Subject: Thermomax TMX300 Controller

From: [email-masked] ("Mike Davies")
Date: Thu, 6 Jun 1996 07:39:55 GMT

Thermomax Solar Panels & the TMX300 controller.

Has anyone got one of these working OK ?

We've had ours about 18 months, and because we don't know anyone else 
who has one, and we don't trust salesmen, we don't know what to expect.

We are using it for domestic hot water. In the winter we use a coal 
fired stove which also does the radiators. In the summer we use the 
sun.

To be fair, there was a fault. In the night, convection currents set up 
and took heat out of the hot water tank, up to the panels, which 
switched the pump on, cooled the panels down again, and this would 
repeat throughout the night, so that if at say 11pm the tank temp was 
60C by 7am it was down to say 35C or less. This didn't matter too much 
in the winter because we had the Rayburn going, but now it is summer...

A few weeks ago I moved the pump and non-return valve to the hot side 
of the hot water tank, and in the process discovered the non-return 
valve was duff, so replaced it, and now overnight we only loose 2 or 3 
deg C. :-)

The problem now is that we seem to loose a fair bit of temp at the end 
of the day, say between 5pm & nightfall. :-(

At the end of a typical day, with the sun coming off the panels, a 
typical set of readings is Collector 63C, Tank 58C, Return 54C. The 
tank reading is really meaningless because we don't use it to switch 
any auxiliary circuits, it just nice to know how hot the hot water is. 
We have the controller set so that the collector must be above 40C to 
switch on, also the difference between the Collector & the Return (DT) 
must be > 8C.

With the return sensor on the return pipe (as it was installed) a 
typical set of readings through the evening would be

Time      17:30  18:00  19:00  19:30  20:00  20:30  21:30  23:00
Collector    63     63     60     54     52     49     45     26
Tank         58     57     56     54     52     50     47     46
Return       54     54     52     46     45     43     38     25

As you can see, we have effectively lost 11C in just the 4 hours up to 
21:30, and then it all seems to stabilise, with virtually no more heat 
loss overnight.

Has anyone any suggestions as to what can be happening here ? It can't 
be convection currents. I've knocked that on the head, so the pump must 
be switching on and extracting heat like it used to overnight. For the 
pump to switch on, Collector must be above 40C & DT must be > 8C, so 
either the return is cooling quicker than the collector, or, what ?

Which leads me to the original question. Has anyone got one of these 
things. What setting are you using, for the collector temp and DT, and 
do you experience this drop too ? The return sensor was installed on 
the pipe returning to the collector. Is the return sensor in the right 
place ? Should I move it to reflect the tank temperature, i.e. on the
tank ?

Mike

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From: [email-masked] ("Mike Davies")
Date: Fri, 7 Jun 1996 07:37:11 GMT

i.e. on the tank ?

Mike

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From: Chris Laughton <[email-masked]>
Date: Sun, 9 Jun 1996 14:25:16 +0100

In article <[email-masked]>, "\"Mike Davies\""
<[email-masked]> writes

>Has anyone got one of these working OK ?

I've fitted about ten systems here in the UK, and all now work good
although I am critical of the manufacturer's suggested layout which is
designed for ease of selling and installation rather than optimum
performance. The collector tubes themselves are high effeciency but it
is important to  optimize of the rest of the system plumbing, control
and integration with the auxillary heating  or else you may as well just
fit cheaper flat-plate collectors.

I find good quality research on these matters hard to find although I am
aware of a few projects in Europe but no hard results yet. To do it
myself would be expensive and time consuming so much of my conclusions
(below) are reasoned rather than scientific. 

 You don't say what type or size of store you are using (one vertical
copper twin coil cylinder ?), the surface area and position of the solar
coil, thickness/ U-value of cylinder insualtion, length of flow and
return pipes and quality of pipe insulation, flow rate (pump type and
speed setting), sealed system or open vented (presumambly sealed ?),
depth of insertion of tank sensor and glycol/water ratio  all of which
are relevant. 

