I've provided a lovely new cold feed going exclusively to a new combi. A quick question.

[Mr.B]

I'm making plans and preparations for a new combi to replace a heat only vented system. Likely a 32kw Vogue with plenty pressure, flow and gas. As we speak I'm not committed and have made no promises and haven't invested much money into the venture. Lots of head scratching and indecision though for all the usual reasons.

An installer has suggested I shouldn't worry about leaks due to pressurising the 28 year old 10mm / 15mm /22mm copper small bore gravity system (Ideal Classic fitted in 1996) so I shall hope for the best in that regard.

Several years ago one of the contributors here (can't remember who) suggested that, if it's possible, it would be advisable to give a combi its own exclusive cold feed direct from the point where the blue 25mm MDPE enters the house and run it in 22mm using a full bore Pegler lever ball valve to isolate it. He said it would minimise the effect of other outlets interfering with the hot water outlet when showering, for example.

In short ... I've done exactly that.

The likely installer has been once and, to be honest, wasn't that bothered either way about what I just outlined but, obviously, he'd run with it.

It happens that the mains fed 15mm supply from the street (with a drop tight Caleffi 535 series pressure reducing valve in the line) to all the house sink and toilet cold water outlets would also feed the two showers ... therefore it strikes me that whilst it's great giving the combi its own dedicated feed (which, incidentally, would also have its own pressure reducing valve in line too) there's still scope for the opening of a second outlet whilst showering to drop the shower flow a bit . So, was it a bit of duff advice that I followed to the letter?

It wouldn't be especially difficult to extend that exclusive, dedicated cold supply that I described to one (or both) shower cold inlets so that the shower gets its hot AND cold feeds from my new supply.

I haven't lived with a new Vogue 32kw combi that has good pressure, flow and gas supplying it so have no personal experience to call upon hence making the enquiry in the hope someone may have done something similar in the past.

Finally, I'm not expecting to be able to run two full strength showers at the same time - only one at a time for 99.9% of the time but the boiler advertises 13 litres per minute and my current gravity shower gives a 5 litres per minute shower which we've been happy with.

Ta.

 

[Mr.B]

I wish I hadn't asked....

 

[terryplumb]

You have wasted your time ,there will be no appreciable difference,every tap / shower/ other outlets both hot and cold are all supplied by the 15mm mains in.

 

[Mr.B]

Received, thank you.

Yes, I accept that they come from the same mains in (although i was under the impression that 25mm MDPE was a bit more than 15mm) ... but, yes, there's only the one main in.

I did listen to the contributor who offered the suggestion a long time back and took it literally thinking that as the feed that I supplied to the proposed combi came away immediately after the MDPE in 22mm and was solely for the boiler's use it might be less subject to being affected by opening other taps. If this was pointless then it was pointless. We live and learn.

Obviously the combi does have to be fed from somewhere and if I hadn't done what I did it would have necessitated running it from elsewhere ... but it would be in 15mm, taken from the cold water rising main running through the house (my supply in 22mm is about 6 metres in length, so even if it is pointless it's not the end of the world.

My question was, however, should I take the cold to the showers from my 'purpose made run' or take it from the 15mm that supplies the rest of the house?

Ta.

 

[terryplumb]

Your original post is confusing as you talk about 25mm MDPE ,and later in the post a mains fed supply of 15mm from the street.
Whatever it is ,you don't seem to have a dedicated feed to the boiler ,it's simply one of a number of branches from the incoming supply isn't it ?

 

[Johntheo5]

Static pressure + dynamic pressure & flow will tell a lot, I've seen a figure of 15LPM at 2.5M dynamic as the minimum required.

 

[Mr.B]

I agree I made that difficult to comprehend. I was saying (not very clearly) that 25mm MDPE comes in from the street under the kitchen sink - and the house builder immediately reduced to 15mm for the rest of the house ... but I have Tee'd off at the blue MDPE and directed my new 22mm feed to the combi in one direction whilst the 15mm feed to the rest of the house carries on as it always did in the other direction. I have very recently replaced the 18 year old Caleffi (non drop tight)533 series pressure reducing valve for a new (drop tight) Caleffi 535 series PRV. When I moved in here in 2005 the incoming was just under 7 bar hence the PRV.

