Poor heat output from radiators

[Old UKPF 92634]

are you sure normally vaillant on the last two numbers on the model is the kw

It's an ecoMAX 635E, I had a typo in the initial post.

 

[king of pipes]

Is that a system boiler meaning pump inside the boiler casing ? output on that boiler is 34.9kw flat out 22 rads say 2kw each some more some less 44 kw + undersized low loss header with a bigger pump Dab Evosta 40 -70 7 metre pump should do it get the professionals back and get em to sort the emersion heater leak out not that hard to do .

 

[Sparkgap]

No, they've really never been great. The boiler is rated are 35kW. I'm not sure what specifications are relevant regards the pump, there's a graph which shows up to 1900 l/h.

Sounds like it could be the bypass letting by (is there a means of shutting it off to check?). I'd also be wary about using the boiler pump for the whole system, but not knowing the layout couldn't give a definitive answer. Most system boiler internal pumps don't have a particularly high head (I think the Vaillant is about 2.5m) and you might find your system is higher resistance, hence not getting the flow to distant rads.
Another thing: doesn't the Vaillant have an internal bypass fitted? If that's letting by there will be less head available.
Assuming it's lack of oomph from the boiler pump you might need to put in a low loss header with a larger pump on the system side.

 

[Old UKPF 92634]

Thanks for all the feedback. I've got someone booked to service the boiler next week, I'll talk this over with them when they're here.

 

[Old UKPF 92634]

Another thing: doesn't the Vaillant have an internal bypass fitted? If that's letting by there will be less head available.

I've just looked at the manual, yes

3.12.2 System by–pass
An automatic system by–pass is included within the boi-
ler. The boiler is suitable for use in systems with ther-
mostatic radiator valves and no additional by–pass is
required.

Is it usual to also fit an external bypass, or could this be the source of the problem?

 

[king of pipes]

No its not unsual its let's the boilers pump still have a circuit should the rad TRVS close or get turned off when the boiler reaches its set tempreture and switches off the pump continues to run to reduce the tempreture in the heat exchanger the bypass in the boiler is only a small pipe so not so good works better having a external one fitted , you could try replacing it or even better move it near the boiler personally I would still say your boiler is undersized .

 

[Stigster]

Why is a 3 bar pressure relief valve being used as a bypass, if that is what it is in your picture?

Edit - I see that it is a bypass valve. Haven't seen that type used round my way before.

 

[ShaunCorbs]

Why is a 3 bar pressure relief valve being used as a bypass, if that is what it is in your picture?

Edit - I see that it is a bypass valve. Haven't seen that type used round my way before.

honeywell bypass mate

 

[Old UKPF 92634]

So I've just had an engineer out to service the boiler and talk about the issues I'm having. His opinion is that there is a flow problem and the system needs a power flush. It may also need an additional pump on the return given the size of the system but that should be reassessed after the system has been cleaned out.