You can. Dunno if this helps but we had an open vented system with the boiler in the loft. The cold water storage tank was on a platform and the header tank was above the boiler which was fitted about a foot or so above the loft floor.
Noob configuration idea for best of both worlds!
- Thread starter Laggy
- Start date
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im a bit confused Muggles, are you saying you’ve done the setup in my OP and it doesn’t matter that the mains going into the cylinder drops the pressure on the boiler like others have said, and also what do you mean you don’t connect the hot, what’s feeding the shower? If it’s the cylinder then this isn’t the same as mine..and like you said it’s a straightforward systems boiler setup no?
Static pressure and dynamic are 2 different things. The 2 dynamic figures (flow and pressure) need to be known before any decision can be made about what approach to take.
I’m going to say 65psi and 18LPM, so that I get to hear your advice
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If you are achieving 4bar and 18L/min dynamic then a high recovery unvented is definitely a great option.
If you are looking to minimise space use then @muggles approach is a great option. A high recovery coil with a cylinder close to the boiler on HW priority will give ample HW with min recovery time. All you need to decide is the max HW usage you may have (bath or extended shower use) to determine the size of cylinder.
Heat loss over 24Hrs with a modern highly insulated cylinder is very low so that isn't really a factor. If space is the key to you then that and usage would drive the cylinder size. If you have a sustained 18L/Min @ 4 bar then there will be very little impact on flow when multiple outlets are used. That would be reliant on 22mm distribution pipework from the cylinder to as close to the outlets as possible to maximise flow. it would also open up the ability to use the HW outlet from the boiler too.
To give an example, recently installed a 24Kw sys boiler with a 200L unvented. Mains is 23L/Min @ 3.5 dynamic .... 2 bathrooms .... 2 showers ... with both showers running and hot and cold tap in kitchen there is still 16L min delivered at the outside tap. It has a 32mm mains, drops to 22mm distribution pipework until it exits the floor when it drops to 15mm (no flexi's/reducing ISO's are used anywhere on the system) m12 hard pipe for the mixer taps. The dynamic figures are key to the whole system working as it does. It is a true dynamic figure where the flow is sustained even with multiple outlets being used. Without that mains and the pipework/fittings there would always be a danger of a noticeable reduction in flow.
edited for duplicated text
If you are looking to minimise space use then @muggles approach is a great option. A high recovery coil with a cylinder close to the boiler on HW priority will give ample HW with min recovery time. All you need to decide is the max HW usage you may have (bath or extended shower use) to determine the size of cylinder.
Heat loss over 24Hrs with a modern highly insulated cylinder is very low so that isn't really a factor. If space is the key to you then that and usage would drive the cylinder size. If you have a sustained 18L/Min @ 4 bar then there will be very little impact on flow when multiple outlets are used. That would be reliant on 22mm distribution pipework from the cylinder to as close to the outlets as possible to maximise flow. it would also open up the ability to use the HW outlet from the boiler too.
To give an example, recently installed a 24Kw sys boiler with a 200L unvented. Mains is 23L/Min @ 3.5 dynamic .... 2 bathrooms .... 2 showers ... with both showers running and hot and cold tap in kitchen there is still 16L min delivered at the outside tap. It has a 32mm mains, drops to 22mm distribution pipework until it exits the floor when it drops to 15mm (no flexi's/reducing ISO's are used anywhere on the system) m12 hard pipe for the mixer taps. The dynamic figures are key to the whole system working as it does. It is a true dynamic figure where the flow is sustained even with multiple outlets being used. Without that mains and the pipework/fittings there would always be a danger of a noticeable reduction in flow.
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Thanks for taking the time to advise all of that..
.. but I’m a little confused. I want to feed the bath and shower with the DHW side of the combi.
I don’t get why, if I’m using a combi boiler, I wouldn’t take advantage of the heat on demand function for the shower, have a small cylinder heated on the central heating side, have that cylinder feed the taps, rather than have a bigger cylinder feed both the shower and taps and have the on demand side of boiler doing nothing..
..if I have 18LPM at mains and boiler will restrict that to say 11 LPM to get the temp rise (an assumption) then surely I can have flow to replace the cylinder when the tap is opened which yes will reduce flow to the DHW side but the boiler couldn’t use the full amount anyway.
If I feed the bath using the cylinder rather than the DHW side of boiler then I’m going to run out without going big on cylinder surely.
And it’s not space I’m worried about btw, it’s the added cost to the very cheap £700 I mentioned.
.. but I’m a little confused. I want to feed the bath and shower with the DHW side of the combi.
I don’t get why, if I’m using a combi boiler, I wouldn’t take advantage of the heat on demand function for the shower, have a small cylinder heated on the central heating side, have that cylinder feed the taps, rather than have a bigger cylinder feed both the shower and taps and have the on demand side of boiler doing nothing..
..if I have 18LPM at mains and boiler will restrict that to say 11 LPM to get the temp rise (an assumption) then surely I can have flow to replace the cylinder when the tap is opened which yes will reduce flow to the DHW side but the boiler couldn’t use the full amount anyway.
If I feed the bath using the cylinder rather than the DHW side of boiler then I’m going to run out without going big on cylinder surely.
And it’s not space I’m worried about btw, it’s the added cost to the very cheap £700 I mentioned.
Not necessarily, the figures quoted in a boilers specs doesn't always mean that supply into the boiler is flow limited to that amount, all it means is that it would only obtain the 35deg rise in the specs at that flow rate, any more flow through the boiler then the 35deg rise couldn't be achieved, if you see what I mean.
Need to check if the Rapid is flow limited or not. Of course if there is plenty of supply then of course you could use the HW output of the combi but if you are just getting the combi because of cost then it would all be down to whether the mains can deal with the demand.
Do also remember that a combi runs at max output to deliver its HW, invariably much more than it would use to heat the cylinder coil, so more in gas use.
There is nothing wrong with a combi feeding some tap supplies and also reheating a cylinder. It’s just a standard s-plan setup
It usually used in properties that have a long distance between draw offs so you may for instance have a long bungalow with a kitchen, utility and boiler at one end fed from the combi side then a hot water cylinder at the far end feeding the bathrooms.
It doesn’t sound like you really need the extra complication in your case, personally I’d recommend you either just go the combi route or take @muggles advice and fit a hot water priority system like this one…
It usually used in properties that have a long distance between draw offs so you may for instance have a long bungalow with a kitchen, utility and boiler at one end fed from the combi side then a hot water cylinder at the far end feeding the bathrooms.
It doesn’t sound like you really need the extra complication in your case, personally I’d recommend you either just go the combi route or take @muggles advice and fit a hot water priority system like this one…
Nice boiler but wake the dead compared to Eco rf etc
My preferred option if going combi route is to let boiler supply hot water to kitchen/ utility and cylinder to showers and baths
They aren’t bad when running. It’s the spaceship whooorp noise they make when they initially fire that annoys most people…
thanks both, I’m not concerned about noise but thanks for the info
thanks I’ll do that
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