Posted by: @ronin92

I'd like a sanity check.

I have an iBoost driving the top electric element of a water tank that is also on the gas central heating.  When I compare the four years prior to installing the solar panels+diverter to the 3 years following, gas consumption fell ~2700 kWh.  The difference is very clear with the pre- and post- annual consumption figures to be tightly clustered.  Nor am I aware of any change in household behaviour.  But my estimates of the annual amount diverted is something around 1000-1300 kWh.

Evidently, I'd expect direct heating through an element to be more efficient than indirect heating via CH but I have hitherto assumed the difference would be minor.  Could heat losses from my condensing boiler and pipework to water tank be so extreme?  Is it just a summer thing?  Or have I just made a mistake somewhere.  If true, this radically changes my valuation of diverted electricity that I had previously just assumed it just 1:1 offset gas consumption.

It would never be a 1:1 ratio because your gas boiler is not 100% efficient. It could also be that your estimate of the amount of energy diverted is not that accurate. Dependent upon the size of your tank, the level of insulation and the temperature to which the water is being heated, there may be a slight difference in heat loss.

You should take comfort from the fact that you are not burning gas and are getting free energy from the Sun.

I've done a bit more investigating.

The diversion data comes from the iBoost.  I don't think it can be too far out, certainly not to the extent that makes my summer gas use look reasonable.

I think the situation being compared is continuous DHW-only heating by the boiler during summer vs diverted electricity-heated DHW generation during daylight hours only.  So what is known about gas boilers making DHW in summer?  A Carbon Trust report that actually examined this in homes indicated that perhaps only 40% energy consumed in boilers actually makes it into DHW delivered to the users (i.e. leaving HW cylinder).  Although 80% of fuel to the boiler actually makes it into the primary pipework, losses in primary pipework and hot water tank are a large chunk of this.  Since diverted electricity heats only the top of my tank and that water is used soon after, unlike the boiler maintaining a hot cylinder continuously, it may explain why my summer DHW production is so inefficient.

This episode also forced me to look deeper into my boiler.  When my last one died, the heating engineer fitted a Vokera Mynute 25HE, a 25 kW condensing boiler.  From smart meter records, the boiler did not output more than 6kW average over an hour even in deepest winter.  I just tested it on DHW duties and it heated the tank from cold in 20 minutes and switched off.  The spec sheet says min heat output at default conditions is 6kW or so.  I think I have an very overspecified boiler and I really doubt it is operating in condensing mode in summer.  Unfortunately, I have been unable to measure the return temp accurately with my IR thermometer to confirm this.

Anyway, it does appear that using electricity to heat water in my cylinder in summer is more efficient than using the boiler and this brings the cost of using either rather closer together than I had hitherto believed.  I should therefore value diverted electricity at some multiple (2x?) of the gas price when working out the economics of my panels.