Please consider standard rant ranted: https://edavies.me.uk/2016/07/grid-intensity/

Can't immediately see an issue with increasing the return temp (within sensible limits), when the gas burner is not in operation. Only issue if it were in operation might be that you would reduce the ability to condense.

I'm wondering about how you control the circulation pump (which appears to be internal to the boiler), separately from the gas burner. Apart from perhaps a short overrun on the pump, would the boiler not shut down the circ pump when the call for gas heating stops?

If you can control the circ pump separately, then why not put the elect heater in the flow line? I'm sure you'll have thought about these already, so I'm genuinely interested in your thought process. I often end up tweaking systems, to meet specific interests/setups of clients - always happy to steal others' ideas.

Hi both,

Thanks for your thoughts!

The boiler is non-condensing. Given that it works and the house is due to be pulled down in a few years, and I don't really want to be burning things at all, I don't intend to upgrade to a condensing unit.

The pump is in the boiler and will only fire up when there is a call for heat. I do that rad-by-rad with Radbots, ie empty rooms don't call for heat as much or at all. So that's where hot water will end up going.

And so I don't mind if the pump stops when the call for heat stops. There is no more need for heat. The stat in the Willis heater will cap the water temperature in the Willis. There is a safety cutout too.

I think that the return is (marginally) better than the flow so as to shut down the gas sooner if the Willis heater can do the job entirely by itself, ie heat demand <= 3kW and all other conditions met. Quite likely when only one rad at a time is on.

Radbot calls for heat room by room already, and making use of the existing rad poses no extra fire risk nor takes any extra space.

Note that right now the National Grid has had to issue orders to (be ready to) turn off renewables in GB, so the 'using excess RE better is better than gas' case is already with us. I do separately time the use of (eg) our dishwasher to best match my available generation in summer or grid RE status in winter.

Fitting an ASHP would be tricky here, partly because of planning and house layout. I'm not utterly ruling it out, but I'd like to try this cheaper shading over from gas with the boiler that I have.

I hope that answers your questions!

Rgds

Damon

Not sure I buy the PR stuff in the press about excess renewables- when I check the stats on earth.org I see that there is still lots of gas and import on the grid - currently 5GW= 22% of grid and I have never seen this come to zero (did you?) or anywhere close.

So it's not correct to say that 'using excess renewables is better than burning gas' because the marginal generation unit right now is either UK gas or (indirectly) German coal fired.

(Edit to add: earth.org shows that grid intensity is currently 89% correlated with gas fired generation, indicating that gas is currently the marginal source.)

This has been conflated with negative prices by the press. Negative prices don't mean there's excess renewables - they mean that generators with long-term contracts are having to be paid to break them - a commercial effect not a carbon effect. If not enough contracts are curtailed, then National Grid will intervene, as they have given notice.

A better signal to switch on an electric heater would be when you detect that both gas generation and electricity imports are zero. Edit: or that grid intensity is currently weakly correlated with gas or imports

Damon, as you know (but the press don't!) the ODFM notices don't mean there's 'excess renewables'. They mean that excess generation overall (incl fossil) has been contracted and NG are looking for the cheapest way to persuade some generators to switch off. The renewable generators have low marginal costs so the ODFM is potentially a cheap way for NG to persuade them to turn off, even though they have been contracted to generate. As you mentioned, the marginal generation is still gas fired.

Sorry for the wrong URL, and thanks again for maintaining the electricity data, definitely my go-to source for these kinds of thoughts!

@djh and others: another very basic question or two since no one has recoiled in horror so far!

1) "External immersion heaters are designed to directly heat domestic hot water (DHW) in a vented system."

My rad circuit is not vented so far as I understand it, but there is an expension vessel there and an emergency pressure relief value, so I am assuming that this would be OK. But please say if not.

2) "Because the Willis Immersion is such a rapid water heater, it is not recommended for use in hard water areas."

Hardness is not going to be an issue as new mains water is not getting in as a source of calcium except for top-ups.

But are there other corrosion or similar issues that I should think about?

Rgds

Damon

Just a thought Damon, but instead of a Willis heater with its foibles, could you not use trace heating on the return, or flow or both and possibly switch them as required. I've no idea on cost comparisons though.

I think trace heating is a red herring, but any other inline heater would be possible if there are problems with the Willis (which I'm not aware there are).

There are people who use Willis-like (maybe actually Willis) heaters as their sole source of heat for UFH so they're not obviously completely incompatible with non-DHW applications. Whether they'd work well with steel radiators and the associated inhibitors, etc, and whether the UFH systems I'm thinking of are vented (unlikely) or unvented (more likely, I think) are separate questions.

It could well be that those UFH systems are unvented but the total volume is less than the limit for special rules for unvented systems.

excellent! looking forward to hearing your results and thoughts once its been running for a bit