How does a system boiler work?

Hi,
It's maybe time to upgrade our 21 yrs old vented CH/DHW system. It works fine and passes the emissions with flying colours, but when serviced recently the Scottish Gas guy (who has serviced it most of the last 11 years) showed me some paint scorching on the inside of the Ideal Classic boiler casing.

Don't want a combi so looking at a standard heat only boiler as a straight swap, but it'd be nice to have better DHW flow and maybe run a decent mixer shower from the system, so considering system boilers instead, since it seems they work from mains rather than a tank, thus giving mains pressure at DHW outlets (I understand this would also require a new cylinder). But I'm a bit confused to how a system boiler actually works.

Our existing boiler has only a flow and return at the boiler, and DHW is heated via a coil within the cylinder. How does a system boiler do it's thang? Is it the same but working from mains, or is there more to it?

And if we were to change to a system boiler (or other unvented system?) what might we need to consider? Our house is plumbed in Speedfit push fit fittings, would they be up to the job? And what about our 21 yr old rads? The taps and cisterns all have the baffle things removed so I assume would all need them fitted? And is my assumption correct that the cold taps would also be mains pressure?

Thanks for any help you guys could give, and/or feel free to suggest something better.

I’d ask for a second opinion on the ideal boiler first!
I smell a rat.

Before choosing an unvented cylinder, you first need to get your incoming mains tested to ensure it is actually up to running it. There's absolutely no point having an unvented cylinder if the mains isn't powerful enough to drive it.

System boilers and heat only boilers work in essentially the same way. The two major differences are that a system boiler has a pump built in, whereas on a heat only it's external, and a system boiler is usually fitted to a sealed system whereas a heat only can be fitted to either sealed or open vented systems. Your existing system & rads should be OK but there's no real way of knowing until you try it.

As Jeff says though, the Ideal Classic is an excellent boiler and you shouldn't be in a hurry to replace it. You can keep the Classic and have an unvented cylinder fitted (mains water supply permitting) if your main driver is to improve your hot water performance

I certainly wouldnt be replacing an Ideal Classic boiler, one of the best boilers Ideal ever produced, nothing on the market these days that will last as long as what you have, remember Scottish gas are exactly the same as any other privately owned company, but they have massive pressure on them by their managers to sell products when in customers homes, your boiler will be fine

muggles said:

System boilers and heat only boilers work in essentially the same way. The two major differences are that a system boiler has a pump built in, whereas on a heat only it's external, and a system boiler is usually fitted to a sealed system whereas a heat only can be fitted to either sealed or open vented systems.

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So there's still only one flow and return to a system boiler, like I have just now? And it still heats the DHW via a coil in my (new unvented) cylinder?

I note the general consensus that it maybe isn't last orders yet for my Ideal Classic. So what's with the light scorching on the inside of the casing - just an age thing and nothing to worry about? Are new casings still available for the old girl?

Oh, and thanks very much for your help guys

alan333 said:

And it still heats the DHW via a coil in my (new unvented) cylinder?

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Yes, it does exactly what your existing boiler does.

If you want mains pressure hot water, an unvented cylinder is required. That could be supplied from a new boiler, or the boiler you have now.

There are so many ways boilers can heat the house and DHW. The cylinder can have the hot coil at mains pressure, the tank at mains pressure or both using a header tank, with the latter a pump can actually result in better shower pressure than mains, you are not allowed to pump direct from mains.

Having some sort of cylinder means you have more options, this shows where solid fuel, electric, and solar are combined, and it can also include a boiler, well maybe boiler wrong name, in the main if it boils we consider it faulty.

With my oil fired boiler there is no problem, but gas boilers have some thing not often found with oil and solid fuel, they can extract the latent heat, and to do this the return water temperature is critical. The return water needs to be cool enough, and this alters the whole concept.

So lets look at the basic idea, water leaves the boiler, and it is presented to the TRV at each radiator, if the TRV is open water flows through the radiator at a rate set by the lock shield valve, and if the TRV is closed then the by-pass valve opens, if the return water is cold the boiler runs flat out, but as the return water warms up the boiler modulated at first (turns down output) and once at minimum it starts to switch off/on, the ratio of off/on is called the mark/space ratio and the anti cycle software in the boiler alters the mark/space ratio to match demand.

If the boiler turns off, any heat in the boiler is lost through the flue, and if the boiler turns itself on it does so at minimum output, but if turned on by an external off/on control it turns on at maximum output.

So we want the external control to work as little as possible or be a modulated control like "OpenTherm" and the return water temperature is very important.

The problem is if we disregard the above, it still works, but not in an efficient way. So installers can walk away leaving an inefficient system and the client is unaware.

I am only an electrical engineer, and I will not claim to know all the required knowledge to install an efficient system, a heating and ventilation engineer has spent at least 3 years getting his degree, to my mind no degree means not an engineer, so in my trade as electrical engineer I did a 4 year apprenticeship followed by three years to get my higher educations status. I am not cheap to employ, well not strictly true, I volunteer at my local heritage railway, so actually work for no wages, on real boilers, which actually produce steam, in fact super heated steam. However your unlikely to find some one willing to work for free, so some one with the ability to design your system will not come cheap.

The main problem is the cost of an efficient system often needs 25 years or more to break even on a basic system, so we are looking at a compromise, unless installed when building the home so you have a 25 year mortgage to pay for it, in the main we are looking at 5 years, as interest rates on money borrowed means more than 5 years not economical, this means looking at a simple system.

So until my old boilers started to leak and there were no parts available to repair it, it was not worth replacing. Fitting electronic TRV heads maybe, the savings when not heating all rooms are huge, but as to which TRV head, and which wall thermostat/programmer that is another question.

No good my saying what I think is best, I got it wrong and fitted Nest Gen 3 only to find support for the Enegenie TRV heads has been withdrawn.

flameport said:

If you want mains pressure hot water, an unvented cylinder is required.

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or a vented cylinder with a second coil. Advantage is that being vented it does not require annual checks/servicing by a G3 registered plumber

bernardgreen said:

or a vented cylinder with a second coil. Advantage is that being vented it does not require annual checks/servicing by a G3 registered plumber


View attachment 246629

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Cold taps being fed from header tank???

Dereekoo said:

Cold taps being fed from header tank???

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The kitchen sink is fed direct from mains. The bath, the basins and toilet cisterns are fed from the cold water storage tank.

The header tank for the boiler and radiators is not shown in the diagram

Thanks for the replies guys, certainly food for thought. We were considering selling the house and moving on, but
we've had a re-think and we'll probably be here for a while yet, so now thinking of things we'd like for the longer term, like a good shower in the en suite.

What would you guys do?
Go the whole hog and do both the new boiler and the unvented at the same time, and forget about it for the next pile of years? Or do the unvented now and worry about the boiler later on?

I do value your advice guys, thank you very much for taking the time.