Unvented Cylinder with Combi?

[Steve118]

Hey All,

We're planning some major works to our house next Feb and I'm trying to plan out our hot water solution.

We currently have 1 shower room (deluge and hand) and 1 bath with shower rail. These are both on the first floor and we will be doing a loft conversion where we will add another shower.

Currently we run off an Ideal Independent+ C35 to supply the rads and hot water and my wife and I both get up and the same time and cant have a shower since one will be cold (I know! first world problems eh)

Since we're about to have our second child, add an additional shower in the loft and have to relocate the combi with the downstairs renovations anyway I've been looked at switching to unvented cylinders but have a few questions.

A few bits of additional detail that may help:

• I dont know what pressure I get from the mains but a quick 10l bucket and stopwatch test gave me a rough flow rate of 22l/min from the kitchen cold tap.
• All three bathroom/showers are located above/next to each other on one side of the house
• When we moved in about 5 years ago we ran new alkathene pipe from the street to the house, I need to double check but from memory I think it was 25mm

So a couple of questions and apologies if these are obvious but I've only ever had combis before.

1. If my flow rate is 22l/min into the house and I use an unvented cylinder the hot water flow rate can never exceed this since the tank must be replensihed at the same rate otherwise you'd have a vacuum right? So if we ran all three showers at once they would have, for example, 7l/min(ish) flow rate each correct?
2. Assuming the above is correct and 22l/min is a pretty decent flow rate into the house and given modern combis can run at 18l/min why would you go to the expense of UC/System boiler if a combi can almost match it? Is it that 18l/min is in ideal circumstances but you rarely get that or is 22l/min into the house is not that good after all?
3. Our Combi is about 5 years old and has plenty of life left so could I use this instead of a system boiler as it seems a shame to junk it. In other words could I have an UC heated by electic immersion that is only plumbed into the showers bath and then just use the exiting combi to run the rads and the remaining hot water taps? Be interested to know if this is a stupid idea or not and what the pros/cons might be. (The long term plan would be to replace the combi with a sytem boiler when it dies in X years time but dont know if doing the above locks me into UC/Combi, in other words would I have to buy a specific type of UC that couldnt take its supply from a SB?)

Many Thanks
Steve

 

[DP]

Why not let the boiler supply a shower and kitchen +utility room, split the heating side to heat the cylinder and radiators
If you have two floors, create three zones, upper, lower and cylinder zones, or go for Honeywell Evohome
Present boiler will work well

 

[muggles]

22l/min isn't great. No reason why you should have to run an unvented cylinder off an immersion heater - that's an expensive way of heating water - just heat it off your existing boiler instead

 

[flameport]

1 - Yes, although the actual available flow may well be more than 22, it should be determined at an open pipe as any tap will have some restriction in it. Mixer types especially as they often have thin flexible pipes to connect them.

2 - 22 is not really that good particularly with a new 25mm supply. As above it may well be more than that.
Some combis can do 18, plenty cannot.
Flow rates are specified for a certain temperature rise and will be less for higher output temperatures. Flow rate will be less in the winter as the incoming water will be at a lower temperature.
Combis have heat exchangers which block with scale in many areas.
Combis are water wasters due to the delay between turning a hot tap on and the boiler actually heating water. If you use the preheating function to avoid that, they waste gas instead.
Depending on the boiler location, you may not want it roaring into operation every time a tap is turned on.

3 - Any boiler can be used to heat a cylinder whether combi or not. Any indirect cylinder can be used. Your existing boiler is perfectly capable of heating a cylinder.
No idea why you are considering an electric immersion - it will cost 3-4x more than gas.

Deluge / rain head type showers require vast amounts of water to operate properly. In many cases at least 15 litres/minute, some even more than that.

 

[shambolic]

It's easy to do. Just use a zone valve on the cylinder for the other shower and kitchen water.
Use the combi for your main shower as a 400l cylinder will only give you 15-20 mins if you have a large rainwater 25l/min shower.
I have this and unvented cylinder and underfloor heating using an ideal vogue 40kw combi

 

[Steve118]

Thanks for the feedback, I thought I'd read that combi boilers couldnt be used to heat therefore the electric immersion idea. I'm clear now that an unvented cylinder is the way to go so I'll get in someone who knows what their doing to take it from here.

Cheers
Steve

 

[DP]

It's easy to do. Just use a zone valve on the cylinder for the other shower and kitchen water.
Use the combi for your main shower as a 400l cylinder will only give you 15-20 mins if you have a large rainwater 25l/min shower.
I have this and unvented cylinder and underfloor heating using an ideal vogue 40kw combi

Shambolic, I am gobsmaked. Most houses only need 12- 15hw for heating and 2kw for cylinder. I hope you have range rated that boiler.

 

[Terry lambert]

You may have had the cold main renewed but probably only upto the internal stop cock which will most likely be 15mm from this point, this might explain your flow rate

With the unvented you would extend from the 25mm in 22mm copper to your cylinder position, from there a balanced cold in 22mm to your appliances and 22mm cold feed to your cylinder.

this should give you a higher flowrate

As said I would still use the combi to feed one shower, cold feed to combi would be fed off of the 22mm balanced cold and reduced at boiler.

How easy all this is would depend on how accessible your pipework is and what was in place before your combi, as in if it was a gravity system converted, then it may be that you can connect onto the old 22mm hot and colds

 

[muggles]

Shambolic, I am gobsmaked. Most houses only need 12- 15hw for heating and 2kw for cylinder. I hope you have range rated that boiler.

2kW for the cylinder!? You're starting to sound like Tony...a modern unvented will take around 20-25kW quite happily

 

[DP]

Muggles, one starts believing the great man when one reads such stuff regularly

 

[Terry lambert]

if you were trying to heat the cylinder from say 10c. Them you may well getting 20 odd KW through the coil.

As soon as the cylinder starts to warm the less you can conduct through the coil, the boiler modulates down as the return temp rises, you won't get anything like 20KW through the coil.

How often is the cylinder at 10c though

 

[Dan Robinson]

How often is the cylinder at 10c though

At least once a day in my house.

 

[Terry lambert]

At least once a day in my house.

You wash ?

 

[Dan Robinson]

Cylinder is sized and controlled so that it discharges close to cold and reheats in a long burn.

 

[shambolic]

Shambolic, I am gobsmaked. Most houses only need 12- 15hw for heating and 2kw for cylinder. I hope you have range rated that boiler.

Eh? You can't range rate it
I have 15 rads and 12 manifold under floor (can't remember kw reqd without checking plans). Trust me it's fine and the boiler has a great turn down ratio anyway.
Photo of one room in my house