On 03/03/2021 21:25, John Rumm wrote:
On 03/03/2021 17:12, rick wrote:
On 03/03/2021 14:27, John Rumm wrote:
On 03/03/2021 10:59, rick wrote:
After another 6 days withÂ* no heating ..... boiler repaired under
maintenance cover (burner, burner door, seal & electrodes), but
thinking it's time to look at replacing .... becoming pretty
unreliable.
Its now 22 years old.
Its a 30kW Heat only wall mounted gas condensing boiler - connected
to a pressurised thermal store.

I need a standard/Heat only ..... can't use as system boiler (as
thermal store control need to manage pump)


Want to replace with 'similar' .... I did like idea of Vaillant
boilers with SS heat exchangers - but only offer these in System
boilers.

They do the 400 series heating only models (mostly open vented, but
some also suitable for sealed system).



I contacted Vaillant support they were very clear that Heat only
boilers cannot be connected to a pressurised thermal store, they have
to be used open vented.

Be careful with your terminology here though. Since cylinders can be
vented and unvented, and they can be connected to primary systems that
are also vented and unvented.

So there is nothing stopping you having an unvented cylinder (i.e. mains
pressure cold feed), heated via an indirect coil connected to an
unvented primary heating circuit.

Much as the (more common) sealed system primary heating a vented
cylinder is also ok.

The time you may have a problem is with a sealed system thermal store[1]
connected in a "direct cylinder" setup connected directly to a boiler
(i.e. the CH primary water also fills and mixes with that in the store,
and it uses a coil or external plate Hx to extract the energy from the
water stored water).

For the latter case you will need a boiler that can be used with a
sealed primary system. Vaillant claim[1] that some of the 400 series
*can* be used in sealed systems (although obviously you would need to
provide an external expansion vessel, filling loop and over pressure
relief valve)

[1] and to be fair, many traditional vented boilers can be used in a
sealed system with the addition of the right external bits.

[2] And even then it not uncommon for people to build heatbanks and
thermal stores using unvented cylinders - but they don't have to be used
that way.




In this instance the boiler is connected via primaries to Thermal Store
- it circulates the contents of the store around the boiler.
There is a pump, expansion vessel and 2 port anti heat-syphon valve on
the prinmaries
No direct connection to DHW or heating
The store is pressurised.
Inside the store is a horizontal baffle .... below which there is a heat
transfer coil for the Heating circuits
Above the baffle is a multi-plate heat exchanger through which is fed
mains water = providing the DHW for the house.

Vaillant advise that only their system boilers can be connected to a
pressurised thermal store.
I explained set up to them .......

They were clear that their OV boilers cannot be connected to a thermal
store.