Heat only (conventional) boiler temperature settings 12 February at 4:19PM in Energy 23 replies 590 views

doodling

Hi,

With some Vaillant controls, Vaillant boilers can deliver different flow temperatures for hot water and central heating (but not at the same time obviously), with or without hot water priority.  This can't be achieved with just the boiler on its own as so far as I am aware there are no Vaillant boilers with multiple hard wired call for heat inputs.  Vaillant do an interface box (VR66) which effectively allows you to connect an S-plan system with separate heating and hot water demands (albeit a Vaillant programmable room thermostat is also required), but pretty much no-one installs them.

Dolor

doodling said:

Hi,

With some Vaillant controls, Vaillant boilers can deliver different flow temperatures for hot water and central heating (but not at the same time obviously), with or without hot water priority.  This can't be achieved with just the boiler on its own as so far as I am aware there are no Vaillant boilers with multiple hard wired call for heat inputs.  Vaillant do an interface box (VR66) which effectively allows you to connect an S-plan system with separate heating and hot water demands (albeit a Vaillant programmable room thermostat is also required), but pretty much no-one installs them.

You can also buy a sub board from mainland Europe that makes the Valliant Opentherm compatible; however, I believe that fitting it invalidates the Valliant UK guarantee. When Opentherm initiates a hot water demand it requires a boiler temperature of 93C which the boiler moderates down to its HW set temperature (which some boilers do but not all).

It all worked brilliantly on my old Atag boiler. Buying a house with a Worcester Bosch boiler has been my biggest regret.

vic_sf49

Rodders53 said:

A combi boiler will definitely have separate HW and CH circulating water temperature control.

Hoping not to derail the thread, but my combi boiler (err, Worcester Greenstar 28i junior) has the HW set at 55 degrees, and that can't be changed, says the manual.

Flow temp (heating) can be adjusted by the dial on the front panel.

Sterlingtimes

BUFF said:

Do you also have a room 'stat(s) [perhaps Wiser]? do you know which Wiser hub you have & how many channels it has?

Thank you. I have a two-channel hub: heating and hot water. There are 14 iTRVs. Two larger living areas span two radiators each, and in those rooms, a room thermostat is married with the relevant iTRVs.

BUFF

Sterlingtimes said:

BUFF said:

Do you also have a room 'stat(s) [perhaps Wiser]? do you know which Wiser hub you have & how many channels it has?

Thank you. I have a two-channel hub: heating and hot water. There are 14 iTRVs. Two larger living areas span two radiators each, and in those rooms, a room thermostat is married with the relevant iTRVs.

are you sure that it isn't the 3 channel hub? https://wiser.draytoncontrols.co.uk/smart-heating-kits

Sterlingtimes

BUFF said:

are you sure that it isn't the 3 channel hub? https://wiser.draytoncontrols.co.uk/smart-heating-kits

Yes, it is a two-channel hub controlling heating and hot water. I bought an extra room thermostat.

Where I have two radiators with iTRVs in one room, a room thermostat takes over effective precedential control of the related iTRVs. 

Sterlingtimes

Rodders53 said:

S plan does not (cannot) signal two temperature demands to a boiler.  
There's only the one call for heat wire in a conventional S-plan and all motorised valves are wired in parallel for that 'call'.

Thank you again Rodders. Can I conclude that it is an illusion when I set the Vaillant to separate temperatures for hot water and heating?

If this is the case, I would set the hot water and heating to the same temperature to avoid any uncertainty. 

The Vaillant can switch off the heating in the summertime but leave the hot water on. Is this another illusion, or will this be effective?

Is the boiler just too clever for an ordinary S-plan system?

Dolor

Sterlingtimes said:

Rodders53 said:

S plan does not (cannot) signal two temperature demands to a boiler.  
There's only the one call for heat wire in a conventional S-plan and all motorised valves are wired in parallel for that 'call'.

Thank you again Rodders. Can I conclude that it is an illusion when I set the Vaillant to separate temperatures for hot water and heating?

If this is the case, I would set the hot water and heating to the same temperature to avoid any uncertainty. 

The Vaillant can switch off the heating in the summertime but leave the hot water on. Is this another illusion, or will this be effective?

Is the boiler just too clever for an ordinary S-plan system?

It is not an illusion as such. The boiler reacts to the source of the demand. The way that I managed it with your type of boiler was to set a HW heating period outwith that of the programmed heating period. My logic for that was that when the boiler flow temperature was high for HW reheating, the boiler return flow temperature was outside of the condensing band. By going for separate times, and a low CH flow temperature, I was ensuring that the boiler was condensing when CH was demanded.

My present property has just one set boiler flow temperature which I have set at 55C. I heat my HW via excess solar or from the grid during my cheap EV tariff period. I have found that whilst a gas boiler might take 8kWh to re-heat the cylinder: electricity takes about 3kWh. The difference in efficiency between gas and electricity is a factor but there are also pipe runs to consider. Moreover, cold boiler water initially takes heat out the cylinder until the boiler flow temperature exceeds that of the cylinder.

Rodders53

I don't know.
I'm an Engineer (E&E) by training and retired... and sometimes enjoy reading manuals.

As your boiler has no way of knowing when HW or CH is demanded except via a special Vaillant box installed by your installer (as described by Doodling up the thread) then you really should / must ask the installer.

If your boiler shows the actual flow water temp (as well as the set CH/HW temps) during operation in your shoes I'd:

Switch off all CH and HW demands.
Set CH flow to 40-45C.  HW to 75C.

Switch on HW demand (if necessary increase tank stat to >55C)  --- observe the actual flow temp.
Switch off HW demand.  Switch on a CH demand.  Observe the actual flow temp.

That'll confirm if the boiler knows what is being called for and if the flow temperature changes between the two values set.

oldagetraveller1

vic_sf49 said:

Rodders53 said:

A combi boiler will definitely have separate HW and CH circulating water temperature control.

Hoping not to derail the thread, but my combi boiler (err, Worcester Greenstar 28i junior) has the HW set at 55 degrees, and that can't be changed, says the manual.

Flow temp (heating) can be adjusted by the dial on the front panel.

Page 8, section 3.1.4 of the manual suggests otherwise. Setting hot water temperature, the left hand control knob.