April 20, 2020 by gc100

[Jeremy Harris]

If the tank has an immersion heater then all that's needed is to wire that via one of the diverter boxes that are available.  The Apollo Gem is a pretty good one, as it allows a remote sensor to be used, which can be very handy: https://www.apollosolarelectric.co.uk/

[gc100]

10 minutes ago, Jeremy Harris said:

If the tank has an immersion heater then all that's needed is to wire that via one of the diverter boxes.  The Apollo Gem is a pretty good one, as it allows a remote sensor to be used, which can be very handy: https://www.apollosolarelectric.co.uk/

 

Thanks Jeremy. Won't there be a problem if both the ASHP controller and say the Apolo Gem are wired to the immersion? 

[dpmiller]

why would the ASHP need to control the immersion? Are you concerned about legionella?

[ProDave]

This appears to be your cylinder https://www.thefloorheatingwarehouse.co.uk/acatalog/Mitsubishi-Ecodan---Monobloc-Air-Source-Heat-Pump-with-Pre-Plumbed-Standard-Cylinder---Coastal---Protected--PUHZ_W___BS_____Comb.html

 

Just a random link chosen not a recommendation to buy it from there.

 

It looks to have a standard nominal 3kW immersion heater with no pre connected wiring so it looks to be completely free for you to use as you wish.

[gc100]

5 minutes ago, dpmiller said:

why would the ASHP need to control the immersion? Are you concerned about legionella?

 

I thought this was a requirement for BC that it comes on every 2 weeks? (and was part of the ASHP setup)

 

1 minute ago, ProDave said:

This appears to be your cylinder https://www.thefloorheatingwarehouse.co.uk/acatalog/Mitsubishi-Ecodan---Monobloc-Air-Source-Heat-Pump-with-Pre-Plumbed-Standard-Cylinder---Coastal---Protected--PUHZ_W___BS_____Comb.html

 

Just a random link chosen not a recommendation to buy it from there.

 

It looks to have a standard nominal 3kW immersion heater with no pre connected wiring so it looks to be completely free for you to use as you wish.

 

Ok as above - I don't know much about these systems and thought the ASHP controller would also control the immersion . From what you are both saying this is not the case?

 

So the immersion would just (normally) be wired up to a separate switch (or timer for legionella control)?

 

 

 

[ProDave]

My own LG heat pump does have the ability to control the immersion via a contactor.  Although I wired the contactor in I have disabled that function. 

 

My own PV dump controller just has the SSR that switches the immersion connected in parallel with the contactor so it would still operate should i wish to use it.

[gc100]

1 hour ago, ProDave said:

My own LG heat pump does have the ability to control the immersion via a contactor.  Although I wired the contactor in I have disabled that function. 

 

My own PV dump controller just has the SSR that switches the immersion connected in parallel with the contactor so it would still operate should i wish to use it.

 

Thanks Dave. Whats a SSR?

[ProDave]

Solid State Relay.  The device that pulses the power to the immersion heater.

[Roundtuit]

1 hour ago, gc100 said:

 

I thought this was a requirement for BC that it comes on every 2 weeks? (and was part of the ASHP setup)

 

 

Ok as above - I don't know much about these systems and thought the ASHP controller would also control the immersion . From what you are both saying this is not the case?

 

So the immersion would just (normally) be wired up to a separate switch (or timer for legionella control)?

 

 

 

Correct. Although if you've no PV immersion controller, you'd probably use the ashp's legionella control programme.  I've got the Apollo Gem, and the immersion heater is wired directly in to it, independent of the ashp.  It has it's own programable legionella cycle and boost facility if you need it.

 

[PeterW]

3 hours ago, gc100 said:

 

I thought this was a requirement for BC that it comes on every 2 weeks? (and was part of the ASHP setup)

1 hour ago, Roundtuit said:

Correct. Although if you've no PV immersion controller, you'd probably use the ashp's legionella control programme.  I've got the Apollo Gem, and the immersion heater is wired directly in to it, independent of the ashp.  It has it's own programable legionella cycle and boost facility if you need it.

 

 

where ..??

 

There is no requirement in the Building Regulations unless it is a business, trade or other commercial premise ..?

[gc100]

2 hours ago, Roundtuit said:

Correct. Although if you've no PV immersion controller, you'd probably use the ashp's legionella control programme.  I've got the Apollo Gem, and the immersion heater is wired directly in to it, independent of the ashp.  It has it's own programable legionella cycle and boost facility if you need it.

 

 

How does the Apollo stop over heating of the water or do standard immersion elements take care of that?

[ProDave]

The immersion heaters thermostat opens when it gets to max temperature.

 

Some of the more clever units can then dump the power somewhere else.

[Roundtuit]

3 hours ago, PeterW said:

 

where ..??

 

There is no requirement in the Building Regulations unless it is a business, trade or other commercial premise ..?

