Hi All
Really been round the houses during the last few weeks, thinking about using some green technology in our new house. We are part way through a refurb and my fafing about is now starting to delay the project !

We've binned the woodburner-connected-to-the-cylinder idea because it was relatively complex and, if i’m honest, we probably will only light it at the weekends etc. Overall I don’t think it will really make that much difference to our heating costs / carbon footprint.

So I’ve been considering Solar panels. We’re using a dual coil cylinder like this

http://www.santon.co.uk/292.htm

plan is for a 300L tank.

We would use the second coil for the connection to the solar panels on the roof. These are the things that I am currenty worrying about.

1) My experience of megaflows is once you draw off, for example, a bath full of hot water cold water rushes in and cools the remaining hot water down. You try and top up the bath after 10 mins and you just get luke warm. So if I’ve got a lovely tank of hot water heated via my solar panels during the day and someone has a shower at 5pm we will will probably end up with a tank full of water too cool to use for anything else without firing up the boiler.

2) 300L is too large and to heat this with solar panels I will need to buy panels I don’t really need. Tank supports the following ( 2 adults + 4 children ).

En suite bathroom (2 x baths a day) + 2 sinks
Another Bathroom ( 1 x bath a day ) + 1 sinks
Kitchen sink
Utility room sink
WC sink.

This is only half the house BTW.

So i’m thinking 300L is overkill. Maybe 200L/250L would be a better fit ? I normally think bigger is better but
If 1) is true then it is doubley daft to heat 300L before cooling it down after using just 60L on a bath.
Any advice would be excellent as I’ve really got to move forward and I’m meeting the plumber on Thursday. He already thinks i’m insane...which is partially right actually.

Thanks

Jeff

Hi

Slightly off the initial question but does anyone out there have similar issues with pressurised cylinders ? Could mine simply not be set up correctly. ie partial run off causing drop in temp of whole tank.

I was thinking I could always use a thermal store instead of the pressurised cylinder ? Just for hot water. That way I wouldn't undo all the good work by the solar panels by running a bath. I would lower the temp of the store ( by one baths' worth of heat via the exchanger ) instead of letting 60L of mains pressure cold water run into the tank.

am i making sense ?

Thanks again

Jeff

PS - 3 baths a day is a LOT. You will need loads of solar panels to keep up.

Hi All

Thankyou for all your comments. This is a really great forum. Really helpful.

@Julian - Thanks for your info. I will google later but which make of tank do you have i.e s/s ? Yes - Megaflow is a brand. I use it incorrectly all the time. i.e hoover, selotape.

Any idea what your mains pressure is? I'm wondering if mine is too high and this causes the stratification to break down. i.e lower it down to 1 or 2 bar may be an option.

@dimengineer - 3 baths is alot but we are 2 adults and 4 children ( all girls! )
Typical day is
1x morning bath or shower (wife)
1x evening bath (me)
1x evening bath ( all 4 children ! ) and it's hell !

Really like to hear more about your system. Are you able to run your system completely without the boiler during the summer months ?
Does your cylinder cool down like mine ? ie new cold diluting hot ?



I should mention that we really only have a west facing roof ( or a flat roof ) so that also has a huge impact on how many panels we are likely to require. South is shaded for most of the day by trees on the other side of the road.

On the thermal store side of things I would be going for a pressurised model.

Thanks again. Any more advice from the day shift would be gratefully received.

You should have a pressure reducing valve on the cold inlet to your cylinder. The output pressure is typically adjustable between 0.5 and 5.0 bar. Turning it down will reduce pressure at taps but will also reduce mixing in the cylinder.

When you fit solar you will fit a thermostatic mixing valve on the hot outlet to regulate temp. of water reaching the taps. This will help reduce the amount of water being drawn from the cylinder, which might also help reduce the mixing a little bit.

I'd have thought 300L is about right for a 6P family. I'm guessing there will be a time when 4 x children can't get in 1 bath, so your DHW demand will increase.

For a 300L cylinder I'd typically recommend about 5m2 of south-facing flat plate collector or 4m2 evacuated tube. If your west facing you may want to increase this or just accept you will gain less energy over the year.

we get a little bit of sun between the trees. We are on a steep hill side ( warminster road in Bath ) and the trees are actually above us on the slope. It doesn't help that they are huge ! I need to start watching the path of the run closely during this summer. I also have a flat roof which I could use but I reading that you don't get great results of you mount panels at this angle.

