Confused and slightly worried about "Heat-only" boiler set up

[Circuital]

Hello all,

My new house has a Baxi EcoBlue Heat Only boiler (pictured), complete with a hot water cylinder, an F&E tank and two cold water tanks in the loft. I've run into problems with cold radiators recently and managed to sort it by running Sentinel X400 through the system. Having had to clean, drain and dose the F&E tank, i've taken an interest in the way the whole system works but stupid as it sounds, I can't get my head round how the boiler works!

Since the boiler heats both the water going to the hot water cylinder and the water circulating around the central heating, is there a chance of the Sentinel getting into the hot water supply? If not, how does the boiler keep the hot water and the central heating water completely separate? And why do they call it a "heat only" boiler?! This seems to suggest it only looks after the central heating, surely?

I appreciate this is will seem like a stupid question but i've tried reading up on it and still can't get my head round it. Maybe it's because it's ridiculously late and i'm tired!

I appreciate your time as always!

 

[bernardgreen]

The water in the cylinder is heated by water from the boiler flowing through a coil inside the hot water cylinder,

"boiler" water cannot therefore mix with the water in the cylinder.

 

[dilalio]

Hi @Circuital

No question is stupid if you don't know the answer

It's all to do with heat exchange.

Your boiler is called a Heat Only (or regular) boiler because it doesn't know what it's heating... It just heats water as it passes through the heat exchanger where fuel (ie gas) is ignited and the heat created by that is transferred to your system water, in pipes that are routed over and/or around the flames... Like an old fashioned kettle on a gas hob.

Where this heated water is then sent is controlled by components outside the boiler casing (again, hence "heat only").

In typical modern systems...
Motorised valves control where the heated water is sent.
A Circulation pump is used to get it there.
And programmers tell room/cylinder thermostats to control when heat is required and when it is not.

The hot water cylinder can be considered as just another radiator on the system, which is only sent heated water from the boiler when domestic hot water is asked for (via a motorised valve).
This "boiler water" travels through a "coil" of pipe inside the cylinder, and back out to return to the boiler, where it is again heated by passing over the flames and the cycle repeats.
Surrounding this "coil" in the cylinder is your domestic water which is heated by the "hot coil"... It's heat exchange again, but this time via boiler water (in the coil) to domestic water (around the coil).
The 2 waters never meet (unless there's a fault).

The domestic water is topped up via either a cold water storage tank (higher than the cylinder) or via mains water... Depending on the type of cylinder and installation one has.

As hot water is drawn from the top of the cylinder, it is replaced with fresh cold water (at the bottom) to be heated up via the coil, to continue the supply of domestic hot water as it is needed, albeit with a time delay to allow the heat exchange to "recover" the spent domestic hot water, if it is all used up through high demand (ie lots of showers consecutively).

Hope this helps explain things to you

 

[mrStat]

Dilalio beat me to the post button

The only thing I will add is that there are so many different types of system and cylinder - that caution should be applied by other readers as there are cylinders out there that appear to have a coil but should not have cleaners added as they can mix with the hot water (Known as Primatic). These will often not have a F&E tank present as they fill directly from the CW Storage.

 

[dilalio]

Known as Primatic

Confangled things that they are!

 

[Mottie]

You all beat me to it.

 

[bernardgreen]

(Known as Primatic). These will often not have a F&E tank present

an F&E tank and two cold water tanks in the loft.

so unlikely the OP has a Primatic (or other bubble separated ) cylinder. That said it is worth checking for any labels on the cylinder that suggest it is a bubble separated cylinder.

Pink is water to taps. Red is water from boiler, the bubbles of air prevent ( in theory ) the red and the pink mixing.

 

[dilalio]

so unlikely the OP has a Primatic (or other bubble separated ) cylinder.

caution should be applied by other readers

 

[Circuital]

All fantastic responses which have contributed to both my learning and my peace of mind, thank you all very much!