>valve was duff, so replaced it, and now overnight we only loose 2 or 3 
A faulty check valve is not an unusual problem. I am not sure what you
mean by 'hot side' but I presume you mean the pump and check valve were
on the return, now they are on the solar primary flow pipe.  I prefer
both items on the return where it is cooler, allows air bubbles to
escape easier and fascillitates draining down.

>typical set of readings is Collector 63C, Tank 58C, Return 54C.

Yes, typical figures. It would be interesting to use the tank sensor
temporarily on the flow pipe just before it enters the store which will
let me know what pipe losses you have and the performance of the solar
coil. I have had to replace a sensor once because it read 10 C. less
than the other two when all put in a container of hot water ! Lesson
here was, never trust the sensors.

>switch on, also the difference between the Collector & the Return (DT) 
I use 35C as default but suggest 30|C in winter if pipe losses are low
and you have a high performance solar coil. It is a toss up between pipe
losses and parasitic electric pump losses. Cold water comes in, at say
10C, so if you've got a roof manifold full of +20C worth of heat it got
to be worth 2 minutes of pump energy to bring it down or else a cloud
may come and you've lost it for good. Thermomax say that this low
threshold is a result of their experience, which is a bit evasive. Other
controller manufacturers such as  the German Resol don't have such a
feature but use plenty of other tricks to pull down more heat. If you
were cynical you may suggest that this low threshold simply makes the
finished system appear to work better, to the uninitiated, because the
pipes get hotter when the pump is on in the morning.
 Your Delta T of 8C is  large, I usually use 5C as default for similar
reasoning as above.. 

>21:30, and then it all seems to stabilise, with virtually no more heat 

All in all, not too bad. Temperatures may flucuate on the tank sensor
due to stratification and eddy currents in the cylinder. You are likley
to be getting losses up the DHW vent pipe if it is not insulated. This
can also be prevented by Teeing the DHW draw-off immediatley at the
cylinder top, keeping the vent just above horizontal for as long as
possible in the airing cupoard before  before going true vertical to the
CWS tank in the loft. Similar story for the Rayburn primary vent and
cold feed if they are located in the cylinder cupboard ( and you have a
twin coil cylinder). Don't forget the solar primaries can only 'steal'
heat from the lowest and coolest part of the store. Also heat losses are
greater at higher temperatures. What is your exterior and internal air
temperatures over these times ? Possible closing of windows near
cylinder later at night ? I would plump for improving insulation at top
of cylinder. The decay of the return sensor does seem a bit slow however
>either the return is cooling quicker than the collector, or, what ?
You may be getting one-pipe thermosyphoning up to the solar return pipe
but unlikley as the return should be at the lowest/coolest point in the
cylinder and this pipe only needs to be 15 mm.. Is the return sensor
well insualted ?  As your return sensor is in a pipe with low thermal
mass,  I would expect a rapid decay compared to the collector for the
same ambient temperature. This I consider a flaw in the normal running
of the system. The return should represent the ability of the solar coil
to transfer heat to the secondary water hence the return sensor should
be inserted about halfway up the hight of the solar coil, actually
immersed in the secondary water, close to but not touching the coil
itself. The Thermomax method can promote short cycling during hot
weather. 

>fired stove which also does the radiators. In the summer we use the 
As an aside, it is usually possible to temporary increase Rayburn heated
storage by a mid position valve, controlled by an adjustable pipe stat (
although not with a solar sealed system)  which is useful if you are
doing a lot of cooking in winter and require extra hot water.

-- 
Chris Laughton   Tel/Fax 0151 606 0207   The Solar Design Company
57 Wood Lane, Greasby, Wirral, L49 2PU.

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From: [email-masked] ("Mike Davies")
Date: Fri, 7 Jun 1996 07:37:11 GMT

i.e. on the tank ?

Mike

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From: Chris Laughton <[email-masked]>
Date: Sun, 9 Jun 1996 14:25:16 +0100