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Renewable Energy: GSHP, UFH & Buffer Tank/Thermal Store

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  1.  
    • CommentAuthoradh
    • CommentTimeFeb 1st 2010
     
    Hi all,

    Hopefully someone with a little knowledge of this setup can advise on my best course of action. Currently our plumber is about to start the first fix. First a basic rundown of the system:

    Danfoss Diplomat 8ES GSHP 8.1kw with integral 180L DHW tank (unvented)
    NuHeat underfloor heating on all three floors.

    To enable more efficient running of the heat pump we require a buffer tank which will be situated in the loft. This is to reduce cycling of the heat pump and has been recommended by Danfoss. I would also like additional DHW capacity and therefore propose to have a thermal store for this purpose. The question is whether to have a combined thermal store and buffer tank or two separate tanks. Due to space issues, I would prefer a combined unit. The plumber says he can do either but didn't really say which was better as he says either method is workable. However I'm confused about how the combined unit operates as both a thermal store and buffer tank. Can anyone explain how this works? Can anyone recommend a good manufacturer to approach or have any specific models they would recommend?

    Our plumber has also suggested running the downstairs bathrooms off the DHW tank integrated with the GSHP and running the upstairs two bathrooms with the tank in the loft. Has anyone any thoughts on this arrangement?

    I'm also tempted to have heated towel rails in the bathrooms but due to our heating being fed at 45 degrees, the rails would have to be fed via the DHW system which entails a separate circulation system and special towel rails which will not corrode or rust - NuHeat have a specific range of towel rails for this and a specific circulation system. Our plumber has suggested it might be more efficient to have the towel rails running off the heating system but boosted to the required temperature with electric elements. Anyone any thoughts on these two methods?

    Thanks for any advice and feedback from those who already have a system installed like this would be most appreciated.
    • CommentAuthorDantenz
    • CommentTimeFeb 1st 2010
     
    If it's additional HW capacity you want then why think about incorporating a thermal store into the system. Hot water delivered off a TS will mean having to heat the TS to higher temperatures which of course you don't want to do off a heat pump. Find out from danfoss what the required HW cylinder heat exchange surface area is and just connect another HW cylinder in series with the integrated one. Sometimes you can get away with using a twin coil unvented cylinder and joining both coils together. Combined buffer tanks and hot water cylinders/TS's often result in constant cooling of the stored hot water as the buffer volume is beneath the cylinder. Depends whether you want the towel rails to contribute to heating or just for drying/warming towels. I certainly wouldn't connect them the HW side otherwise again your heat pump will be running at higher temps for longer. Take them off the heating circuit and add electric elements for summer use.
    • CommentAuthorwookey
    • CommentTimeFeb 1st 2010
     
    Seriously consider solar thermal either now or in the future when speccing any tank. Heat pumps are good for low-temp heating water butmuch more inefficient for high-temp DHW, so getting 50-70% of the DHW from solar thermal can greatly improve your overall efficiency. That simply means getting a tank with a solar coil or stratifier in it so that you have somewhere to plumb the solar in in due course.

    If you want to understand the pros and cons of different tank designs for combi-systems (as combined heating and hot-water systems are known in Europe) then the Task32 EU research is very illuminating. That's actually about solar thermal combi-systems, but much of it is about tanks and the interaction between heating and DHW water in the storage vessel.
    http://www.iea-shc.org/task32/index.html Task26 also has info on storage tanks. Sadly much of the more detailed research is only available on a pay-per-paper basis.

    The short version of how combined buffer/tank DHW systems work is 'stratification'. A small upper volume is for DHW and the lower volume is for warm heating water. This covers a wide range of actual layouts. Somtimes the DHW is actually in separate tank (called 'tank-in-tank' systems). More often it is just the upper part (in which case a heat-exchanger is use to take out the heat because the water used is non-potable). The DHW and heating volumes may be variable or, more usually, fixed portions of the tank volume. For this to work efficienctly it is important that addition and removal of heat from the system does not mix all the water up and spoil the stratification.

    Here is a list of many of the tank suppliers around: http://www.navitron.org.uk/pmwiki/pmwiki.php?n=SolarThermal.Tanks
    • CommentAuthorjezza22
    • CommentTimeFeb 2nd 2010
     
    I've just commisioned my new system which is a DPS Excel heat bank and have the tank at 55 deg c. The DHW production is amazing with 3 bar hot water at the taps (too hot even mixed down to 48 deg c ish). I've used efficient new aluminium rads which heat-up very quickly and as I've oversized them the 55 deg c water temp is plenty hot enough). UFH will come off the lower part of the tank when I install it.

