Choice of intelligent switches ?

orrery

danesol wrote: »

Completely agree with this, but I will add that its a subjective situation depending on multiple factors and I have noticed with surprise sometimes how this unit works outside the normal Feb to Oct period, given you have the right conditions outside to cause an excess the unit can utilise.

Indeed, many factors. I've just got the transfer over to economy 7 (about 2.5 years after 1st requesting it) so much of the car charging will now be overnight, unless the day is completely cloudless. This will leave more surplus for the diversion to the immersion.

In reality, this will make the biggest difference in the darker months as the surplus is probably more than adequate in the summer months.

The diversion is rather an odd process. There is a point where the diversion is working flat out and then it goes "water hot" and stops - on a sunny day 11am or before. However, it appears as though the tank conducts heat away from the hot water (and down into the part of the tank with the heating coil which is below the immersion) and the immersion starts up again - this process repeats in ever shorter bursts throughout the day, and through into the next day if the water isn't used.

EricMears

orrery wrote: »

The diversion is rather an odd process. There is a point where the diversion is working flat out and then it goes "water hot" and stops - on a sunny day 11am or before. However, it appears as though the tank conducts heat away from the hot water (and down into the part of the tank with the heating coil which is below the immersion) and the immersion starts up again - this process repeats in ever shorter bursts throughout the day, and through into the next day if the water isn't used.

It sounds as though you might need a 'destratification pump' ?

A destrat would take hot water from the top of the tank and pump it back in at the bottom allowing rather more of the surplus generation to be used.

Depending on how you look at it, I either don't need one or already have several since a Heatbank DHW circuit operates by extracting water from top of tank, feeding it through a heat exchanger then returning to bottom. Cold mains water is fed into the HX and heated before heading for taps (via a mixing valve). UFH circuits work by extracting some water from top of tank and mixing with returned water then sending the mixture around pipework and returning to bottom of tank.

pinnks

Hi Z, Indeed I have added such jacket. No longer quite so cheap as you suggest but still only about a tenner if memory serves.

orrery

EricMears wrote: »

It sounds as though you might need a 'destratification pump' ?

Fitted, failed and removed.

We couldn't prevent the air accumulating - the flow wasn't adequate to guarantee clearing the bubbles out. It would run for a good few weeks then inexplicably stop working with an air blockage. Neither me nor the plumber could sort it out. He'd never seen the issue before - I suspect that when the immersion was getting very hot there was some out-gassing and this eventually took out the pump bearings.

Hence fitting the 30" immersion - I now use the second circuit to drive radiators in spring/autumn.

EricMears

orrery wrote: »

Fitted, failed and removed.

We couldn't prevent the air accumulating - the flow wasn't adequate to guarantee clearing the bubbles out. It would run for a good few weeks then inexplicably stop working with an air blockage.

I think it's unlikely that you'd get that much 'air' out of the water in the tank. Rather more likely that somewhere in the external circuit you had a poor joint that was sucking in water. Probably one of the joints on the suction side of the pump (on the other side you'd have got a puddle of water) or possibly a faulty pump sucking in air through the bearing gland(s).

danesol

zeupater wrote: »

Hi

So, in summary - you seem to have absolutely no idea what you're diverting but are confident on what's being saved ...

Regarding "Are you always this argumentative/subjective ?" ... well, considering the paragraph above, by utilising mathematical modelling I'm obviously trying to bring an objective viewpoint to claims which seem to be purely based on subjective interpretation ....

Pretty simply, a typical domestic cylinder (~114litres) would need to consume approximately 7kWh of electricity to heat from completely mains temperature cold to around 70C. Considering that most cylinders wouldn't be completely cold at the start of the heating cycle then probably less would be common ... so a sanity check based on a (say) 5kWh daily diverted energy provision to a mostly depleted typical DHW store, every day - month by month, summer & winter - totally independent of weather conditions and daylight hours gives what ? ... 1825kWh (5x365) ... this is almost almost spot on with the 1800kWh/year calculated as the likely diversion required to achieve a claimed £94.90 annual saving as calculated in post #500 earlier on this thread ..... sanity check therefore suggests that the claim would be pretty hard to achieve, after-all there are plenty of days where our 4kWp system doesn't achieve an excess which would allow diversion of 5kWh to thermal storage, some of those even being within the past couple of weeks ...

HTH
Z

As already stated I'm not really interested as long as I get FREE hot water daily and export nothing back to the grid with the GCH switched off and still get paid my FIT and export 50% tariff too :-) ---- - what more do you want or need than that ??

