Is customer asking too much?

[Juliancanoe]

170L cylinder. 30x47mm tubes. South facing. North Wales. Mid April 2010. Customer has sent me this (was my first install in Oct ish 2009)...

Hi J,
We turned the heating off for a few days last week and the solar's worked okay, though I am wondering why, even in strong sun, the temp in the tank never rises above 60 degrees. Any ideas? We've found that we need to turn on the boiler to bump up the temp in the early mornings, but perhaps that wouldn't be necessary if the tank got hotter? Hope all's well.

I replied...

Cylinders usually kept at 65 so not bad. I would suspect that we are still fairly early in the year (April) and this will rise as we progress into summer. I will however ask the question on the Navitron forum.


Any thoughts? Many thanks!

[dhaslam]

The rule of thumb  says about five litres per  tube  maximum tank size.   However you have to also take into account the usage of water relative to tank size.   If hot water is used up in the evening  there isn't going to be adequate hot water in the morning.   By June the panel will start heating much earlier, at a low angle,  but still  will struggle if the cylinder is too small.   At this time of the year the mains water is  colder than mid summer so  the first Kwh or two  is used just to bring  water up to the summer starting temperature.   

[ericw]

I suspect the key is the water usage.
According to EU M324EN the average European family usage is the equivalent of 100 litres @ 60 degrees.
But I'm sure that if the house is occupied all day and no changes in life style have been made this number can be easily exceeded.

To heat the 100 litres of water from 15 degrees (assumed cold water temperature) to 60 degrees takes 4,500 litre-degrees or 5.3KWh (1kWh = 850 litre-degrees)
As the peak output of 30 tubes is around 1.5Kw they ought to be capable heating it during a reasonable day.

[brackwell]

I have 30x47 feeding a prefeeder tank of 160L on south facing roof on the south coast. A advantage of this is i can monitor the solar performance without being interferred with by any other heat source.  Only last week did i stop heating my main tank of 120L  as it was only then that my preheater was consistently going above 45C.   This morning the top of main tank had cooled down to 35C and top of feeder tank to  45C  and therefore gave main tank 5 mins of immersion heater  (much cheaper than gas boiler due to start up losses )

I guess in conclusion your customer is expecting a little too much but in any case the extra heat is minimal cost.   If his water is cooling down from 60C to less than 40C (perhaps the lowest usable) then it is because of higher usage than the system can sustain and/ or lack of adequate lagging. Use less and lag more !

Ken

[wookey]

My heat dump has come on 3 times already in the last 2 weeks because it got to 90C. But we've got 40x47mm tubes, 2 people, 114l tank, low usage and thermosiphon top-loading which all conspire to produce high temps. So, we expect it to be over 60C by now on sunny days but clearly this is not going to be true for every system.

Your customer may find that adjusting usage times is the best way to keep boiler usage down. i.e evening/lunchtime showers, not morning ones. Still, if it's at 60C in the evg then it still ought to be hot enough to use in the morning too, although possibly not for the whole family. More lagging if not.

[Ancient Brewer]

I have 2x20 x 47mm heating a 210L tank in East Anglia. Yesterday the system was about 1 degree from energising the heat dump circuit. Bottom of tank was 68 degrees with circulating water at 75.

That said I had a dirty weekend and didn't use much water

[langstroth2]

As a comparison, today:

start average at 9am: ~26C (i.e. all the HW drawn off)
end average at 5.30pm: ~64C

A little bit of HW use around lunch time.

140L tank
2x20x47mm tubes.

So assuming your customer is in fact using all their HW by the morning, seems probably about right?

I've had days of 70C plus, but there was usually some >40C water left in the tank.

[GavinA]

If this was your first install, is it possible that you left the controller settings to switch the pump off when the tank got to 60?

could be the systems working fine, could be there's something like that going on, either way it's surely worth a quick visit even if just to keep the customer happy (as well as easing any niggling doubt in the back of your head). It does sound like the customer's not being very realistic though in expecting the solar to do all the hot water usage in mid april.

[Drawmer]

I suspect the key is the water usage.
According to EU M324EN the average European family usage is the equivalent of 100 litres @ 60 degrees.
But I'm sure that if the house is occupied all day and no changes in life style have been made this number can be easily exceeded.

To heat the 100 litres of water from 15 degrees (assumed cold water temperature) to 60 degrees takes 4,500 litre-degrees or 5.3KWh (1kWh = 850 litre-degrees)
As the peak output of 30 tubes is around 1.5Kw they ought to be capable heating it during a reasonable day.

You will see from my sig. that we have a somewhat 'under tubed' system.
I have had to re-train my wife (AKA discussion, heated debate, "of course I know how a thermostat works") to not keep using hot water during the day.
If she reverts to type, and rinses everything under the hot tap after every cup of tea, then our system's performance is only 'ok'.
If I can persuade her to just put stuff in the dishwasher during the day, then the system works fine.
Right now at 18:09 we have 210L at 62C. Boiler not been on for a few days now.

[wyleu]

That said I had a dirty weekend and didn't use much water

Plumps up cushion and settles back, listening expectantly...

[Amy]

I guess its give and take. It does make sense to align your useage behavior to the best availability of hot water.

Learn to be more accomodating.