Unable to run bath of hot water after boiler change

[sargan]

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? (maybe specific settings to work with a Thermal Store)

 

[ShaunCorbs]

Have you had viessmann out on a warranty call ?


My guess is your having flow issues / lack of flow as it’s modulating it’s sensing heat build up on a cold store this shouldn’t happen

 

[Timmy D]

Have you checked flow and return pipe temp at boiler while filling a bath?

Where did you measure 27c?

 

[sargan]

Have you checked flow and return pipe temp at boiler while filling a bath?

Where did you measure 27c?

When you start to run a bath the cold water flows across the heat exchanger and initially gives plenty of hot water out of the mixing valve. The flow & return are within a few degrees of each other around 40C .... which presumably is the average temperature at that time in the store.
The thermostat on the store kicks in when enough HW has been used, but the boiler does not modulate up ...... stays at a minimum setting, and bit by bit the primary flow & return drop until they reach ~27C at that point it modulates up to full burn and continues to run then until flow & return reach 70C
Measuring the flow & return tempo on the primary tails about 8" away from the boiler.
The Thermal store should be controlling the boiler .... i.e call for heat and it runs.

It 'appears' once the boiler has reached 70C, the control modulates it down, and will not modulate upwards untill return is below 27C

 

[John.g]

Looks like the primary circ pump is not circulating water, there should b a big dT between the boiler flow and return temps, but if too high, say > 30C, may stop firing, or, just modulate down as you say, when the boiler flowtemp reaches its SP, this might be caused by the pump flowrate being too low, what pump is it and its setting, also where is the store stat located (for starting/stopping the pump.

 

[sargan]

Pump is fine, have found 2 issues.
1. On one install - Thermostatic mixer valve faulty, was OK at first call for heat, but then shuts down. Replaced valve and now getting plenty of hot water.

2. The boiler not going into high power mode is an issue, Viessmann were engaged and they advise boiler has issue with warm water temp return by Thermal Store.
Keeps it running at lowest modulation.
They have a fix .. boiler has 2 modes of operation, Heating …. Fully modulating …. and DHW … they have a ‘cylinder demand boost box’. … as soon as DHW flow signalled by flow switch, it will put boiler into ‘full power mode’ until flow Switch opens again.

Viessmann are sending 2 of tHess units, and installing them for free.

 

[ShaunCorbs]

But the return should be cooler than the flow until you start to get upto temp as it’s just a cylinder in reverse

You should need a cylinder demand box as it’s not a weather comp system is it ?

Also which Viessmann’s boiler 100 or 200 ?

 

[John.g]

A bit strange how warm water return affects it, the return temperature on rads can be anything from say 35C to 50/55C and has no effect. If it is running on the lowest modulation then it seems a bit of a coincidence that its also running at its SP temperature. What are the boiler flow/return temps? If the boiler minimum output is say 5kw then a flow/return of 70/50 = flowrate of 3.6LPM, 70/60 = flowrate of 7.2LPM, very unlikely, still looks like a circulation problem but the cylinder demand boost box may rectify this?.

Where is the cylinder store temperature thermostat located?.

 

[sargan]

But the return should be cooler than the flow until you start to get upto temp as it’s just a cylinder in reverse

You should need a cylinder demand box as it’s not a weather comp system is it ?

Also which Viessmann’s boiler 100 or 200 ?

 

[sargan]

Viessmann advise it as the correct solution for Thermal Store … they are well aware that there is no temperature compensation unit fitted.

Their chief engineer was on the phone for over an Hr to the Viessmann guy at the house.
They checked all parts of the system incl plate heat exchanger performance, and it was well in excess of minimum required.
Flow rates checked.
Pump flow. etc.

They came up with solution of ‘cylinder demand boost’ will provide the full power when DHW needed. Boiler return to normal modulating mode when no DHW call.

Both boilers are ‘Vitodens 100 W‘ heat only models.