New regular boiler (heat only) in a sealed system??

[BenH]

Hi, I am replacing our 11 year old boiler (due to H.Ex. leak) with a new one. This is in a "sealed" system with red expansion vessel in airig cupboard & a megaflo (no tank in loft). However the exisiting boiler is Vaillant ecoTEC 4xx boiler - which is satated by vaillant as an "open vent" boiler (has no internal pump). So at least in theory I have been running with the "wrong" boiler for 11 years...??
I have asked for quotes from 2 local plumbers and initially they both recommended to replaced like for like and stay with the same boiler.
This is a large house mainly with UFH (3 manifolds and 3 small rads upstairs).

Just heard from one of them that he now realised I need a "system" boiler and therefore I should fit an ecoTEC plus 6 (not 4!)

Question(s):
Can I install an "open vent" boiler in a sealed system without invalidating new boiler warranty?

If I can not and it must be defined as a "system" boiler, what do I do with the exsiting CH pump (in airing cupboard)?
Can the existing pump be made to work with the internal pump of the system-boiler?

Is there a risk the 2 pumps will create too much pressure in new boiler?

Thanks in advance!

 

[ShaunCorbs]

Open vented boilers can be fitted on either a open vent eg gravity system or a sealed system eg with a red expansion vessel etc better in my opinion as they don’t use expensive and manufacture only components eg pump, expansion vessels and diverters

You would have to remove the external pump

 

[BenH]

As per the install manual (pdf) you are correct. Thank you.

 

[Vee]

Stick with heat only. You most probably still require an additional expansion vessel due to system volume if its got underfloor heating and you wouldn't gain anything by conversion to a system boiler apart from more things to go wrong.