Heat only boilers and confusion

[kasser]

Trying to get to grips with all the terminology here and what can be done with what.
Heat only = open vent boiler?

I suppose nothing stops me from sealing it up and sticking an expansion vessel to it? This is the same as a system boiler then? Is it cheaper then to buy a system boiler instead of adding an expansion vessel to the heat only? Even with a system boiler you may need an extra EV anyway, so what's the advantage of picking a system boiler in the first case if you're going to add an EV to it?

Is there anything stopping me from having an open vent heating system and an unvented hot water cylinder with a heat only boiler? Or vice versa?
There's no reason I can't have an open-vent hot water cylinder with a system boiler?

If I'm right in my above thinking, a heat only boiler can be used as a system boiler with the right components, but a system boiler cannot be used as open-vent boiler due to the expansion vessel? I suppose you could, by ignoring the EV, but then you're paying more for nothing?

This is confusing me more:
https://www.vaillant.co.uk/for-installers/products/ecofit-pure-open-vent-boiler-26116.html
If you look under Specifications and DHW, it mentions a minimum pressure to operate the boiler. Why, what for?


Finally, with a cold water tank in the loft and bathroom just below, the tank is going to be less than 1 m above the shower head when taking a shower. That's 0.1 bar of pressure minus losses due to pipe length and fittings. Not possible to have a shower then? I suppose this was designed to fill a bath, so large pipes, big flow rate, low pressure?

 

[Allgoode]

Forgot to add, if you can look inside a "heat only" boiler beside a "system boiler" you will immediately see the piping difference's and logic to the layout which helps to put the theory in context.

Old scrappy or removed boilers can let you tinker to your hearts content without worry about damage or if you have some spare bits leftover when you are learning.

 

[kasser]

Youtube is pretty good too.

 

[Millsy 82]

I personally only fit a system boiler if I lack space to fit an external pump expansion vessel etc. In the future it's generally easier and cheaper to replace external parts. On oil I would fit a system boiler as Grant's are awkward and have 2.5 bar pressure reliefs.

Yes you can fit most heat only boilers to most vented/unvented cylinders etc.

 

[chris watkins]

Youtube is pretty good too.

I can highly recommend the Domestic Heating Design Guide if you want to get up to speed with heating it will be the best £28 you will spend.
https://www.cibse.org/knowledge/knowledge-items/detail?id=a0q20000008I7odAAC