Boiler descriptions

[F]

I've been reading past posts to try to get a feel for which condensing boilers are currently on the recommended list. However, descriptions like 'system boiler' and 'heating boiler' seem to have variable meanings.

If I've got it right, a 'heating boiler' has one flow and one return and any pump(s) must be installed within the piping whereas a 'system boiler' has two flow and two return, one pair for central heating and one pair for domestic hot water, both of which have a pump already installed within the boiler casing.

Anyone care to clarify/correct me?

[Tony Bryer]

Generally true (though some boilers have two flow and return connections)

You're confusing a system boiler and a combi. A system boiler includes a pump and will have flow and return connections like any other boiler. A combi adds main cold water in and DHW out to serve the hot water taps, the hot water being heated when required.

[F]

I understand combis (I think!): they heat cold water straight from the mains and send it straight to a hot tap. The heated water isn't stored.

Are you saying that the only difference between 'system' and 'heating' is the presence of a pump within the casing?

[Dave Plowman (News)]

System boilers are everything in one box, so beloved of our resident self appointed pro heating engineer, Drivel, who can't do even simple pipework without leaks. And wiring just plain terrifies him.

[Doctor Evil]

Er, er, I don't want to confuse matters, but in some, hot water is stored, but all inside the same one-box case.

A system has a pump and a pressure vessel. A heating boiler "genrally" has none. Although some makers may incorporate a pump in some cases.

A system boiler is an on-demand combi without the water section.

[Doctor Evil]

No. The DHW is not catered for, combi's do that.

[John Rumm]

To some extent that is true - they are not exactly defined terms.

That is usually the case - the components required to make a complete system need to be added; typically a pump, sealed system expansion chamber, and any valves and controls circuits required.

A system boiler may only have one flow and return (and a heating only boiler may have two!). It will almost certainly have a pump, it will probably have the expansion vessel. What else it contains will vary from one make/model to the next. As a general rule however you will need to add fewer external components to form a working system.

Any better?

[TheScullster]

Not according to Worcester Bosch!

Their definition of a system boiler seems to cover boiler, pump and expansion vessel but not necessarily with water heating combi facility.

From their website:

The Greenstar 28 HE System is suitable for a medium/large family house. As this boiler uses a hot water storage cylinder, the cylinder capacity determines how many bathrooms can be used.

Phil

[Christian McArdle]

A system boiler also includes the components for sealed pressurised operation, such as pressure relief, expansion vessel, filling loop, pressure gauge etc. Some don't include a pump, although the vast majority do.

Christian.

[Ed Sirett]

The better models have a store to assist with short term flow rate.

In many cases: yes. However it is common to also find that a system boiler will also include the components for a Sealed system. See FAQ.

[Matt]

Utter Bullshit, go back and read your leaflets you moron

[OldBill]

I'm sorry but as I have fitted 2 system and 1 combi I agree with Dr Evil. System boilers I've looked at were nothing but combis without the DHW section. As they are not so popular they are not so competitively priced as combis so the next one I need will be a combi with the DHW ignored or just feeding the kitchen tap.

[Doctor Evil]

Another lunatic who hasn't a clue.

[Doctor Evil]

Yep a common ploy. Or have the combi do the shower - high pressure.

[Doctor Evil]

They are, a system boiler is very specific.

[Doctor Evil]

That is what I said. Read again.

[Doctor Evil]

Ah Matt, the one who crops with many names to throw abuse. Hi Matt, how is the cylinder business? Feeling the pinch from combi's eh.

[John Rumm]

If that were true then all system boilers would contain the same components. However they don't.

[Doctor Evil]

Pump and sealed system componets.