Going into data modelling, further analysis or supporting maths isnt worth the effort IMHO LOL

zeupater

danesol wrote: »

As already stated I'm not really interested as long as I get FREE hot water daily and export nothing back to the grid with the GCH switched off and still get paid my FIT and export 50% tariff too :-) ---- - what more do you want or need than that ??

Going into data modelling, further analysis or supporting maths isnt worth the effort IMHO LOL

Hi

That's fine, as long as anyone looking at diverting solar to DHW understands the limitations and aren't led to believe that 100% in house solar-generation or displaced-gas savings related to DHW of well in excess £50 would be rare or unlikely ....

There are plenty of people who don't use the GCH for months due to solar, but without doing so with solar thermal (which you can't export - unless in bucket-loads ... ) as opposed to proportional diversion, there will still be days when a considerable proportion of generation will be exported ... for example, we generated >25kWh yesterday, but the DHW only needed 8kWh.t whilst today the cylinder grabbed what it needs (4kWh.t), with the solar thermal entering stagnation (the water on the roof is currently 151C) .... if we were using a proportional diversion system, we would still have exported ~50% of generation yesterday and are on course to be exporting far more than that today.

My particular issue is the inconsistency between re-raising various particularly high theoretical savings values and the following post which was made after actually assessing the value of the savings over a given time period ...

danesol wrote: »

OK UPDATE, the GAS boiler is now back on due to the recent weather... so that's a definite 6mths of using NO gas at all to heat the HWC, saving 1,346kwhs of GAS equal to ~ £ 46 saving

I have a way of measuring the divert now, so should know exactly what the SI diverts from the Solar panels over the Autumn and Winter mths, which will give me the full extent of what actually is been saved.

... where the better two quarters of diversion equated to 1346kWh.t (~£46), which logically means that the expectation would be divert almost as much over the remaining quarters in order to get anywhere near the annual savings which were previously raised, whereas it would be usual for the split to be closer to 75:25 .... simply allowing for a total fuel displacement of ~1750kWh (1346+400) against an overall gas boiler DHW efficiency of ~66% gives 1155kWh of energy diversion which wouldn't be out of step with what many others on this forum (and others) have reported as being in-line with what the BRE would expect .... hence the original question ....

zeupater wrote: »

Hi

So you're diverting somewhere around 1000kWh/year to the DHW then ?

Z

Don't get me wrong, I believe that the benefits of solar DHW heating are really great, but hope that it is fully realised that the expectations of potential customers of products which are being 'promoted' on these boards shouldn't be overestimated ...

HTH
Z

orrery

EricMears wrote: »

I think it's unlikely that you'd get that much 'air' out of the water in the tank...

I suspect that the problem is with the temperature gradient over the immersion. You can (well, I can) easily get spitting of the hot water where (I assume) the very top of the immersion is almost boiling the water but the thermostat lower down isn't yet satisfied.

I may install a new pump with better designed pipework - with the tank outfeed going into the vertical vent pipe to the c/w tank and the re-circ pump pulling from lower down the same pipe, therefore forcing any gas to vent before the recirc pump.

Questions to those with a recirc pump:
1 what power pump do you have?
2 what non return valves are fitted?

EricMears

orrery wrote: »

Questions to those with a recirc pump:
1 what power pump do you have?
2 what non return valves are fitted??

As explained above, I don't have a 'designated' destrat but the Heatbank design incorporates several pumps that effectively carry out the same function. Each of those pumps is an 'ordinary' central heating system circulation pump which I believe usually has a motor rated at around 50 to 100W. No NRV needed in such a circuit.

orrery wrote: »

I suspect that the problem is with the temperature gradient over the immersion. You can (well, I can) easily get spitting of the hot water where (I assume) the very top of the immersion is almost boiling the water but the thermostat lower down isn't yet satisfied.

That of course is exactly what destrat suppliers would suggest their product is meant to overcome.

Immersion thermostats are usually installed inside a metal sleeve attached to the heating element and it's very hard to see how it's not going to be at exactly the same temperature as the element itself.

Dave_Fowler

orrery wrote: »

I suspect that the problem is with the temperature gradient over the immersion. You can (well, I can) easily get spitting of the hot water where (I assume) the very top of the immersion is almost boiling the water but the thermostat lower down isn't yet satisfied.

I may install a new pump with better designed pipework - with the tank outfeed going into the vertical vent pipe to the c/w tank and the re-circ pump pulling from lower down the same pipe, therefore forcing any gas to vent before the recirc pump.

Questions to those with a recirc pump:
1 what power pump do you have?
2 what non return valves are fitted?

Pump is Grunfoss Selectric Type UPS 15-50
Non-return valve is 22mm double check valve DZR, Screwfix 26003

Never had any problems with air intake, Pump is set to minimum power setting.

Dave F