    The point is you dont have to have high water temps with a store!

    PS My aluminium rads are oversized between 25% and 55% depending on the location but they are so high output they are tiny compared to those they replaced.

    Hope this helps
    • CommentAuthoradh
    • CommentTimeFeb 3rd 2010
     
    So if I've got this correct, it's generally better to have the UFH heating kept completely separate from the DHW due to the difference in temperature. It seems that the most efficient option is to have two separate tanks, even though this will require more space. I had looked at solar heating and supplying hot water from a log burner. However the log burner manufacturers say it's very inefficient using a log burner to heat the water. They said log burners were primarily for space heating and should be left as such. Not sure what others think about this?

    At present, with only two of us, I think the integrated tank within the GSHP will be sufficient. However the house has four bedrooms with three ensuite and a family bathroom so if the house was occupied to capacity I think more hot water would be essential. I feel it's more prudent to ensure adequate hot water is allowed for at this stage rather than try and incorporate it at a later date.

    The solar heating is interesting as the tanks will be in the loft, the input for solar would be relatively easy. I couldn't afford to do this now but perhaps in the future provided I have the correct tank installed now. I had thought it would be more beneficial to provide for solar electric to reduce the running costs of the heat pump. Is it more beneficial to provide a greater volume of solar heating for hot water or use photovoltaics to reduce electricity usage from the heat pump?

    Finally has anyone any recommendations for a separate hot water storage cylinder with an input for solar? I have looked at Chelmer Heating for this but they appeared very expensive. Danfoss recommended Gledhill for the buffer tank supplier. Any thoughts on these?

    Thanks for taking the time to reply.
    • CommentAuthoracroiso
    • CommentTimeFeb 3rd 2010
     
    Seperating the DHW and buffer means that you should always have DHW available; as a GSHP will usually priritise DHW. If combined, during times when there is a high space heating load then you may find that the DHW performace/temp is effected.

    In your case this probably won't be an issue as I assume that the diplomat will always prioritise the integral DHW tank(?). If it does then you could possibly use a combined accumulator to act as a buffer and to pre-heat the DHW before it enters the 180L store. You'd need to look at the GSHP controls to see if this would work.

    I'd definately recommend PV over solar thermal when using a heat pump. Each kWh of heat that the ST system replaces will save you about 2p (assuming elec. 8p/kWh and COP4), where as each kWh the PV system replaces will save you 8p and earn you 41.3p through the FIT's.
    • CommentAuthorCoxy
    • CommentTimeFeb 3rd 2010
     
    Andrew,

    We are in a similar situation to you with a Nu Heat UFH and Steilbl - Elstron GSHP setup. I am in Somerset and our tackers said that they recently did a 3 story house with Nu - Heat UFH.

    I am going for a seperate expansion vessel and TS and I am opting for the Nu - Heat option of pumping the Towel rads from the TS (I think). Have you tried asking Nu - Heat or Danfloss what is the best option?

    Sean

    www.littlehipcompany.com
    • CommentAuthoradh
    • CommentTimeFeb 4th 2010
     
    Hi Sean,

    I've been in touch with both companies. Danfoss seem to think having separate tanks is more efficient but has the obvious drawback of requiring more space. Nu-Heat are promoting their towel rads being fed from the DHW side but, as others have said, this is very inefficient for a GSHP. From my limited knowledge, keeping the supply temperatures from the GSHP as low as possible maximises the COP. Everytime you raise the temperature for DHW, you are reducing efficiency and it's costing more to run. I think in reflection I'd rather feed the towel rads from the heating side, at 45 degrees, but then boost to working temperature via electric. It's not ideal as you're using more energy but using DHW to provide what is essentially space heating seems a waste.

    I'm no expert though so don't quote me!! I'm still researching and the links in this thread have provided valuable contacts and reading. It seems that there's a lot to this type of setup and getting the combination right could mean the difference between a very efficient economical setup or the opposite!! :shocked:
    • CommentAuthoradh
    • CommentTimeFeb 4th 2010
     
    I forgot to mention that we're also having a log burner in the downstairs kitchen/diner. It has been suggested that we could utilise this to heat additional hot water in the winter. However I've talked with the log burner manufacturer (Clearview) and they said it can be done and they provide back boilers for this purpose but it reduces efficiency of the log burner. Their take was that log burners are primarily for space heating and using them to heat water isn't efficient. Anyone any thoughts on this advice and whether it would be beneficial to have the option of heating water from the log burner? This would only provide additional capacity in winter though as we won't have it running in summer.
    • CommentAuthorCWatters
    • CommentTimeFeb 4th 2010 edited
     
    Posted By: jezza22 The point is you dont have to have high water temps with a store!