So, to confirm your assertion ... no it's not a dedicated feed, it's a branch taken immediately after the 25mm blue plastic.

 

[Mr.B]

Static pressure + dynamic pressure & flow will tell a lot, I've seen a figure of 15LPM at 2.5M dynamic as the minimum required.

I have 4 bar from the street and 36 - 38 ish litres per minute with no other taps open. The 4 bar drops by approx 0.25 bar if I fully open an adjacent tap.

 

[Agile]

Taking a dedicated supply directly to the combi is a very good idea for you.

Having such a high supply pressure if there was any noticeable difference in flow rates at the showers when other house taps are opened then I would suggest a PRV just before the boiler inlet just set at about the lowest pressure you ever get at that point.

 

[Mr.B]

Taking a dedicated supply directly to the combi is a very good idea for you.

Having such a high supply pressure if there was any noticeable difference in flow rates at the showers when other house taps are opened then I would suggest a PRV just before the boiler inlet just set at about the lowest pressure you ever get at that point.

There's some difference of opinion between your (Agile) opinion and that of terryplumb, who said I wasted my time. Obviously I went to the trouble of providing the supply to the boiler with the best of intentions thinking (hoping) it would serve the purpose of reducing any interference of shower pressure and/or flow whilst the other outlet was in use. Either way I shall definitely use the supply having gone to the trouble of providing it.

I made my original post on the basis that two heads are better than one and I was trying to see whether the cold supply/s to the shower/s should be taken from the aforementioned (controversial) 22mm supply or just taken from the 15mm rising cold water main that feeds all the other outlets in the house. The routing of either choice involves similar work so there's no great physical advantage either way ... neither route is especially difficult.

I have planned to provide a, readily accessible, Reliance 312 22mm drop tight pressure reducing valve on the 22mm supply ... along with a full bore Pegler lever ball valve in order to be able to isolate the supply to the boiler should the need arise.

I'm grateful for the advice received. Thank you.

 

[Agile]

There is no difference of opinion with Terry. He was only saying it was not required.

However as you have already done it then I accept that is your current situation.

It would be slightly better if you connected the shower cold input to just before the boiler.

 

[Mr.B]

All received, thank you very much. But Terry was saying it's a waste of my time - and you're saying it's a very good idea for me.

I'd say there was a difference of opinion .... but we won't dwell on the matter.

Thank you again.

 

[terryplumb]

Consider this...the cold feed into the combi is 15mm ,the domestic hot water outlet from the boiler is 15mm. The water pressure to the boiler is the same if the pipe is 15 or 22 mm. The flow rate that you quoted of 13 litres per minute of hot water from the boiler can be comfortably supplied by 15mm pipework,it will not be any greater because you used 22mm. As you have installed that ,fine, all well and good . But you haven't gained any advantage ,in my opinion.

 

[Mr.B]

^^^^ Thanks for taking the trouble to come back to this.

I can obviously see what you're saying and why you're saying it. I can't and won't disagree because I don't have any arguments to offer ... but if I were were forced, on pain of death, to find some sort of logic for doing as I did I guess I'd say maybe the positioning of the 22mm branch being right at the junction of the incoming MDPE plus the increased bore almost right up to the boiler might help prevent other outlets temporarily reducing the feed to the boiler during the course of a shower.... but I may well be talking nonsense. It wouldn't be the first time.

When Tony says ..... Taking a dedicated supply directly to the combi is a very good idea for you. I can only listen to him and think presumably he has some reasoning for saying it. Perhaps he'll return and say. If I could remember who it was that made the suggestion here in the first place I'd willingly say, but it was good while back and, whilst I can just about remember what I had for breakfast, I definitely cant remember who offered the advice. But, like I say, I appreciate your thoughts.

 

[terryplumb]

Your shower will get it's hot water from the boiler ,and a little cold will be added to achieve your desired showering temperature.
Two showers running at the same time is really asking too much from a combi ,but you seem to realise that already.
All the best.