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.

 

[John.g]

Came across a few notes from a thread on here, a while ago,re a Telford 300L TS.
The gist of it was that a old fixed 17kw (gas) boiler runs almost continuously when heating the TS, the cylinder stat is in the "bottom" of the TS, set to 65C, it switches in at 60C, a hysteresis of 5C, the boiler runs with flow/return temps of 72C/60C using a grundfos Alpha2 set to max CC3 curve. By calculation, 17kw output wiyh a dT of 12C = flowrate of 20.3LPM, 1.22m3/hr which the 6M Alpha2 will deliver at a 4.5M head.
The boiler flowtemp is set to 80C to avoid the boiler cycling as it reaches 70C as soon as the stat cuts in at 60C.

Suggest you increase the boiler SP to 80C and see if the boiler ramps up. Also check setting of the circ pump as the boiler Hx may have a fairly high pressure loss and requires a high head to get a high flowrate and dT.

 

[sargan]

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?.

Stat is on bottom 1/3rd of the store.

 

[sargan]

Came across a few notes from a thread on here, a while ago,re a Telford 300L TS.
The gist of it was that a old fixed 17kw (gas) boiler runs almost continuously when heating the TS, the cylinder stat is in the "bottom" of the TS, set to 65C, it switches in at 60C, a hysteresis of 5C, the boiler runs with flow/return temps of 72C/60C using a grundfos Alpha2 set to max CC3 curve. By calculation, 17kw output wiyh a dT of 12C = flowrate of 20.3LPM, 1.22m3/hr which the 6M Alpha2 will deliver at a 4.5M head.
The boiler flowtemp is set to 80C to avoid the boiler cycling as it reaches 70C as soon as the stat cuts in at 60C.

Suggest you increase the boiler SP to 80C and see if the boiler ramps up. Also check setting of the circ pump as the boiler Hx may have a fairly high pressure loss and requires a high head to get a high flowrate and dT.

That looks like same issue .. store stat set to 60. Boiler temp set to 70C so 10 degrees hysteresis, (this is as per design guide & system manufacturer advice)
The boiler goes into low power cycling once store temp is about 45C… never full power.

As part of investigation, Viessmann found that setting boiler temp to 80C does forced boiler to go into full power burn, this is part of logic in their advice to fit ‘Cylinder demand boost module’. Which will force boiler into no -modulation mode when DHW demand.

As interim work around it is set at 80C

I asked why it worked OK with Baxi, the Viessman expert on phone advised he had previously been designer with Baxi for 15yrs, and the early Baxi condensing boilers were very simple and basic modulation … the far better control & efficiency in the current Viessmann gives much better modulation, but is an issue with the way return temp from thermal stores.

 

[John.g]

The baxi Hx may have had a much lower pressure loss than the Viessmann, looking at the pump curve below the residual head with a "6M" pump is ~ 2.8M which means the Hx pressure loss is 3.2M at 20LPM. If your pump can deliver 20LPM at this 3.3M head then assuming the minimum output of your boiler is 5kw, the boiler dT will only be 3.6C, and even with the boiler SP at only 70C then assuming a flow temp of 60C (dT of 10C) the boiler should still run at almost 14kw? theoretically a 30kw boiler with flowrate of 20LPM will run at 30kw but with a dT of 21.5C and that is why you "must" set the boiler SP to 80C?. But, just now even with the boiler SP at 70C then you should get ~ 14kw if the flowrate is ~ 20LPM.
Can you post the make/model of the circ pump and its present setting, it may only be pumping 8 to LPM just now, its vital to get a feel for its flowrate.

Ideally, IMO, the stat should be located just above the flow outlet to the pump, that way the store should never go below 60C as long as the boiler can keep up with the demand.

 

[EvilDrPorkChop]

Was always my understanding you ran a thermal store at 80oc+ anyway. Why are you running it at 60oc? When you've got a HW and heating demand, surely the store isn't hot enough to cope with all the demand.

 

[John.g]

Because of the dT through the boiler and you are heating from the top down then you have to set the store stat well below 80C?, as above, as long as the demand is below the boiler max output of 30kw then once the stat falls to 60C you will be filling the store at ~ 80C and up to 85C when the stat opens at 65C. (this assumes a flowrate of 20LPM).

