Solar Panels with Unvented Cylinder

[Teknet]

Hi all I’m in the process of buying a house which has the traditional central heating/HW system whereby there is a cold water tank in the loft and vented cylinder in the airing cupboard. I am looking to replace the cylinder with an unvented megaflo cylinder as the HW pressure out of the taps isn’t good. Is it straight forward to do this if you want to keep the rest of the system in the house the same?

Also I’m looking at getting solar panels installed with a battery wall. I was thinking that I could get a cylinder which is connected to the gas boiler but also more importantly have more than one immersion coil so that essentially the cylinder water is kept warm from the electricity generated by the solar panels. Is this possible or is there a better way? I figured some sort of system whereby the cylinder is just trickle heated from the electricity automatically once it goes below a certain water temperature?

Thanks for your advice!

 

[Madrab]

I am looking to replace the cylinder with an unvented megaflo cylinder as the HW pressure out of the taps isn’t good. Is it straight forward to do this if you want to keep the rest of the system in the house the same?

2 key things need to be know - Mains Dynamic flow and pressure - without that being at least 2+bar and 20L/min then it's not worth it. That's presuming that it needs to feed more than one Hot and Cold outlet at the same time. The mains needs to be tested properly, both at the take off of the district mains - your outside tap - and at the your mains stop tap.

Yes - you can get an indirect unvented with 2 immersion elements, where the main element could be powered by a PV powerbank. Would be easy to set the thermostat on the immersion powered by the PV system to whatever temp level you wished. Immersion elements are a 3Kw draw as a standard, though I guess the supply could be down rated by relay if you didn't want it to draw that amount of wattage.

 

[ericmark]

You can get mains hot water pressure without pressurising the cylinder. However, the systems can be rather costly, and I am no plumber, so I will let the experts expand on this.

There are a few methods to use PV solar panels to heat domestic hot water (DHW).
1) The iboost+ can use spare solar power to heat the DHW.
2) The Willis system allows heating the cylinder from the top down.
3) You can run two immersion heaters from the iboost+
4) If you get payment for excess solar, the payment for export during peak times, can be more than the cost of off-peak, so may be better to use a simple timer.

At the moment and for the whole of the summer, the iboost+ has been a gain for me, and it actually longs how much saved, today, yesterday, and last 7 days, typically this was logging around 2.5 kWh to 3 kWh per week, so saving per year only when sun is shinning, so around £62.50 per year, if you look at cost of iboost+ seems unlikely it will pay for its self.

Clearly down to how much DHW is used, and also battery size.

So far this month, not a single kWh exported. However, the battery is charged with off-peak, and used at peak time, so even with no solar, I am gaining, but what I am looking for is the battery to be fully charged around 1 pm to 4 pm to have spare for DHW, my panels peak at around 5 kW, and I started with 3.2 kWh battery, now 6.4 kWh the larger the battery the further into the evening before it runs out, but also the less time in the afternoon that there is spare solar.

I use around 10 to 16 kWh per day, to have enough solar or battery to cover the whole is not worth it, it would cost too much for the installation, so you need to hit some happy medium. Good luck.

 

[Teknet]

Thanks for all your replies… I might be wrong but it almost sounds like what I wanted isn’t really worth it? We are a family of 2 adults and 2 kids so it’s not like we need a constant supply of HW as no one’s in during the day anyway apart from weekends. In my current house we literally just stick the HW on for 30mins just prior to having a shower and that works fine for us. Would it be better that I just stick to using the gas boiler to heat water?

 

[Madrab]

Yeah -

You can get mains hot water pressure without pressurising the cylinder. However, the systems can be rather costly, and I am no plumber, so I will let the experts expand on this.

Heat banks - a modern take on what is basically a thermal store with more modern kit.

Would it be better that I just stick to using the gas boiler to heat water?

It really comes down to cost over requirement.

We are a family of 2 adults and 2 kids

If you are a young family then if this is a future proofing exercise then I can just about guarantee that the kids will need more. Using the PV to power an element isn't a bad idea, especially if you have a power bank and if you use it when that is available then it could offset gas usage and over years it would eventually pay for itself but it would be a long term venture to recoup the costs for the PV.

PV and a powerbank is certainly a positive step to reduce the carbon footprint and it will reduce direct energy costs but again unless it's a long term venture then it comes down to how long it would take to recoup that outlay.

 

[Murdochcat]

Why do you think you would need a tank with 2 immersion heaters?

 

[ericmark]

Would it be better that I just stick to using the gas boiler to heat water?

I was rather surprised at results with an oil boiler, unlike modern gas, the oil boiler is on or off, so time running = kWh output. So summer would use oil boiler 4 times a week, at ½ hour each time, but would only run approx 20 minutes, water warm enough to wash hands but not as hot as it gets in winter. On fitting solar I used the iboost+ which shows how much used, so could compare, oil approx 25 kWh per week, electric 2.5 kWh per week, so even with electric being 5 times the price, still cheaper for me with electric, this of course with my house, with tank two floors above the boiler, so will vary house to house.

Why do you think you would need a tank with 2 immersion heaters?

The iboost+ heats the top immersion first, and only once that is hot does it switch to lower immersion heater, so two immersion heaters allows a fast re-heat time, but still stores a lot of energy in the water for cloudy days.

