Hoping for some insights on expected performance vs actual

[Shtukka]

Hi folks, new UK member here, but I've seen some excellent advice in this forum. Any help and advice gratefully received!

I have 2 x9-evacuated tube thermal solar system (along with PV) on a south facing roof at 45 degree angle - all fitted prior to me moving into the house. In the past 3 months I've found out enough to interpret the numbers reported on the control box. I think I'm seeing some unexpected behaviour which boils down to poor performance and modest contribution to actually heating the hot water.

I'd appreciate a steer on whether or not it's financially worthwhile getting a specialist to come and service the system. A quote from Rayovac (who were either the installers, or were at least involved) was for 250 pounds.

On a sunny September day, I recorded the following around 11am.
* Collector 40
* Tank 26 (i.e. we'd used up most of the hot water). This sensor is at the top of the tank.
* Return 43. I thought it odd that the return would be warmer than the collector but thought nothing of it.


Then at 1pm. Sun has continued to shine and the collector has warmed up but not by much. I watched the pump come on and go off a few times as the C-R delta moved around the 5degree parameter.
* Collector 54
* Tank 29
* Return 49


Final check of the day around 4pm and the sun's been shining but evidently at more of an angle. I'm disappointed to see that the reading from the top of the tank has increased by only 14 degrees after a full day of about as much sunshine as it's going to get.
* Collector 45
* Tank 40
* Return 40

(Unsure of my tank size, but I'd call it a standard domestic UK tank with the usual compact foam insulation around it. Panels are very close to the hot water tank.)

My concern is that the Collector just doesn't get particularly hot. Even on a baking hot day back in August it only reached 60 degrees, after I'd carefully cleaned the algae off the tubes. Is this a sign that they're somehow faulty? Or are they limited from getting to the higher temperatures I might expect because the system's working as designed and pulling heat to the tank, and my system is just a bit small for the tank to get properly hot. Over winter the contribution will presumably be negligible.

Any other advice, or checks I can do myself?

[LucMan]

You can find out the capacity per day in BTU of your collector array at the ICC-Srcc. The capacity of your storage tank will be required to calculate the out put of your system. Calculate the output of your system with the following.
1 btu required to heat 1 pound of water 1 degree fahrenheit. Water weighs 8.35 lbs per gallon. BTU = gallons x 8.35# x delta T
Compare the calculated out put of your array to the tested output of your array ( from the srcc site) to determine if your system is working properly.
There are several possible problems with evacuated tube systems, the most common is air in the collector loop that needs to be purged from the system,( requires a purge pump).
Next the tubes could have lost the vacuum, or the heat pipes could have frozen and split losing the heat transfer fluid.
The tubes can be checked with an IR thermometer, there are several threads on this site on how to check the tubes and the heat pipes if required.

[scottishjohn]

Hi,
i am also a uk person I have run solar thermal for last 18years -it was removed recently only cos i am selling house and half the tubes were blown and estate agent said it would drop price ==go figure.

i bought this system from SSE --pity they didn,t spec things right --the air bleed unit was made from plastic --so that melted when my sensor packed up about 10 years ago .
at same time the temp got so hot it melted the foam insulation off the pipes --thats at least 160c and i,m in scotland
anyway that replaced with metal item etc and it ran fine for along time
it consisted of 2x20tube thermomax evac panels and 300litre solar tank which had solar input -hot water coil at top and underfloor in centre of 6ft tall tank
i used it not only for hot water but also to help run underfloor heating which retro fitted to a chip board floor
In those days no nice insulated panels with slots for pipes were available
i routed all floors dropped in pipes then covered with 8mm plywood to put strength back in floors
for your system to be showing such poor performance something is wrong
you say its a std h/w tank ?--so how you putting solar heat in ?
with my tank I also used a thermostatic water mixer on hot water --no point in using hot water at 80c ,just to put cold in it at tap --regulated my out put down to 50c

I regularrly saw 90c on collector and 15c less on return form tank --so something wrong with your set=-up
coil not big enough to trasfer all heat --pump running too quick ?
but none of that accounts for low collector temps --would suspect problems there as well
my 300litre tank in the morning after unning undefloor all night would be 20c at bottom and 35c at top --good sunny day and it would be maxed out at 96 by lunch time --if little ot no underfloor requirement--then sit doing nothing for next 6hours --so the key is as large a tank as possible so it always has somehere to heat up when it can .
but you have more basic problems.
the new house will get same treatment but a 500 litre buffer tank minimum coupled with air source hear pump and controlled so when solar cannot do underfloor heat pump will