@DoubleSpeed

Im assuming you are talking about a gas combi boiler? When you say efficient do you mean most cost effective?

The greenest option is to keep your water heated via the diverter, which is totally renewable and even if you needed to heat with grid electricity in the winter, that could also be a green tariff. Depending on how often you need to use the grid, it might also be the cheapest.

In terms of efficiency, the electric immersion heater is almost 100%, a modern gas boiler is a bit less efficient depending on the temperature you heat your hot water to.

The cost equation would be comparing the cost of grid electricity you required over the year to the cost of gas required over the year, factoring in the increased cost of the new boiler. There might be additional complicating factors such as if you export your excess electricity on a metered tariff.

Have you thought about a heat pump instead of replacing the gas boiler? The grant has gone up to £7500 now 🙂

Posted by: @doublespeed

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We have PV Solar with an old boiler/hot water tank and a diverter and this works great however as our boiler is old we are looking to update it to a modern more efficient one. We can keep the hot water tank and just modernise the boiler so can still divert to the immersion however we also have the choice to go to a combi with the Direct Hot water on demand and have no cylinder. The cost of changing to this system is more however we then only heat hot water when we need it.

Does anyone know which would likely be more efficient/cost-effective/green etc.; Retain a Hot water cylinder with a Solar Diverter to Immersion or a combi with the Direct Hot water on demand and have no cylinder?

There are a number of options, which may depend on what you are trying to achieve.

I am not 100% certain, but would it be possible to keep your present arrangement and replace the old gas boiler with a new one?

Going to a gas combi for both CH and DHW would mean not being able to fully utilise your solar PV.

Probably the most efficient, and possibly cost effective, would be the gas combi boiler for CH, and a new hot water cylinder with inbuilt small ASHP for DHW production. In that way it should be possible to better utilise your solar PV production, either heating the DHW via the power diverter or powering the small heat pump to do the same more efficiently.

Solar diverter for DHW = free (when surplus solar) *

Heat pump for DHW = 200-250% efficient (by far the most efficient electrical method of producing DHW)

Direct Electric immersion = 100% efficient

Gas combi boiler DHW on demand = 70% efficient. You're wasting a lot of energy burning a combi in short-cycles for hot water. A steady-state combi real world efficiency is around 82% (Government monitoring), but firing the burner for short periods to produce low volumes of hot water is the least efficient way of running a boiler. You'll be heating the street via the boiler flue outlet almost as much as you'll be heating hot water!

* I've found exporting surplus solar at 15p/kWh (Octopus export fixed) and using my ASHP powered from solar PV battery to generate DHW the most cost efficient. Still free hot water, but getting paid a little on the surplus solar rather then losing it via the diverter. The ASHP heating coil in the hot water cylinder heats the cylinder contents much more thoroughly than the 3kW immersion does. It's lower in the tank and much larger surface area, so is a more effective means of heat transfer to the tank water content. If I heat with the immersion only on solar divert, the cylinder thermostat drops in indicated temperature quite quickly after the immersion stat has opened and de-energised, whereas with the ASHP it remains at the set point for several hours after reaching cylinder stat temperature.

@allyfish & @derek-m 
Thank you both for your helpful replies, much appreciated.

Much greener  (lower carbon emissions) to use electricity, or if it isn't at this moment in time it will be over the lifetime of your boiler. In the future it is likely that lots more people will have heat pumps, and it will be necessary to have a hot water tank. I predict that people will regret taking out their tanks!