Forums

Forums
Renewable Heating
Air Source Heat Pum...
why solar diverters...
 
Notifications
Clear all

Joining the Renewable Heating Hub forums is completely free and only takes a minute. By registering you’ll be able to ask questions, join discussions, follow topics you’re interested in, bookmark useful threads and receive notifications when someone replies. Non-registered members also do not have access to our AI features. When choosing your username, please note that it cannot be changed later, so we recommend avoiding brand or product names. Before registering, please take a moment to read the Forum Rules & Terms of Use so we can keep the community helpful, respectful and informative for everyone. Thanks for joining!

why solar diverters for HW instead of the heat pump?

 
Air Source Heat Pumps (ASHPs)
Last Post by Anonymous 11 months ago
5 Posts
4 Users
2 Reactions
1,171 Views
RSS
 AdamK
(@adamk)
Reputable Member Member
Joined: 1 year ago
Posts: 211
Topic starter 22/04/2025 8:37 pm   [#2277]

am i missing something? was watching this podcast and they started suggesting solar diverters for HW even with a Heat Pump. why? heat with heat pump get maybe 1-3 ratio? or us immersion and get 1-1 ratio.



   
Quote
JamesPa
 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 3 years ago
Posts: 4634
22/04/2025 9:16 pm  

The argument I can think of is that solar diverters precisely match the power draw to the available excess solar power, which heat pumps cannot do.  This means that with a heat pump you may be importing some or most of the power consumed, whereas with a solar diverter you are guaranteed not to be importing any. 

The financial equation depends also on your import/export rates so if, for example, you are not getting paid for export then a solar diverter may well make sense even if you have a heat pump.

One approach favoured by many is to time DHW heating at say noon, when you are likely to have solar (in the season) and also the heat pump is likely to be efficient because its likely to be warm.  If, alternatively, you have a night time import tariff which is less than your export tariff it makes sense (at least when there is any export available) to do it on the heat pump at night!

 


This post was modified 12 months ago by JamesPa
This post was modified 12 months ago by Mars

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
ReplyQuote
Mars
 Mars
(@editor)
Illustrious Member Admin
Joined: 5 years ago
Posts: 4474
22/04/2025 9:45 pm  

Spot on, @adamk. This is exactly why, when our diverter failed, we chose not to replace it. It was a bit of a lightbulb moment for me, and part of what led to me creating this calculator:

https://renewableheatinghub.co.uk/which-is-more-efficient-ashp-or-solar-diverter-a-comparative-calculator/

@jamespa raises some valid points too, and it’s always good to look at the wider picture, but in our case, it’s hard to ignore the numbers. To heat our 300-litre tank to 45C, our heat pump uses around 5.81kWh (on a 2.7 COP). The diverter, by comparison, would use 15.7kWh. That kind of efficiency gap makes the choice pretty straightforward for us.


Get a copy of The Ultimate Guide to Heat Pumps

Subscribe and follow our YouTube channel!


   
ReplyQuote



JamesPa
 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 3 years ago
Posts: 4634
22/04/2025 9:54 pm  

I should probably add to my comments above that, whilst I can definitely see an argument for using a solar diverter in some cases - most particularly if you get no payment for export, I removed mine once I got the ASHP.  I currently get paid 15p for export and only pay 7p at night for import, so my cheapest option by far is to heat DHW at night on the ASHP, and export by day if there is excess solar available.


4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
👍
2
ReplyQuote
 Anonymous 5011
(@Anonymous 5011)
Joined: 3 years ago
Posts: 562
06/05/2025 8:26 am  

Posted by: @adamk
↑

am i missing something? was watching this podcast and they started suggesting solar diverters for HW even with a Heat Pump. why? heat with heat pump get maybe 1-3 ratio? or us immersion and get 1-1 ratio.

For me there are few factors why you may want to heat water via excess PV.

1. You don't get any money for export.

2. You do get money, but because you don't or can't have a smart meter you will only get 4.5p kWh 

3. I generally get a CoP of 3 or less doing DHW, so it costs me on my tariff more than either 4.3p or 10p to heat water per kWh of actual heating.

4. If you run legionella cycles you get them for free, mostly, with a diverter.

So for me, I have smart meter, but it cannot communicate due to poor signal area. We have self installed PV so don't get paid for export. Prefer to have cylinder heated to 70 degs whenever it's free to do so. Then night time (cheap rate) heating of cylinder doesn't occur, as cylinder is still hot enough, this saves a few pennies each time. Our cylinder today is still at 64 degs from being heated yesterday, so unlikely to need heating by heat pump today. Normal routine is heat twice per day, so no heat pump heating yesterday or today from one good solar day. 4 cycles of heat pump live saved. Do that 6 months of the year, you add a further year or two on the overall life of the ASHP.

If you have a battery a diverter isn't the most efficient, as battery has to full and excess energy going to grid. I prefer to use a big relay controlled by home assistant to leverage battery capacity just prior to battery being full and divert the full 3kW at the immersion. This ensures nothing goes to grid, if there is work to be done in the house.

 



   
ReplyQuote
Forum Jump:
  Previous Topic
Next Topic  



Share:

Join Us!

Worth Watching

Latest Posts

Members Online

Click to access the login or register cheese
x Powerful Protection for WordPress, from Shield Security PRO
This Site Is Protected By
Shield Security PRO →