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Housing - Renovation: Combining Gas boiler with Solar, UFH, Heat Bank....

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  1.  
    • CommentAuthorcromar
    • CommentTimeOct 6th 2009
     
    Just a quickie and what I hope will be a simpleish topic...

    I'm adding an extension to my 30s house. The heating at the moment is gas fired Viessmann Vitodens 200 boiler supplying DHW via a tiny cylinder and CH.
    My plan is to keep the boiler - too expensive to replace - and have CH running from this still. The new rooms, will be UFH running from the Heat Bank (300l DPS Excel). Also being added will be two solar panels (Vitosol 200) and a wood burniung stove with back boiler all firing into the Heat Bank. The boiler will heat the Heat Bank and all DHW will then come from the Heat Bank.

    All the new build is specified with a lot more than the statutory required insulation.

    Any comments on this set up gratefully received. It seems OK to me and to my polumber who has put in loads of this type of system before. What do you think??

    Dave
    • CommentAuthorCWatters
    • CommentTimeOct 6th 2009 edited
     
    Sounds ok to me.

    I have a heat bank but the model I have only has one thermostat half way up. I believe the Eccel has two stats which provides proper hysteresis rather than just relying on the hysteresis in the stat.

    Plan how you will get the store into position. We had quite a struggle as there was nothing to hold onto. You can't lift it by the pipes etc.
    • CommentAuthorMiked2714
    • CommentTimeOct 6th 2009
     
    Sounds OK to me.

    300l is quite alot. Make sure your panels have enough oomph to give you all solar in the summer and at least a significant amount of DHW in the spring and autumn. There's nothing more soul destroying than saving a few pounds on panels and then having to light up the boiler on a cloudy day.
    • CommentAuthorcromar
    • CommentTimeOct 6th 2009
     
    8m2 of vacuum tube panels... I've been advised I only need 5m2 for a family of 5 but like to overspecify these things
  5.  
    8m2 panel area, absorber area or aperture area...?

    (I don't know either)

    J
    • CommentAuthorcromar
    • CommentTimeOct 7th 2009
     
    Hmmm. Panel area i think but hopefully absorber area is pretty similar. Doesn't look like the panels have got huge margins.
  7.  
    Posted By: cromarabsorber area is pretty similar.


    ...'fraid not I believe.

    J
    • CommentAuthorMiked2714
    • CommentTimeOct 11th 2009 edited
     
    How does 8m^2 translate into no. tubes?

    I vaguely recall guide of 1 navitron 47mm tube per 4 or 5l of store. It would obviously be different for other diameters of tube, also Navitron are thought to be slightly less efficient than other tubes (but robust and economical).

    Edit: At a rought guess, 8m^2 does sound plenty
    • CommentAuthorskywalker
    • CommentTimeOct 11th 2009 edited
     
    Absorber area is always smaller than panel area. What matters is the output of the panel. If you are using el-cheapo tubes, many suppliers not just Navitron, 8m2 equates to around 70-80 tubes. The solar coil will not be heating the whole store so you will end up with the eqivalent of abut 1 x 47mm per 4l. If you are not having a solar coil (DPS prefer drain back systems from memory) then the following advice may need to be modified.

    Navitron persist in suggesting that 1 tube per 7l is enough, this is rubbish & they know it as many people, including myself, have found this ratio to be inadequate. Here and on their own forum 1:5 is the sensible ballpark often recommended & is a good general rule of thumb. Given the summers we have been having & the general lack of performance in cloudy weather I would go with the equivalent of 80 tubes with a 300l store. I have a 210l DPS store (not that it matters that it is DPS) hooked up to 30 tubes and, whilst we get hot water on sunny days it never gets much above 65c from a standing 40c start. At some point I will either get more tubes (60x47mm total or 1:3.5l) or better (single wall) ones simply because with a very small store like mine I need to get to 90c to get through the cloudy days. There are only three of us and we a quite careful with water (son now 9 & 'improving'!!).

    Another option, which I wish I had been aware off, is to get a second solar coil fitted halfway up my store and switch between the two (most half sensible controllers can be configured to do this). In this way you would need less tubs to heat say the top 150l first (on your store) which is a fair amount of hot water and then the rest if there is enough sun. With 70 tubes this would almost guarantee you 150l usable hot water in all but the most dismal summers. With less tubes you would still get good amounts of hot water & potentially reduce heat dump events (not a joy I have ever suffered!) during more scorching (we all wish!) periods.

    Bear in mind that with a solar coil stratification is not maintained in the store so quite often (if you monitor the temps) the top of the store will cool as the temperature of the heated volume equalises. After this the whole store will then heat up. So you can start the day with 60c at the top & 40c at the bottom but end the day with the whole store at 50c (just figures from the air).

    Hope this is some use, other will hopefully help with any extra confusion I have created!!

    S.
  10.  
    Why does not a solar coil maintain stratification - as compared to any other type of coil or to nothing? About to put in a 2000l tank with a 310l 'tank in tank' surrounded by a solar coil, so not just a passing Q!
    • CommentAuthorcromar
    • CommentTimeOct 12th 2009
     
    Thanks Skywalker et al. Mine will be 300l DPS store with 60 vacuum tubes. From various comments here and elsewhere i think this should be OK even though I take your point of recommending 300l and 80 tubes.
    Once it's up and running I'll let you all know how it's doing.
    • CommentAuthorskywalker
    • CommentTimeOct 14th 2009
     
    Gotanew'

    Any heating element (a solar coil is just a heating element) will heat the water in the volume above after first equalising the temperature. This water rises when it is warmer than that in the surrounding volume to a point where water is at the same temperature. As it rises it causes turbulence where water from lower down & thus cooler is mixed with water further up (& thus warmer) cooling that water. So basically the first thing the heat from the coil does is to equalise the temperature in the whole volume by mixing and warming at the same time. A pan of water on your stove or a kettle works in exactly the same way. Some stores (consolar I think) get round this using an upturned funnel in the tank. It may also work differently if you have big volumes/tank in a tank but I do not know.

    If you add hot water to the system, and remove an equal volume of cold, it can work (as long as there is not much mixing/turbulence) to produce stratification - cold at the bottom & hot at the top.

    S.
    • CommentAuthorskywalker
    • CommentTimeOct 14th 2009
     
    cromar

    60 should work (1:5), lots & lots of people say it does.

    S.

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