    I agree. DHW doesn't need to be hotter than about 44C. We're using a mixer to drop the temperature down to that. One dissadvantage of a low temperature store is that more volume of hot and less (or no?) cold is required for a shower. That might be an issue for a big family and small store.

    Posted By: adhHi Sean,

    I've been in touch with both companies. Danfoss seem to think having separate tanks is more efficient but has the obvious drawback of requiring more space. Nu-Heat are promoting their towel rads being fed from the DHW side but, as others have said, this is very inefficient for a GSHP. From my limited knowledge, keeping the supply temperatures from the GSHP as low as possible maximises the COP. Everytime you raise the temperature for DHW, you are reducing efficiency and it's costing more to run. I think in reflection I'd rather feed the towel rads from the heating side, at 45 degrees, but then boost to working temperature via electric. It's not ideal as you're using more energy but using DHW to provide what is essentially space heating seems a waste.

    I'm no expert though so don't quote me!! I'm still researching and the links in this thread have provided valuable contacts and reading. It seems that there's a lot to this type of setup and getting the combination right could mean the difference between a very efficient economical setup or the opposite!!


    I suspect the element in a dual fuel rad is only powerfull enough to heat it significantly with no water flowing. So it' might not be possible to use electric to _boost_ the rad temperature?

    If you only want the towel rads to speed up drying then they needn't be very hot. If you want them to heat the room and get the towels warm they have to be a lot hotter.

    Beware that you need special rads if they are connected to the DHW. That's because there can't be any corrosion inhibitor in the DHW.
    • CommentAuthorCWatters
    • CommentTimeFeb 4th 2010
     
    I wouldn't recommend using a plumber to design such a complex system it really needs a heating engineer who is familiar with the needs of the different technologies to do you a proper design. Is your plumber approved to install log boilers and/or presurised systems? If you roll your own it will be your own responsibility to sort out any design problems. If you get a heating engineer to design it all it should be possible to get him back to resolve any such issues.

    Be aware that a custom made thermal store with addditional coils/taps for log burners and towel rads ets can take 8 week to deliver.
    • CommentAuthoradh
    • CommentTimeFeb 4th 2010
     
    Yes our plumber is qualified to install all the elements but he's not a heating engineer. Timescale isn't a problem - we're in no rush!

    Anyone recommend a good heating engineer in the Gloucester area I could contact for design advice?
    • CommentAuthorJimbo1
    • CommentTimeFeb 4th 2010
     
    Sorry can't help on the installer.
    Just noticed the log burner - currently can't be used with an Unvented.
    Also would recommend copper for heat pump cylinders, as it transfers the heat far more effectively than SS.
    Maybe try McDonald Engineers who manufacture unvented and thermal stores in copper - and I've never had to wait more than 2 weeks for a special.
    They might be able to help.
    • CommentAuthoradh
    • CommentTimeFeb 14th 2010
     
    Well I've been in contact with several companies including those recommended. I have given them all the information and am awaiting replies.

    However from everyone I've spoken to including Nu-Heat & Danfoss, a separate buffer tank is a definite. It's much more efficient and works better than trying to combine two tanks into one. The decision now is which type of hot water tank to go for to provide the additional hot water capacity.

    Danfoss make a hot water tank directly for this purpose. Basically it sits in series with the heat pump integral tank and all hot water drawoffs come off the second tank. There's also no need for a second immersion as the one in the integrated tank serves both. It's straightforward and will do what I need. The only thing it doesn't have is an input for additional heat sources such as solar. I'm waiting to hear back from the other companies to see if they can offer something similar. I'm also looking at a cost of around £2200 for both the DHW tank & buffer tank from Danfoss.

    I've also been thinking about the benefits and drawbacks of solar thermal and photovoltaics. From a simplistic point of view, it seems to be better in the UK to utilise PV. My reasoning is that solar thermal will only provide noticeable gains in the summer months, you won't get any solar thermal in the winter... or do you?? With PV, you should still get some electricity production in the winter, although I would have thought significantly less than summer. However I know the capital expenditure on PV is much higher. I see there are various discussions on this topic already by those more knowledgeable than myself! As I'm starting my plumbing system from scratch, I would like to make the correct decisions on whether to allow for solar thermal at a future date. Is it worth it?
  15.  
    Posted By: jezza22PS My aluminium rads are oversized between 25% and 55% depending on the location but they are so high output they are tiny compared to those they replaced.


    These rads sound interesting. Do you have details?

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