Wertert,

My experience is as follows, based on the last 2 years of operation, SE England. We have a family of 4 (5 in the summer when eldest daughter is home from uni). 250 litre megaflow solar cyclinder, and a 21 x 58mm Kloben tube array.
The panel is mildly undersized for us, in that for the peak 4 summer months we can turn the boiler off, but we do need to press the +1hr boost button a couple of times a week. It provides about 90% of out HW then.
We mainly take showers, with only occasional baths (2 -3 total a week).
The thing about 3-4 baths a day is that solar is a "one hit" heating - you get a tankful and thats it. So your 3 baths will probably drain your 300litres of hot water. What you may have to do is set your heating to fit with your water use patterns and maximise the solar benefits.

Hope this helps

tim

Hi All

Thanks again for your help. I was wondering something on the way into work today.... i'm getting obsessed !

If you take a 250L unvented cylinder heated to 70 degrees and you run off 60L of hot water you get a temp drop caused by the unrush of cold mains water. Imagine in this example that the hot and cold fully fix.

Would this temp drop be the same as a 250L thermal store heated to 70 degrees which was used to run off 60L of hot water ? ( via the heat exchange )

ignore stratification for the moment.

So what i'm asking is if you took the two systems and extract the same amount of energy ( hot water ) would the drop in temperature be the same on both systems.

Thanks

J

Posted By: wertertIf you take a 250L unvented cylinder heated to 70 degrees and you run off 60L of hot water you get a temp drop caused by the unrush of cold mains water.

Would this temp drop be the same as a 250L thermal store heated to 70 degrees which was used to run off 60L of hot water ? ( via the heat exchange )

ignore stratification for the moment.

If the flow rates are the same & you extract the same amount of energy from the two tanks then the energy remaining will be the same. If you ignore stratification & assume perfect mixing then the temperatures will also be the same.

The difference is that in the unvented DHW cylinder case the water coming out of the tank (before the thermostatic mixer) will be at exactly 70 degrees. In the thermal store case the water coming out of the heat exchanger will be at slightly less than 70 degrees because the heat exchanger is not able to heat it from mains supply temperature to tank temperature in the few seconds it takes to flow through the heat exchanger. So less energy will be extracted from the thermal store & it's temperature will be correspondingly higher than the unvented DHW cylinder.

Note that this ignores the fact that if the heat exchanger itself has a capacity of 25 litres then the first 25 litres will be at exactly 70 degrees.

David

I know this post is old, but can I follow up - I will also be posting further questions on a new post shorty.

In this post a couple of you say you dont like mains hot water - can i ask why? why on earth would you not want pressurized hot water?

I have lived with a megflow for years and just moved into a house with tanks in the loft and it is hell - low pressure, small tank runs out etc and the pump drains the cold water faster than it can fill so you left with no water at all.

We have solar so more questions to follow, but just wandering if I am missing something.

ta

Thanks Tony,

Why more dangerous?

It does not really describe why people dont like it though...our pressure has always been above 3 bar maybe we are lucky

J

Posted By: wookeyjust about nothing went wrong.

Love it!

mine never exploded so I reckon thats proof they are fine :-)

Posted By: borpin

Posted By: tonyMains hot water is intrinsicly more dangerous and should be tested annually but not many do this.

I really do think this advice is FUD (Fear Uncertainty and Doubt).

Sound like it doesnt it, but when you imagine 300l at 3 bar bursting it is not a pleasant thought.....:-)

Posted By: borpin

Posted By: narcosi

Posted By: borpin

Posted By: tonyMains hot water is intrinsicly more dangerous and should be tested annually but not many do this.

I really do think this advice is FUD (Fear Uncertainty and Doubt).

Sound like it doesnt it, but when you imagine 300l at 3 bar bursting it is not a pleasant thought.....:-)

Why on earth would you run it at 3 bar? Put a pressure limiting valve on the incoming water supply to just over 1 bar and that is more than enough. Equally how do you ever check all the hidden pipes and fittings?

because 3 bar is better for showers!

You don't check them you just pressure test it!

Posted By: borpin

Posted By: narcosiYou don't check them you just pressure test it!

I'm going to resist....

don't resist - go ahead!

i am in the slightly fortunate position with this house to be replacing the kitchen which is the only hidden pipes..