Sorry, I was referring to the mechanics, not BR requirements.  All I know is that my ashp installer got a bit twitchy when I told him that he didn't need to worry about a legionella cycle...

[Roundtuit]

1 hour ago, ProDave said:

The immersion heaters thermostat opens when it gets to max temperature.

 

Some of the more clever units can then dump the power somewhere else.

Yes, the Gem can divert power to a second 'appliance' when the immersion switches off.  I haven't found a use for it yet, but I think it's used for things like water cylinder de-stratification pumps.  If I had battery storage, I guess it would be battery first, then immersion for any surplus.

[ProDave]

At the moment I have my home made PV diverter set to turn on a little 750W convector heater when there is no other load.  That will come on at mid day even when the immersion heater is running at 100% on a sunny day if there is nothing else on in the house at that time.

[joe90]

With regard legionella I have not bothered with my set up, as @PeterWSays above no legal requirement and other threads here have shown that in a domestic situation there is virtually no risk.

[RazzleDazzle]

One burning question on this: 

 

Similar to gc100, our ASHP supplier suggested diverting excess PV to the HW tank via the ASHP (instead of using an immersion heater) - therefore benefiting from the ASHPs increased efficiency (COP of 2+). This makes sense based on this fact alone, but would the ASHP produce a high enough temperature / Draw enough power to consume all of the excess PV generation?   I believe the unit they're offering has a VFD so could ramp up/down based on supply from PV?

 

Has anyone considered/attempted this? 

 

First question so hello as well!  ?️

[PeterW]

3 minutes ago, RazzleDazzle said:

One burning question on this: 

 

Similar to gc100, our ASHP supplier suggested diverting excess PV to the HW tank via the ASHP (instead of using an immersion heater) - therefore benefiting from the ASHPs increased efficiency (COP of 2+). This makes sense based on this fact alone, but would the ASHP produce a high enough temperature / Draw enough power to consume all of the excess PV generation?   I believe the unit they're offering has a VFD so could ramp up/down based on supply from PV?

 

Has anyone considered/attempted this? 

 

First question so hello as well!  ?️

Don’t do it ..!! ASHP supplier is talking rubbish ..!! 
 

A diverter can transfer seconds worth of excess power to a resistance heater (ie an immersion) that reacts instantly.  Try that with an ASHP and it will not even power up the fan and compressor before the available power is used. 
 

 

[Nickfromwales]

2 minutes ago, RazzleDazzle said:

First question so hello as well!  ?️

Ola.

 

The question is a more complex than it first appears.

I would not want to fire my ASHP, then wait for it to reach optimum operating range, then warm through all the inter-connective pipework, have those associated losses / wear and tear / fatigue etc vs just energising a docile immersion heater which is 99% efficient in what it does. Excess PV is delivered in pockets, so ( IMHO ) is unsuitable for your proposed application.

KISS wins every time.

[Nickfromwales]

Crossed with the old boy....

[SteamyTea]

Back to basics I think.

 

PV will deliver its power to the closest load, even if that load is greater than the PV can supply.  The difference just gets supplied by the grid.  It is not either/or, but both.

 

What you can do is get the PV to initiate that ASHP i.e. when then PV is generating above a set level, say 5 amps.  Some inverters have a relay dedicated to this.

Or you can just put the ASHP on a timer, say between 9:30AM and 4:30PM, and rely on averages to offset your imported energy. This can be seasonally adjusted to take into account different hours of daylight.  Ideally you need to do both i.e. switch on once PV is supply above a limit and between set hour (times PV generation s higher i.e either side of noon and adjusted for your array azimuth).  That way you reduce short cycling of the ASHP (same apply to resistance heater in reality, adding a couple of Wh to an element every few minutes is going to reduce the life of the element, just that it is a longer life and a cheaper part to replace.

 

I think there is too much emphasis on high temperatures for the legonella cycle, do some research and find a proven domestic case that has been caused by the DHW system, then decide.

 

Edited September 24, 2020 by SteamyTea

[RazzleDazzle]

Thank you all. Can't disagree with KISS in this case. Immersion heater via an iBoost (e.g) it is. Strange that more than one ASHP supplier is against the idea - Maybe they think it dilutes the business case for their kit. 

[Nickfromwales]

15 minutes ago, RazzleDazzle said:

Thank you all. Can't disagree with KISS in this case. Immersion heater via an iBoost (e.g) it is. Strange that more than one ASHP supplier is against the idea - Maybe they think it dilutes the business case for their kit. 

And repeat business when it conks out early from improper ( unnecessary ) use and needs parts / replacing . 
There is absolutely zero chance of an ASHP responding in time with the infrequent / short lived pockets of excess during a sunny / cloudy interval day.

Also you can use the PV diversion controller as your duress boost too, giving you the recharge of the ASHP plus the electric input if / when you have very high demand ( house full of guests etc ).