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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthordivergav
    • CommentTimeJun 30th 2013
    I had solar thermal evac tubes fitted about 3 years ago to work alongside a air source heat pump. The first year it wasn't really working hard as we had an old bathroom and no shower connected to the hotwater tank.

    Last year was a disaster year with flooding poor weather and the finishing of the complete refit on the house. The solar thermal was found to be without pressure and so while moving the heatpump the system was drained down and reinitialised with new Glycol. Everything seemed to be working fine, but the temperature it produces is way below my expectations.

    On a good day it makes a maximum of 50 degrees C and mostly makes around 45 degrees. The heat pump has to top up the water pretty much everyday. I had a guy around to look at it and he said it was working fine, but it was an overcast day.

    I think I could leave a tank of water in the loft and it would heat up quicker and higher than the tubes are producing. The guy also said washing the tubes might do the trick. They have never been cleaned, but could some dust really effect the temperature that much?

    What sort of readings should I be seeing from a panel of 16 tubes?


    • CommentTimeJun 30th 2013
    Have a look for a local weather station that has solar data, also how large is your tank.
    • CommentAuthortony
    • CommentTimeJun 30th 2013
    It depends on too many things: how much you use, heat losses of store, heat losses and control of solar panel, orientation and angle of panel, time of day when it is sunny, temperature of cold water etc
    • CommentAuthortony
    • CommentTimeJun 30th 2013
    Temperature is nor really a measure, you could boil a saucepan full, so it is the amount of heat that you extract is the important number.
    • CommentAuthorowlman
    • CommentTimeJun 30th 2013
    As tony suggested there are many variables. When you talk of temp are you refering to the collector or the cylinder,I assume cylinder. Have you had a look at the collector temp? Can you give more detail, e.g. orientation, pipework length- collector to cylinder, cylinder size, etc.
    As a comparison; I have 30 x (1800 x 58mm ) tubes, coupled to a 180l cylinder, South facing at 35 degrees, and with about 5-6 metres of pipework on the flow. On a good day with the 180l at for e.g. 25 degrees top to bottom I can achieve the full tank at 65+ degrees without difficulty. My neighbour with the same set up and a 250l cylinder is the same. On a good day the collector sensor can easily hit 70 degrees.
    • CommentAuthordivergav
    • CommentTimeJun 30th 2013
    Sorry I now realise I should have given a lot more information.

    Ok the tubes are on a south facing roof and the pipework from tubes to the heat exchanger next to the tank can only be a maximum of 5 metres as the panels are basically right above it.

    The temp I was referring to was the temp at the collector. my read out lets me see both temp at the cylinder and temp at the collector tubes. The collector today in really strong sunshine reached 50°C. However, when the air source heat pump came on that jumped up to 70°C and then feel back quite quickly when it went off. The cylinder is a 150l and made by Daikin for the entire system - heat pump and solar thermal, so heat loss should be kept to a minumium.

    It's as if the hot water produced by the Heat pump goes up to the tubes, but heat from the tubes doesn't come down. Although saying that the collector temp is at max 50° so the water even after a full day of sun only scratches 40°C.
    • CommentAuthortony
    • CommentTimeJun 30th 2013
    It sounds to me like you are the person responsible for warming up the planet!
    • CommentAuthorGaryB
    • CommentTimeJun 30th 2013

    Just checking - do you have a secondary return pump on your DHW system? If so, then make sure the secondary pump is turned off during collection as the circulating losses often match the solar collection input.
    • CommentAuthorGaryB
    • CommentTimeJun 30th 2013
    Sorry, forgot to mention I have 2 x 2m2 flat panels on a shallow SW facing roof, plus a 210 litre cylinder. On a sunny day the cylinder will reach 80 deg C (Tmax for my system) and the panels will stagnate. Highest panel temperature observed under stagnation conditions has been 160 deg C,

    Normally the panels only exceed the cylinder temperature by 4-6 deg C. That is how the controller is set up.
    Hang on - your HP is heating your *panels*??

    Can you send a plumbing diagram?

    Possibilities: (?)

    Sensors mixed up (but then I imagine your hand-under-the-tap sensor works pretty well)

    Diff thermostat failed? You describe the heat from the HP going up to panel but the heat from the panels not coming down. If the diff thermostat is functioning correctly, the HP input would render the temp higher at the cyl sensors than at the panel and therefore the pump would be off.

    ?No non-return valve - reverse thermosyphoning>?
    • CommentTimeJun 30th 2013
    Posted By: tonyIt sounds to me like you are the person responsible for warming up the planet!

    Posted By: GaryBIf so, then make sure the secondary pump is turned off during collection as the circulating losses often match the solar collection input.

    Posted By: Nick ParsonsHang on - your HP is heating your *panels*??

    No sh*t Sherlock springs to mind!:bigsmile:

    Get a plumber in quick.
    • CommentAuthorJeff B
    • CommentTimeJun 30th 2013
    divergav - definitely something amiss! We have a flat plate system ( 4 panels, total area approx 8 sq meters)on a 40 degree sloping roof (facing SSE) with a 200 litre primary DHW tank and a 400 litre thermal store which mops up the excess heat when the primary tank is satisfied. On a sunny day the primary tank reaches 60C within a few hours and the collector temperature can easily reach 130C (stagnation condition) although like GaryB the differential is set to 4C.
    • CommentAuthorGotanewlife
    • CommentTimeJun 30th 2013 edited
    As the others have said something is fundamentally wrong here. You need to scan in a simple schematic of what is connect to what and attach it to this thread. After all we deserve some entertaining diversion:rolling:
    • CommentAuthordivergav
    • CommentTimeJul 1st 2013
    Well at least I'm not the mad customer who complains when his solar panels are working properly! Which is sort of how I was made to feel. I'm not expert in any of his and certainly no plummber. So by the sounds if it it could be a number of things diff thermostat (whatever that is), a return pump or a non return valve which is shunting the hot water into the panel. Even if a valve or pump wasn't working surely the temp readout on the panel would be accurate wouldn't it?

    I'll see if I can find the diagram for how I works and post it up but is may take a few days as I have to be away for work.
    • CommentTimeJul 1st 2013 edited
    The differential thermostat just 'looks' at the cylinder temperature and the panel/tube temperature and switches a pump on and off if there is enough temperature difference to store or not enough to store respectively. It works on relative temperatures not absolute (except for freezing and boiling so may have a minimum and a maximum built in)
    Posted By: divergavIt's as if the hot water produced by the Heat pump goes up to the tubes, but heat from the tubes doesn't come down.
    This is not as silly as it sounds. The heat provided by the heat pump could be lost over night by thermosyphoning, where the warm water from your DHW tank rises up to the panels where it cools & drops back down to the tank again. This can be stopped by a correctly installed non-return valve.

    Is your controller set/wired correctly? Is it dumping heat to the collectors when the tank temperature gets above a preset level?
    Posted By: divergavEven if a valve or pump wasn't working surely the temp readout on the panel would be accurate wouldn't it?
    The temperature readout may be accurate, but if the temperature sensors are cross-wired or there's cool water circulating through the panel it won't get hot. Is there anywhere else the heat could be going? Is the system dedicated to DHW? Is there a heat dump radiator?

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