Traditionally we would have the two immersion heaters labelled sink and bath, with a switch on the immersion heater, this was before we lagged the hot water tank like we do today, and we used the heat from the tank to air clothes.

Latter we split the heating with bottom element on off-peak set slightly higher temp to upper element which was connected to peak, so the peak element would only switch on when running out of hot water.

The other method is to fit the immersion outside the tank, with the Willis system, this results in the tank being heated from the top down, theroy wise by far the best system, however unless you live in Ireland, finding a plumber who knows how to set them up, may be a problem?

This is a question for @Madrab is it likely a mainland plumber knows how to install the Willis system? I am sure @Madrab does, I feel from his posts he is a very knowledgeable man, but what about the run of the mill plumber?

My daughter in her Turkish home has solar panel DHW heating, which works well, but my father-in-law had it fitted in North Wales, and it looked the part, but about as much use as a chocolate fire guard. Maybe North Wales just did not get enough sun? However think more likely the installer had no idea of how to set it all up, plus the guy we got to repair it. Plumbers in UK don't fit many solar water heaters, so don't know how to do it.

Same today with heat pumps, I worked on heat pumps in 1980 in Algeria, and also in Hong Kong latter on, but very few in UK, OK Next in Chester had 9 units on the roof, but that was an odd one out, and the whole idea of heat pump was to cool the building not to heat, and the most important thing was all pipes need to be lagged, and one needs pumps and drains for the condensate, which are not needed for heat only systems, I have seen installations where to lagging is completely inadequate. Clearly the person fitting it had no idea of what was required. The Next in Chester had a ceiling collapse due to condensate on the suspended ceiling, caused because a clip was missing on the pipe to condensate pump.

So I tend to stick to electrics, and let the plumbers do their job, even if considered as allied trades.

 

[SpecialK]

Assuming you use gas??? - Gas cost is 6p/kWh and export payments are 15p/kWh. It is a better return to export your solar excess than to heat with gas. It is however nice to have hot water heated by the sun...

I normally switch to octopus agile for the summer months and heat my tank with the immersion when the import rates drop to 0p/kWh.

 

[ericmark]

Gas cost is 6p/kWh

And off-peak costs me 8.95p/kWh so gas needs to be 67% efficient to match electric. This includes the losses on the pipework. So although the boiler may be over 67% efficient, unlikely unless storage tank and boiler are very close, that the gas system will be better than electric.

Electric it does not matter in the 5 hours off-peak how many times the immersion heater switches on/off, there are no external losses. The same is not true for the gas, each time the boiler fires up, it needs to heat the transfer water, pipes, and boiler.

I do not use much DHW, so the 2.5 kWh per week, is what is lost with a poorly insulated tank. I asked for a gas smart meter with mothers house, and was refused, I wanted to see how much gas used, and the meter was hard to access. So have no figures for gas. Oil actually costs me less than gas did, but the amount of oil left at start of this season was so much more to what we had at start of last season, so it does seem using oil to heat DHW was far more expensive to using electric.

Seems we gloss over the losses heating boiler and pipework.

 

[SpecialK]

For me it is 8.5p overnight for 5 hours and gas is 5.62p/kWh. Even with boiler and pipework losses gas is still cheaper for my use case. I have a combi so there are no tank related losses.

The unvented tank just sits there in the winter not used as agile is too risky even with a large battery and solar array.

It would be nice if overnight rates dropped more and the immersion would be worth using again.

 

[Madrab]

This is a question for @Madrab is it likely a mainland plumber knows how to install the Willis system? I am sure @Madrab does, I feel from his posts he is a very knowledgeable man, but what about the run of the mill plumber?

Gosh, there's a blast from the past Willis, haven't seen one of them for years, certainly not a standard - I would reckon there would only be a few that know about them and ever fewer that have ever fitted one. I have only ever fitted 2 of them and they were replacements. They are generally superceded now and are pretty old tech. Could they find a new life given renewables/self generated energy, possibly but there are more efficient single package solutions now and they can only be used with Vented systems, if I remember rightly.

We actually looked at them when I was doing my papers, like 35odd years ago - those were designed to draw cold from the bottom of the cylinder and through convected flow transfers the heated water up to the top of the cylinder. Typically those cylinders only had a single full length element and this was a way to time heat just enough water as a boost for low HW use requirements say for dishes, basins, maybe even a quick shower without heating the whole cylinder.

 

[ericmark]

Willis is marketed as for use with solar

I think the main thing was it would work with tanks with no boss to take an immersion heater. I know quite common in Ulster, but do not seem to have left the island to any great extent. The version with an immersion heater

again it seems only used in Ulster.

 

[Murdochcat]

We have PV, no fit and an Eddi feeding a single lower element.

Works for us when the sun is out , which it hasn’t been doing much for a couple of months now

 

[Madrab]

Willis is marketed as for use with solar

Ah right - that's a different thing. The original Willis was/is basically a metal can with an electrical immersion element in it. Can certainly see the benefit of a solar PV feeding the element but hadn't seen a Solar Thermal version - called SolarSyphons it seems. I guess things have moved on since I've come across them.

Must be honest, it's always good to see old tech being repurposed for modern use, sometimes is shows that we don't need to re-invent the wheel to bring things up to date, though it does look quite a niche product TBH.