 

[sargan]

The baxi Hx may have had a much lower pressure loss than the Viessmann, looking at the pump curve below the residual head with a "6M" pump is ~ 2.8M which means the Hx pressure loss is 3.2M at 20LPM. If your pump can deliver 20LPM at this 3.3M head then assuming the minimum output of your boiler is 5kw, the boiler dT will only be 3.6C, and even with the boiler SP at only 70C then assuming a flow temp of 60C (dT of 10C) the boiler should still run at almost 14kw? theoretically a 30kw boiler with flowrate of 20LPM will run at 30kw but with a dT of 21.5C and that is why you "must" set the boiler SP to 80C?. But, just now even with the boiler SP at 70C then you should get ~ 14kw if the flowrate is ~ 20LPM.
Can you post the make/model of the circ pump and its present setting, it may only be pumping 8 to LPM just now, its vital to get a feel for its flowrate.

Ideally, IMO, the stat should be located just above the flow outlet to the pump, that way the store should never go below 60C as long as the boiler can keep up with the demand.


View attachment 80835

The pump came as part of the system kit … it’s a 3 speed Grundfos UPS 15-50, and is set to speed 1.
The thermostat set in bottom 1/3 PFD of store, about 200mm above the return pipe.
the store has an internal baffle allow differentiation between bottom (heating) and top (DHW) and part of reason it relies on pump to create a ’stir’ when DHW is being drawn off.

 

[ShaunCorbs]

Wack that to speed 2 and try again if that’s the boiler pump

 

[sargan]

Wack that to speed 2 and try again if that’s the boiler pump

The on site Viessmann guy said that would make it worse, needed slower flow to give it chance to transfer heat, but I’m not sure if fast or slow flow is better.

 

[ShaunCorbs]

Problem is you need 0.35 lps (21 lpm) flow through the boiler so it can ramp / max

Viessmann’s ramp at the start and then slowly drop to match the flow / ntc temp for them to keep ramping you need to keep the boiler cool / get the heat away

 

[John.g]

The faster the better IMO, I would suggest speed 3. Speed1 is only a 3.7M pump.

 

[sargan]

Problem is you need 0.35 lps (21 lpm) flow through the boiler so it can ramp / max

Viessmann’s ramp at the start and then slowly drop to match the flow / ntc temp for them to keep ramping you need to keep the boiler cool / get the heat away

I don’t mind trying other speeds … is there a way of getting it ‘optimum’ is this via flow & return temperatures ?

 

[ShaunCorbs]

You will know as it will stay on ideally you want a difference of 20d between f and r on full ramp / 3/4 burn depending on the store size as boiler might be oversized etc

 

[John.g]

The on site Viessmann guy said that would make it worse, needed slower flow to give it chance to transfer heat, but I’m not sure if fast or slow flow is better.

I think your boiler has a minimum output of 8kw so will need a minimum flowrate of 11.5LPM to avoid exceeding 70C from stat cutin of 60C, more importantly, all boilers fire up at ~ 65% output and then modulate down which means a high flowrate or a big dT required, because your dT of 10C (70-60) and probably lowish flowrate then its quite possible that the boiler will repeatedly cycle and not get away until the boiler return temperature (store bottom) drops well below this.
Its also possible (if unlikely) that the circ pump is faulty.

Optimum setting?, flowrate as high as possible but not > 20/25LPM. Boiler efficiency/condensing depends on getting the return temperature as low as possible but you are limited to 60C (just now) whereas a convenrional CH system can easily get down to 35/40C for alot of the time and use priority DHW to recharge the HW cylinder as fast as possible and then revert to CH.

 

[EvilDrPorkChop]

Because of the dT through the boiler and you are heating from the top down then you have to set the store stat well below 80C?, as above, as long as the demand is below the boiler max output of 30kw then once the stat falls to 60C you will be filling the store at ~ 80C and up to 85C when the stat opens at 65C. (this assumes a flowrate of 20LPM).

The store is pumped though. So it's going to be a unified temp throughout the store, It's not going to be like a cylinder with a coil. So I'd say the boiler flow needs setting at 80oc, and the Store stat at least 70oc. 60oc IMO is too low. Pump speed on 3.

 

[sargan]

Currently, the stat is set to 60C as that is the max temp that you can put into the heating circuit (UFH)
I have bought a 28mm UFH temperature Mixing valve.
I'll put that on the UFH take-off and use that to control Max temp into UFH, and then I can crank up Store temp to 70C or more.

The Nu-Heat design does not have a mixing valve (see diag) but I will add one.



Just to be fair, the Nu-Heat system has worked superbly for over 15 years ..... the Baxi Barcelona Heat Only boiler had come to the end of its life, so now trying to get optimum set-up for Viessman boiler.

