Heating & Thermal Store

[RickH]

Heating System
Have 2 houses where identical Heating systems installed (self & neighbour)
In Dec both Baxi boilers were replaced with Viessmann Vittodens 30W due to ageing reliability issue.
System is as per drawing



Hot Water Draw off
Thermal Store operates by the process that as DHW is drawn off (via heat exchanger) the flow switch detects the water flow, and it opens the 2 port valve AND operates the primaries pump, but does NOT provide a call for heat signal. The whole contents of store circulate around the boiler.

The purpose of this run, is to ‘stir’ the water in the store to keep hot water passing over the Heat exchanger and maintain hot DHW draw-off.
Once the thermal store temp drops, the store thermostat closes and ‘Call for Heat’ signal is provided to the boiler.


The Issue
What is happening on both installs is that when DHW is drawn off, the heat exchanger starts cooling and after about 2 mins the water temperature starts dropping. Preventing the filling of a bath for example with hot water.

The cause is the boiler not firing to reheat the store.
Temperatures have been checked with digital laser thermometers…. and it shows that although the boiler is set to 70C (5 degrees higher than the Thermal store thermostat), the primaries are sitting at around 42C
It is proven water is flowing around the primaries … 2 port valve is open and the pump is running, and that ‘Call for Heat’ is at 220V.

The crucial thing ‘seems to be’ once the boiler has reached the 70C on a burn, it modulates right down, and will not modulate up to run until primaries drop to around 27C .. and that is way too low to top up the Thermal Store.

The hysteresis of more than 40C (set temp to restart temp) is far too much, needs to be around 10C
It can take more than 2 Hrs for primaries to cool to 27C and restart a burn.

The Thermal store is not reaching the designed temperature (digital measurement shows the top of store is at 48C … (should be over 60)
Consequently, the Thermal store is not getting up to temperature and is not being reheated as it should.

This was never an issue with previous Baxi 100HE boilers and those were also modulating boilers, the Viessman may be too clever.

Does anybody have suggestions to resolve this, hoping there is a configuration on the Viessmann?

 

[ianmcd]

Get the installers back, the viessmann boilers need to be configured for a thermal store, personally I would never have installed that boiler on a thermal store, but it can be configured correctly

 

[RickH]

Installers due back later in the week.
Can you give any guide/advice on how to configure it for Thermal Store. I can pass it on.

I did call Viessman before buying they said it was compatible.... and it was Viessmann-approved guys who installed it.
Went for Viessmann as it was one of only a few Heat Only boilers that had a stainless steel heat exchanger and 10 yr warranty, plus approved for on pressurised Thermal store. Most were either system boilers and/or Aluminium heat exchangers.

 

[ianmcd]

Never heard of a Vitodens 30, there rae a few that are 30KW is that where you are mistaken, what is the actual model that you have ?

 

[DP]

The cylinder controls the boiler
Cylinder needs to be maintained at a set temperature.

While the horse has bolted, what Baxi did you have.

Problem with your instal I suspect is configuration
Not uncommon boilers are nailed to the wall thoughtlessly.

 

[RickH]

Never heard of a Vitodens 30, there rae a few that are 30KW is that where you are mistaken, what is the actual model that you have ?

Was slightly wrong ... was thinking of 30 kW ..... model is "Viessmann Vitodens 100-W"
Heat Only Type B1GA

 

[RickH]

The cylinder controls the boiler
Cylinder needs to be maintained at a set temperature.

While the horse has bolted, what Baxi did you have.

Problem with your instal I suspect is configuration
Not uncommon boilers are nailed to the wall thoughtlessly.

One install was a Baxi Barcelona (upgraded to 100HE control) the other was a Baxi 100HE both were Heat only boilers.
Any suggestions as to what configuration, is there a specific setting to change ?

 

[DP]

Barcelona, did well to get rid of it. 100e hardly ever got serviced properly so it eventually started playing up to show red lockout light

 

[RickH]

Barcelona had everything other than chassis changed …. SSE we’re here every couple of weeks Swapping parts.
HE 100 last year was consuming so many parts, spending more time down than working. British Gas hugely increased contract price due to No. of call outs and parts fitted.
Several companies would not put either under contract as >10yr old

So both of us decided to replace, we looked and asked heating system provider, and boiler manufacturers.

Viessmann said their model was suitable for thermal store … so hopefully it’s a configuration issue.
Looking for any help here Incan get.

 

[DP]

Place to start is look at the cylinder for a start.

Cylinder temperature drops
Demand is made to boiler to get it to raise cylinder temperature.
Is the boiler reacting to this demand, if not why not.

Some boilers react to demand by running at low power to then ramp up.
Is your boiler doing that?

Easy thing would be get Viessman out saying boiler fault. Viessmann will then sort the issue if boiler issue else walk away if fault beyond the boiler.

 

[RickH]

The store thermostat opertes and provides call for heat ... boiler is acknowledging this as it is is 'burn mode' but runs at low modulation (tick over)
It seems that the return Temp of store (~40C) prevents the boiler ramping up to full power.
It maybe that the condensing software is a lot more interactive ..... whereas perhaps Baxi simply responded to call for heat until the thermostat set temp met.

Viessmann approved Heating guy here today looking at both houses ... he seemed to understand the issue and that was first key step ..... he agrees it has been installed as a basic boiler, and its flow temp (and possibly flow rate) may need to be tuned to work with the thermal store.
Initial thoughts are Tank stat (60) and boiler temp (70) maybe too close.

 

[RickH]

We have a way forward

With the Store set to 60 and boiler set to 70 ….. the primaries were not dropping enough in temperature to replen the used DHW. This was why we saw full burn only occurred when primaries dropped to 27C

The Vitodens-100 W boiler has the option to run 2 different burn modes ….. one for Heating and one for DHW.
To switch it to full power when DHW demand is made, there is an additional unit needed, a ‘Cylinder Demand Boost module’

This will trigger when DHW flow switch activates, (and Store Thermostat closed) and switch boiler into immediate full power burn, and 85C temp and will continue to work in this mode until Flow switch opens ….. at which time it will switch back to Heating mode, and a slower more efficient heating up of the store.
Store stat is in series to all of this so …. No heating at all if stat open.


In parallel, I will also fit a temperature thermostatic blending valve on the heating circuit, to allow us to increase the store stat above 60 ….. using blending valve set to 60 instead.

 

[RickH]

Get the installers back, the viessmann boilers need to be configured for a thermal store, personally I would never have installed that boiler on a thermal store, but it can be configured correctly

of the suppliers I contacted they were one of the few that stated it was approved to be used with a Thermal store.
Are you aware of any ‘Thermal store config’ for these ?

 

[ianmcd]

of the suppliers I contacted they were one of the few that stated it was approved to be used with a Thermal store.
Are you aware of any ‘Thermal store config’ for these ?

If the plumbing and wiring is corrrect then the configuration is done via the boiler control panel

 

[RickH]

If the plumbing and wiring is corrrect then the configuration is done via the boiler control panel

What do you think should be changed - there is no 'Thermal Store setting' Keen to know if there is something that can/should be set.
Current advice is to fit the Cylinder demand Boost module.