A Heat Bank/Heat Dump challenge

[paddylen]

Hi Guys,

A little lateral thinking challenge for you for the Bank Holiday Weekend.
It includes problems others will face on Heat Banks and Condensing Boilers.
All suggestions received will be studied and appreciated.

I have a DPS Pandora Heat Bank for my DHW – excellent device, but I want to add solar Heating to fully  take advantage of the Heat Bank principle.  (the mains pressure DHW runs from an external Heat Exchanger which draws heat  out of the Heat Bank store, leaving the original boiler-heated water in place)

Designed for use with a flat plate solar panel system, it has a built-in expansion vessel and prevision for “solar drain down” connection via three blanked-off bosses in the side. (the stored water would become part of the solar loop and  be heated directly by the collector.  For freezing or overheat protection, the collector is temporarily  drained down back into the store)

I have a couple of Ivan’s best 20 tube panels which are of course, totally unsuited to a drain down system and need to find a way of making the Hat Bank and the Navitron tubes work together.

The challenge is to plan a system using –
•   Two 20 tube panels – one on a 30 deg. SE facing roof, the other vertical on a SW facing gable end wall. 
•   A DPS Pandora 210 litre Heat Bank with Primary loop only.
•   A Worcester-Bosch Greenstar 28HE Condensing Boiler. (Built-in pump).
                 (The CH system has 3 zones + the DHW loop all on a Grundfos  UPS 25-55 pump.)
•   A Resol ES Controller
•   A second flat plate 100 kW Heat Exchanger.

I have a plan (below) I think will work which retains a pressurised solar loop – transferring the heat to the store through the second Heat Exchanger. The Store contents are pumped in a loop from the other side of  the HE, through the store via the two lower bosses and back to the HE.
To provide a Heat Dump, a three port valve diverts this heated  flow through a radiator, back into the top of the store (sealed but unpressurised) and back to pull heat from the HE.
I wanted to use Antman’s  Heat Dump design which uses the coil from the boiler to pull heat out of the store but this type of boiler makes the wiring changes impossible.  My alternative design appears to get the heat out of the store and should cool the solar loop.

Using Arr. 19, the ES controller should look after the solar loop OK but I will also need it to –
•   Run Pump3  when P1 or P2 are active.
•   Activate the MoMo 3 port valve when the HD is required.




What thoughts? Any suggestions for modding the wiring?  What have I missed?

regards,
paddylen

[Antman]

Paddylen emailed me on this and we had several email exchanges prior placing it on the Forum for all to ponder. Unfortunately the drain-down version of the DPS Panndora (rather than the version with solar coil) complicates the control and heat transfer options.

Paddy - ignore my comments re evacuated tube panesl not being suited to drain-down systems. I overlooked the inclusion of a solar Heat Exchanger which allows the solar loops to run full of water continuously - sorry! You new drawing makes everything clear on what is a dull wet day 


My suggestion was to operate the Heat Exchanger pump in parallel with the solar pump. However doing this as a direct connection will result in the Store reverse-heating the Heat Exchanger (and solar loop) in winter when the initial Store temp is higher than the solar loop temp = unwanted loss of fossil heated energy.

Therefore an additional thermostat could be connected in series with the HE pump which only allows the HE pump to run when the HE is at 25 deg C (for example). This will prevent the HE pump running until the solar loop has pre-heated the HE, thus preventing the HE being used to pre-heat the solar loop during initial morning runs in winter.
However on further thoughts, this will prevent the anti-freeze CFR controller function being effective becasue the HE won't have the thermal mass to store heat without re-heating from the Store. Hence it would be essential to have antifreeze in the system.

I cannot see any other way to get the ES controller to operate the exchange pump via a separate relay.

Antman

[paddylen]

Many thanks Antman, a very comprehensive analysis of the situation which covered a couple of points I had missed.  Looks like yours is the route to follow.
I have heard from a couple of  “lurkers” in a similar situation with a Heat Bank, who were following the thread.  I expect we will all adopt your solution.

[ifordesmith]

Guys,

just a quick question and it relates to how much temp drop you might expect to get across the HE as we are only dealing with differential temps of 5-10 degrees.  I am not an expert on HE and do not know how efficient they are in terms of losses, naturally I would assume that you would encase the HE in rockwool insulation to minimise losses.

Cheers

Ian.

[ifordesmith]

Guys,

just a quick update, thought you might like to know that this method of using the HE to isolate the heat bank from the pressurised loop is now being offered directly DPS and if you following the link http://www.heatweb.com/pdf/DPS/DPS_Data_Sheets.pdf  and look on page 12 you will see their config, look familiar Paddy ?

I have asked them about the losses across the HE and will let you know their response.

Regards

Ian.

[ifordesmith]

Just had an update from DPS re heat losses and they reckon they will be minimal across the HE somewhere in region of 0.5 to 1 degree.

Ian.