System boiler wiring connections.

[fixitflav]

But for my boiler to work efficiently in condensing mode I need the return temperature as low as possible and with a relatively small delta T I can only achieve this by turning my boiler thermostat right down to 2 or 3

A weather compensation system does that, it detects outside temperature and in CH mode adjusts boiler flow temperature via an (adjustable) profile. It knows when CH or HW is called, and in HW mode it ignores the weather compensation and reverts to normal control-stat setting. Can't have simultaneous CH and HW a la Y-plan, and it's set up for HW preference. But as ianmcd says, the boiler has to have the necessary features.

 

[jamesBen80]

Is 5 years old not modern enough?

 

[cjcoffin]

I would have thought so. Can you retrofit weather compensation to your boiler. Then you may need to add a relay to disable it while there is hot water demand. This is what we did on a 2009 Vokera, job done.

 

[Madrab]

Is 5 years old not modern enough?

Not for that model of boiler no, the CDI classic regular has simple on and off controls - Switched Live - it doesn't have an E-Bus, which these days is a Open Therm or WB's EMS (BB). Without an E-Bus and the extended sensors/PCB control etc, the smart controls cannot control the system temps in the way you want. Sounds like you may have been sold the last one of that model on the shelf I'm afraid.

If maximising efficiency is important then the system needs to be balanced properly so the desired drop in temps across the rads can be achieved ensuring the return temp is as low as possible and maximise condensing. When it comes to HW then 55Deg as a standard with one day a week up to >60Deg, if legionella is at all a concern, though it shouldn't be for a well maintained regularly used HW cylinder. Under those conditions then a return temp of 50Deg should be achievable, which is well within the boilers condensing range.

 

[jamesBen80]

Not for that model of boiler no, the CDI classic regular has simple on and off controls - Switched Live - it doesn't have an E-Bus, which these days is a Open Therm or WB's EMS (BB). Without an E-Bus and the extended sensors/PCB control etc, the smart controls cannot control the system temps in the way you want. Sounds like you may have been sold the last one of that model on the shelf I'm afraid.

If maximising efficiency is important then the system needs to be balanced properly so the desired drop in temps across the rads can be achieved ensuring the return temp is as low as possible and maximise condensing. When it comes to HW then 55Deg as a standard with one day a week up to >60Deg, if legionella is at all a concern, though it shouldn't be for a well maintained regularly used HW cylinder. Under those conditions then a return temp of 50Deg should be achievable, which is well within the boilers condensing range.

Thanks for the reply, I was starting to thing that I had the last one on the shelf before they went to E-Bus. The radiators were all balanced when the boiler was fitted and a few rads moved around about 5 years ago, Could it need doing again? I'm still only getting a temperature drop of about 6-7C at best. Some of the rads were over sized as the company installing them told me this would help to improve efficiency, especially in 3 rooms where there are large bay windows. 2 of these room have 3x 3 column rads connected in series around the bay, so the temperature drop across them should be quite large, if I understand it correctly.

 

[Madrab]

I think it's important to understand what is to be achieved when balancing the system.

2 things primarily:-

1) the system heats up quickly and as equal as possible
2) maximise efficiency by attaining specific drops across the radiators to achieve a given space being heated up quickly and to a specific target point whilst allowing the boiler to condense properly

The trick with balancing is to obtain the required temp across the whole space as quickly and as evenly as possible and when that's achieved - 20deg is considered a standard being the target temp when it comes to deltas. The system needs to warm up evenly and when it reaches the set flow temp then the system should modulate to maintain that temp - system/rad controls dependent. Whilst the system is working to achieve the target space temps then ideally the return temp to the boiler would be <= 55deg, being the optimum dew point temp to maximise condensing.

To balance the system correctly to obtain the required return temps there needs to be a optimum pump/flow settings and the radiators throttled to control the flow through the rad so the rad can release enough heat into the space and allow the return temp to be at a point that the boiler can condense.

There are a number of factors and some of these can be difficult for a DIY'er to either understand or implement - pump flow curves can be difficult to understand and system size dependent but pump speeds are important, as are correct settings of the lockshields but that is all configured in concert with correctly sized radiators for the space it's heating up (Heat loss calcs and the correct types of radiators too - column rads can be terrible at space heating). It is quite an extensive process to get it correct and there's a lot to consider and even then a lot of it can only best endeavours, due to site specific limitations/hardware etc.