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Gas Boiler and Solar PV Diverter (EDDI)- Help me understand

  • April 30, 2025
  • 6 replies
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Hi, can anyone help my understanding of this system please. 

I have a solar PV and battery system that is working great, with plenty of excess energy.  We fitted an Eddi unit to divert any excess to the water tank with the hope of reducing the gas bill, associated with heating the tank. 

Everything is working and the excess energy is send to the tank and heats until “max temperature achieved”.  However, I have left my gas water heating schedule on and noticed that even though the tank is already at temperature the boiler would fire up for the duration of time the schedule is set to run - should that happen? and is that using gas to heat an already hot tank??

Given the now amazing weather I thought I would turn the gas schedule off and set the Eddi unit to boast the tank using the stored (battery) energy in the morning and again in the evening, while continuing to use the excess to top up.

Despite the energy going to the tank in the morning for an hour (05:00-06:00), I have woken up to warm, but not hot water to wash with.

Also in the evening, by the time the rest of the family have has their showers, I have had to take a cold shower.  I did notice that the boost hadn't happened last night - could that be that at the time it was set to heat, the tank was already at temperature so it didn’t start the cycle?

Sorry for the long post, I just want to make sure that I get the savings and dont use the gas, if the tank is already at temperature.  

Forcing the boost with the Eddi unit has resulted in 2 cold showers for me out of 2 nights.  I must be doing something wrong.  

Best answer by BPLightlog

Updated on 12/06/25 by Abby_OVO

There is an option to force boost on a schedule, which is what I am using as well as the excess.  It worked on 2 of 3 scheduled boosts.  
 

I just can’t get my head around the boiler still firing up when the tank is already at temperate. 

Take a look at this from the manufacturer 

 

Sorted it - I think.

 

As long as the electric thermoset is set higher than the gas thermoset, the boiler does not come on if the water is at the right temperture. 

 

6 replies

BPLightlog
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  • BPLightlogPlan Zero Hero
  • Plan Zero Hero
  • April 30, 2025

Hi, as far as I’ve read, those devices only divert generated excess, not using any battery stored energy.

Early morning isn’t going to offer much excess generation and so it probably needs to be operating more during the day. If the tank is hot enough, the excess will then go to the grid.

Bring me Sunshine ~ E&E {Solar PV, Battery Storage, Hybrid EV, ASHP}
Emmanuelle_OVO
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  • felsteaduk
  • Author
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  • April 30, 2025

There is an option to force boost on a schedule, which is what I am using as well as the excess.  It worked on 2 of 3 scheduled boosts.  
 

I just can’t get my head around the boiler still firing up when the tank is already at temperate. 
 

 

BPLightlog
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  • BPLightlogPlan Zero Hero
  • Plan Zero Hero
  • Answer
  • April 30, 2025

Updated on 12/06/25 by Abby_OVO

There is an option to force boost on a schedule, which is what I am using as well as the excess.  It worked on 2 of 3 scheduled boosts.  
 

I just can’t get my head around the boiler still firing up when the tank is already at temperate. 

Take a look at this from the manufacturer 

 

Sorted it - I think.

 

As long as the electric thermoset is set higher than the gas thermoset, the boiler does not come on if the water is at the right temperture. 

 

Bring me Sunshine ~ E&E {Solar PV, Battery Storage, Hybrid EV, ASHP}
Emmanuelle_OVO
Abby_OVO
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  • felsteaduk
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  • April 30, 2025

Thanks for all the comments.

 

One question I still have is that if the excess energy has already heated the tank by 16:00 and the has boiler was on schedule to heat the tank at 16:00 should I expect to hear the boiler fire up and run?  Because that is what I see / hear. I thought it would see it was already at temperature and not bother to fire up.  

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  • felsteaduk
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  • Carbon Cutter***
  • April 30, 2025

Sorted it - I think.

 

As long as the electric thermoset is set higher than the gas thermoset, the boiler does not come on if the water is at the right temperture. 

BPLightlog
Emmanuelle_OVO
Abby_OVO
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  • mavison
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  • April 30, 2025

The thermostat setting needs to be higher for the eddi than the boiler.

But it also depends on where the thermostats are. 

We have a similar iBoost system, and it effectively uses the immersion heater thermostat which is relatively high in the tank, but the thermostat for the gas hot water is lower on the tank. So the iBoost heats the top hotter, but the gas heats more of the tank to a lower temperature. During all summer months we turn the gas boiler off completely, and rely on the excess solar hearing the water whenever it can. We only occasionally have to boost it using supplied power or gas. When the water is hot, and the battery is full, any spare kw is exported.  However, it also depends on your hot water usage, so some experimentation is required.

Emmanuelle_OVO
Abby_OVO
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