 

[EvilDrPorkChop]

Currently, the stat is set to 60C as that is the max temp that you can put into the heating circuit (UFH)
I have bought a 28mm UFH temperature Mixing valve.
I'll put that on the UFH take-off and use that to control Max temp into UFH, and then I can crank up Store temp to 70C or more.

The Nu-Heat design does not have a mixing valve (see diag) but I will add one.
View attachment 80841


Just to be fair, the Nu-Heat system has worked superbly for over 15 years ..... the Baxi Barcelona Heat Only boiler had come to the end of its life, so now trying to get optimum set-up for Viessman boiler.

Is there no mixing valve on the UFH manifolds themsevles?

 

[John.g]

The store is pumped though. So it's going to be a unified temp throughout the store, minute or twoIt's not going to be like a cylinder with a coil. So I'd say the boiler flow needs setting at 80oc, and the Store stat at least 70oc. 60oc IMO is too low. Pump speed on 3.

Agree that it should be set to 80C & pump speed 3 but find it hard to get my head round the unified temperature bit when heat is being drawn off but agree the whole store will reach 80C with no heat demand. If the whole store is at 60C and assume the 30kw boiler is set to 80C and the flowrate is 20 LPM., the boiler fires up and because the dT through the boiler will be 21.5C. the boiler SP will be reached in a minute or two so the boiler will ramp down to 27.9 kw to maintain the 80C SP, the whole 300L cylinder will then heat up in 15 minutes, the cylinder bottom (stat) will then rise fairly rapidly and when it reaches 65C and the boiler will stop firing.
If there is a demand for say DHW then DHW flowswitch will start the store circ pump and when the cooled store water beneath the DHW Hx cools to 60C then the boiler fires up to to restore the boiler SP to 80C but you will then have a temperature gradient through the store?, is that the way you see it?.

 

[John.g]

If the store temperature falls to 60C without any call for heat what starts the store circ pump and does a call for Ch only start it?.

 

[sargan]

The sequence which is quite key ...
In the event of DHW draw off, the flow switch starts the boiler pump, but does not provide a Call for Heat ... so boiler does not fire.
This pump run 'stirs' the store to keep replenishing the plate heat exchanger.
Allowing the stored energy of water in the Themral store to provide heat without constant firing of the boiler.
When the store temp drops sufficiently the thermostat operates, then it provides call for heat to boiler.

My concern with Viessmann suggestion of using 'Cylinder Demand Boost unit' is that I would lose all of that functionality.


Think it needs an AND function, if flows switch operates AND thermostat is closed then switch to DHW Boost mode , once DHW demand stops (no flow switch) boiler return to normal mode.

Not sure if Boost Unit can accomodate that ....

 

[sargan]

If the store temperature falls to 60C without any call for heat what starts the store circ pump and does a call for Ch only start it?.

Had just put explanation of that in the thread .... comment #30
When stat open and flow switch active - pump runs to provide stir
If stat is closed ... pump will run and provide boiler with call for Heat signal. (orrespective of flow switch condition)

 

[John.g]

So the DHW boost mode ‘full power mode’ will presumably change the boiler SP to 80C and return it to 70C or whatever when DHW demand ceases?.
The only benefit I see in (if) that is that the store standing losses will be slightly lower than maintaining it at 80C at all times.

Have you tried the pump on increased speed and increased the store stat SP??.

 

[ShaunCorbs]

As soon as you use hot water the boiler should start up with a max store drop of 5 degrees

 

[sargan]

So the DHW boost mode ‘full power mode’ will presumably change the boiler SP to 80C and return it to 70C or whatever when DHW demand ceases?.
The only benefit I see in (if) that is that the store standing losses will be slightly lower than maintaining it at 80C at all times.

Have you tried the pump on increased speed and increased the store stat SP??.

My ‘understanding’. (No guarantee I‘m right)
is that the boiler max burn temp does not change.
What changes is that the ‘cylinder demand boost’ switches boiler to Hot Wafter mode ..
which means direct full power, no modulating.
when ‘cylinder demand.boost’ ceases boiler switches back to ordinary mode, but still set to 80 as max temp.

I have increased pump speed to ‘3’. I am unable to increase Store above 60C until I fit the temperature blending valves … candy risk damage to UFH pipes.

 

[ShaunCorbs]

Too late as tbh the ufh shouldn’t be running at 60 or even 50 you sure there isn’t a blending pump set on the ufh picture ?