House setup - what to do with pv excess

Electricity is the sole source of energy in my property... 

It powers the Heat pump (Nibe F1245) for heating and hot water. It powers the pump for the well water and an air pump for the waste water treatment. Its used for cooking, cleaning etc etc etc - you get the picture.

I've had a Emonpi working since the start of the year and my usage is just under 24kwh in a 24 hour period. Does that sound reasonable? Think I read on the openenergy monitor blogspot a target of 11kwh (

In December I have a 4kw solar PV array installed. The Nibe F1245 has an internal immersion for dhw incase of compressor failure / to assist with legionella cycles.

Prior to my emonpi discovery I spoke with nibe about using a solar iboost with the internal immersion. Apparently its not possible as the Nibe computer has to control it. The only work around would be to signal the computer when to turn it on but that would be a 3kw load - no variation.

So my question is what to do with the pv excess...?

Can I make use of a mk2 diverter?

My current thoughts are to put an dhw tank and immersion in the loft on the heat pump dhw cold feed pipe so to preheat all water. The down side to the this would be the cost of the tank and the requirement to relocate the current cold water tank to a higher position in the loft in order to feed the new dhw preheat tank. (the cold tank is currently on the loft floor).

Alternatively my heat pump has a 'ballast' or load tank next to it. This has a spare 28mm connection at the bottom. The tank is on the underfloor heating circulation so does such a small but powerful immersion exist that I could utilise here?

Hope the above makes sense and many thanks for reading


pb66's picture

Re: House setup - what to do with pv excess

A couple of passing thoughts

1) would it be possible to add a changerover relay to the feed of your existing immersion with the common to the immersion, a MkII on the normally closed and the Nibe feeding both the normally open and the relay coil so that in the rest position you have "MkII diverteted surplus" but when the Nibe feels it is appropriate it can override and provide full power.

2) perhaps a external immersion heater unit could work either wth the "ballast" tank or as an etension to the existing DHW setup.

I assume there is no way to fit a second element to the existing tank?



graemetherunner's picture

Re: House setup - what to do with pv excess

Thanks for the reply...

1) I suspect its possible but... probably not easily, and I wouldn't really know where to start or even get to the feed for the immersion - any one know the Nibe heatpumps well enough?


2) interesting thought another unit... it would have to be small though as externally to the tank I've not a huge amount of space left in the cupboard... and I thought I was being so smart when it was installed in there!

The only 'hole' left in the tank is a 28mm.. its at the bottom though so perfect location.

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