Noob configuration idea for best of both worlds!

[Laggy]

Cheap Combi boiler (Intergas Rapid 32, £700)

Shower and Bath (used different times) fed direct from DHW side of combi

small unvented cylinder (less space, less stored heat wastage) on central heating side of combi, Zone 1, feeds all sink / basin taps

Rads on Zone 2.

No electric shower
No affect on shower by using taps
Very little chance of running out of water
No massive cylinder needed
And cheap


what’s wrong with this and why can’t I find it anywhere?

thanks!

 

[Mr Chibs]

I've got this boiler, it's great... just fit it normally.

 

[bernardgreen]

Turning a hot tap on will mean cold water flowing into the unvented cylinder, this flow will reduce the pressure of the cold water feed to the combi boiler.

That pressure drop could be enough to affect operation of the boiler and hence the temperature of the water flowing to the shower

 

[Laggy]

I've got this boiler, it's great... just fit it normally.

ah that’s good to hear.. do you experience much effect on the shower when someone opens a tap? I don’t want that, it’s a busy household.. thanks

 

[Laggy]

Turning a hot tap on will mean cold water flowing into the unvented cylinder, this flow will reduce the pressure of the cold water feed to the combi boiler.

That pressure drop could be enough to affect operation of the boiler and hence the temperature of the water flowing to the shower

Interesting, no such thing as a free lunch comes to mind..

..but wouldn’t my setup be good if I have a higher mains flow (18LPM) than the boiler can deal with, so it’s a way of removing that bottleneck so that I can demand higher hot water flows through the shower and tap at once?

 

[bernardgreen]

if I have a higher mains flow (18LPM) than the boiler can deal with

What ever the flow rate is there will be a pressure drop when a tap is opened, there wouldn't be any flow if there was no pressure drop along the pipe from the street.

If when drawing 18 Ltr/Min the pressure at the boiler is above the pressure the boiler needs then it is likely that the flow to the shower will not be significantly affected by other taps being opened while the shower is in use.

 

[Laggy]

What ever the flow rate is there will be a pressure drop when a tap is opened, there wouldn't be any flow if there was no pressure drop along the pipe from the street.

If when drawing 18 Ltr/Min the pressure at the boiler is above the pressure the boiler needs then it is likely that the flow to the shower will not be significantly affected by other taps being opened while the shower is in use.

So I should not bother with the cylinder then you reckon?

 

[Mr Chibs]

ah that’s good to hear.. do you experience much effect on the shower when someone opens a tap? I don’t want that, it’s a busy household.. thanks

Yes, you know when a tap is on if you are in the shower, as it runs a little cooler and not as powerful.

I’ve made it a criminal offence to turn the tap on when I’m in the shower, so all is good

it’s quiet as a boiler, turned off the ‘learning hot water’ feature as two showers back to back seems to make the second one cooler. Been fine since I turned this off.

 

[bernardgreen]

My preference is to have a cylinder supplying low pressure hot and cold to basins and bath and with a shower coil in the cylinder feeding mains pressure hot to the shower.

 

[Madrab]

What is your mains dynamic pressure and flow?

 

[muggles]

With an Intergas you can easily set it up for priority hot water and have a smaller unvented which feeds all the taps.

I did this for a customer last year. Despite the new cylinder only being 120 litres, they never ever run out of hot water...

Before

After...

 

[flameport]

less stored heat wastage

Heat losses from modern unvented cylinders are tiny, and the differences between a large and small cylinder are almost irrelevant.

Most heat loss is from the pipework.

 

[Laggy]

My preference is to have a cylinder supplying low pressure hot and cold to basins and bath and with a shower coil in the cylinder feeding mains pressure hot to the shower.

View attachment 260554

Thanks, I can’t really do that as boiler is in loft so I can’t get the gravity I need (noob here so apologies if wrong)

What is your mains dynamic pressure and flow?

Hmmm not sure, I know that I get 18 litres a minute on an outside tap as I’ve tested it with a bucket. As far as pressure I only know that it is “high”, as in for the purposes of this topic you can assume whatever the average engineer working on this would consider high, and if I got that wrong it’s my fault

Heat losses from modern unvented cylinders are tiny, and the differences between a large and small cylinder are almost irrelevant.

Most heat loss is from the pipework.

interesting and thanks !

 

[Laggy]

With an Intergas you can easily set it up for priority hot water and have a smaller unvented which feeds all the taps.

I did this for a customer last year. Despite the new cylinder only being 120 litres, they never ever run out of hot water...

Before
View attachment 260559


After...
View attachment 260560

Is this the setup I was originally talking about?

did you do this for the reasons I mentioned…ie to allow shower and taps to be used simultaneously whilst using a combi to provide never ending water?

 

[muggles]

Yes, essentially. You don't actually connect the hot water pipework on the boiler to anything in this example - it's effectively a system boiler with some clever controls.

The small cylinder has a large coil in it which allows the boiler to put its full power into the cylinder to heat it up very very quickly. Water for the taps is taken from the cylinder, and the cylinder is heated as the water is drawn. Hot water is effectively endless as the reheat time after a draw-off is only a few minutes, and of course it is heating whilst water is being drawn too.

The other advantage is this setup is that you can have two flow temperatures from the boiler, which makes it more efficient. High temperature for heating the cylinder, but a lower OpenTherm-controlled temperature for the heating so that the radiators are only as hot as they need to be to maintain a stable room temperature. Lower flow temperature = less gas usage