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Heating and cooling: How to prioritise Solar Thermal over boiler?

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  1.  
    Hi all,

    I have a lovely Grant solar thermal set-up, but am struggling to work out how best to prioritise the heat from them over the boiler. Currently the boiler is set to top up the thermal store when the store's thermostats ask for heat. This happens throughout night and day, and so when the sun shines, the store is already up to temperature and isn't adding much heat.

    How can I control the system so that there is hot water for morning showers, but leave the solar to do its thing during the day, and if it hasn't done its thing by, say 5pm, then the boiler kicks in?

    Is a simple 7 day programmer the easiest/best way?

    Many thanks in advance,

    Tom
    • CommentAuthorcrusoe
    • CommentTimeJun 21st 2012
     
    Lovely eh? Have to take your word for it in absence of a full description...:devil:

    Easy solution - just listen to the weather forecast. If good, leave the boiler off and vice versa. If only it was so simple. IMHO the best interim solution, until someone does an accurate predictive weather device, is to heat only the water you want in the morning before the sun is up, then drain the tank for showers, washing etc, so that IF the sun comes out to play, you have something to heat.

    This may mean raising the 'normal' FF stat position (again, depends on your set-up - I am talking blind here) so that not too much of the tank is heated.

    Listening to the forecast the night before during stable weather patterns can seriously help. Or shower at night, again, draining the tank, leaving only enough for a wash in the morning. In a ideal world, the solar store is lareg enough for several days' hot water and the boiler is OFF until you manually put it ON, precluding much of the problem you describe.
    • CommentAuthorMartinH
    • CommentTimeJun 21st 2012
     
    Two possible approaches - or a combination of the two

    The solar coil is set in the bottom of the store and the boiler should be plumbed to only heat the top half, so that there is always some capacity to store solar heat. This assumes that the system you have stratifies well.

    The other way is to, as you suggest, put it on a timer. Another variation is to let the thermostats ask for heat but have a manual switch which interrupts the supply to the boiler so that it does not come on, despite the stats calling for heat. If you keep an eye on the temperatures displayed on the controller (and on the weather forecast) you can then switch on the boiler as required.
    •  
      CommentAuthornigel
    • CommentTimeJun 21st 2012
     
    I set the boiler to bring the water up to temperature at 6 in the evening.

    If its been a sunny day then there is no need for it to come on, if its been rubbish then the boiler does come on if the top half of the tank is below required temperature..

    If you have a good solar set up then a sunny day should heat up the whole tank via solar which should last more than a day and have the boiler stat half way up the tank so it only comes on when the top half is below temp.

    I have this set up and the boiler comes on very rarely during the summer (except perhaps this last month).
  5.  
    I have used a 'hold off' pipe stat on the return line from the solar coil which interupts the call for heat wire from the thermal store upper thermostat. When the solar was working and the stores is warm it prevents the boiler from cutting in to heat the hot water. Don't put it on the flow line, as it could get to hot for the stat.

    You have to tune the 'hold off' stat to suit your requirements. I set it to about 45/50 degrees.

    But a simpler solution is to set you timer to boost the hot water with the boiler in the evening around 9pm each day, If the solar has worked well the store stat shouldn't be calling for heat at that time.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJun 22nd 2012
     
    Do you know when you use most hot water? That will make a difference to how you use a timer. Also what temperature do you want your hot water at? How large is your thermal water store?
  7.  
    SteamyTea
    I don't really use much hot water at the moment as I'm the only one camping in the 5 bed house whilst I finish the renovation. I'm then going to rent it out, probably to a family, so need to plan ahead for them. I'm after a solution which the tenants won't have to fiddle with or think about, or if they do want to fiddle then something like a timer might be the easiest. I could be wrong, but I don't expect they'll be checking the forecast and turning the boiler on and off each day, and I suspect they'll want hot water in the morning and evening mostly.

    Blakeecotec
    I like the sound of your "hold off" stat concept. Do you have a link to one of these products and maybe a bit more info as to how to wire it in etc?

    Thanks all,
    Tom
    •  
      CommentAuthorSteamyTea
    • CommentTimeJun 22nd 2012
     
    Have you used a solar calculator to see what can be expected for your region, seems a good place to start.
    5 bed house but unknown occupancy is a tricky one. Maybe two cylinders, a low temperature one heated by solar only and a secondary one in series for the final boost from the boiler/electric.
  9.  
    Hi,

    Could you clarify as you use the term "store" - is this the Thermawave store ie specifically a vented thermal store fed directly from the boiler and are you also supplying any radiators circuits / UFH. Although space heating would be less of an issue during summer (well for some).
    Cheers, Mike up North
  10.  
    In winter, we have the timer set to top up our cylinder at 9pm, so there's plenty of water for morning showers. In the summer we to for a totally manual approach and if it hasn't been sunny and one of us wants a bath or shower we look at the thermostat half an hour in advance (or the night before for morning showers). If it needs it, we push the boost button and the boiler comes on of an hour (or less if that's all it needs). It's surprising how seldom we have to do it - although it's been a bit more than usual this month.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJun 22nd 2012
     
    Posted By: YouGen-CathyIn winter, we have the timer set to top up our cylinder at 9pm, so there's plenty of water for morning showers.

    Have you ever monitored the heat loss overnight? And for that matter how long it usually takes to heat the cylinder up.
    • CommentAuthoralec
    • CommentTimeJun 23rd 2012
     
    certain controllers "suppress" the boiler when the panels can contribute to the cylinder...the boiler "asks" the panels if there is heat in them to contribute to the cylinder and if so does not fire..the cylinder is in effect monitored by the boiler to ensure that you don't run out of hot water...and if there was a sudden draw off the boiler fires..
    • CommentAuthorRobinB
    • CommentTimeJun 28th 2012
     
    Posted By: TheOldSchoolHouseIs a simple 7 day programmer the easiest/best way?


    I would have thought so. If its for a rental property, and the cyclinder's well lagged I'd have it come on late afternoon.
    • CommentAuthoralec
    • CommentTimeJun 30th 2012 edited
     
    the trouble is the hours for the winter won't be the same as the hours for the summer...thats why boiler suppression is so important...but frequently overlooked..especially if solar is to become mainstream...

    at the moment its only the dedicated few who change time clocks...
    • CommentAuthordjh
    • CommentTimeJun 30th 2012
     
    Posted By: alecthe trouble is the hours for the winter won't be the same as the hours for the summer

    I guess that depends on lifestyle. I get out of bed and come home from the office at pretty much the same time all year round.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJun 30th 2012
     
    Posted By: djhI get out of bed and come home from the office at pretty much the same time all year round

    I try and sleep all day at work as well, I find the company is more productive :wink:
    • CommentAuthordjh
    • CommentTimeJun 30th 2012
     
    :bigsmile:
    • CommentAuthoralec
    • CommentTimeJul 1st 2012
     
    Posted By: djhI guess that depends on lifestyle. I get out of bed and come home from the office at pretty much the same time all year round.


    I was referring to the sun....useful solar energy for thermal is available until about 9.00pm in summer and 3 in winter...

    thats the opportunity...
    • CommentAuthoralec
    • CommentTimeJul 1st 2012 edited
     
    and in the morning I suspect useful solar gain is from 5.00 am in summer and froml about 9.00am in winter...

    not having boiler suppression just decreases the amount of heat you can collect...
    • CommentAuthoralec
    • CommentTimeJul 1st 2012
     
    I admit its marginal...but energy savings is all about the marginal bit!

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