I am having my house replumbed with a new CH/HW system. I was expecting that the plumber would use 22mm pipe for the hot water to the combi boiler. However he has used 15mm pipe, and most of it plastic.
Can anyone point me to some information so I can calculate the pressure losses. The 15mm pipe may be OK. I've no idea about plastic pipe. Unfortunately I have to be at work while the plumber is here so I cannot speak to him in person. If there is a real problem I need to know so that I can take a week off work to sort it out.
The house concerned is 4 bed, 2 bath with a 20ltr/min combi boiler. The boiler is in a attached garage.
That's normal: you'll find the connections to the boiler are 15mm for mains cold water in and DHW out, and 22mm for CH flow and return. (Anomalously the gas inlet is often 1/2" BSP although most of the gas pipework has to be in 22mm.)
Plastic pipe is fine (except that the connections to most boilers are still specified to be in copper for the first metre or so). What plastic loses on the slightly smaller internal diameter it probably makes up for by making smooth bends.
Even at that relatively (for a combi) high flow rate the boiler itself is likely to be the limiting factor to the flow and delivery pressure achievable.
If you were having an unvented cylinder or heat store fitted, and especially if your incoming mains pressure were on the low side or your pipe runs had to be _very_ long, then 22mm pipework would be indicated.
Thanks very much for taking the time to reply to my post.
The connections to the boiler are 15mm but the installation instructions states that "A 22mm hot water distribution pipe to the first branch is recommended ...".
The boiler is quite a long way from the rising main. I estimate there will be nearly 20m with 2 right angle joints and 2 tees.
Why I am concerned is that, when I moved in, the mains flow rate was quite poor (about 16 litres/min). When we removed some kitchen cabinets we discovered that somebody had cut the 22mm supply pipe just after the stop c*ck and routed a 15mm pipe under the sink, put another stop tap in on the 15mm pipe, and then rejoined to the 22mm pipe further up. Thereby introducing about 2m of 15mm pipe.
When the 22mm pipe was reinstated with a reducing tee to the sink the mains cold flow doubled. Therefore I am now quite skeptical that 15mm pipe is adequate. I have tried to get the mains pressure tested, but all I got from the experts was "It's fine".
Anomalously the gas inlet is often 1/2" BSP although most of the gas
And sometime in 28mm. I believe the calulation of the pipe size is down to the number of bends and length of the gas pipe along with the power rating of the gas appliance.
There is a flow restrictor in the combi which reduces flow far mor than any length of 15mm pipe.
I find it quite surprising that you noticed a difference in cold wate flow by removal of a short length of 15mm pipe.
If you have problems then maybe your local water mains pressure i down. All that the water bord have to do is supply water to your tap there is no requirement for you to get a good shower.
The solution for poor water pressure is to store sufficient wate inside your house and pump it with a 3 bar heavy duty pump either t your combi or to an unvented cylinder or through a thermal store
The problem with mains pressure systems: combi, unvented cylinder, thermal store, is that one appliance may rob another of pressure. The most critical is the shower. You don't want fluctuations on this for scalding and discomfort reasons. Companies like Gledhill recommend a dedicated supply from the stop c*ck to the appliance (combi) only. Also from the mains stop c*ck they recommended a dedicated cold supply to the shower.
A 15mm pipe may be adequate, but it is the balancing of the mains water that is important.
I would recommend:
A full bore stop c*ck.
2.. A dedicated 22mm supply to the combi from the stop c*ck.
Tee off at the stoop c*
ck and run all cold outlets from this, except the shower (s).
Take the cold for the shower off the combi cold feed just before the combi. Any pressure fluctuations on this line with be pretty equal on cold and hot.
Insert in-line restrictors on some appliances like the dishwasher, w/machine etc. They don't require full flow to operate. Also on basin taps, etc. In short ensure the hot for the combi takes all the supply and the shower is least effected.
If necessary a pressure erqualising valve to the shower or one built-in to the mixer. (about £20 in B&Q)
Having one pipe from the stop c*ck snaking around the house supplying all appliances and the combi merely being one of them is foolish, unless you have 10 bar pressure and 1" supply, or the likes of.
Plastic pipe restricts flow and pressure. The inserts reduce a 15mm pipe bore to 12mm.
I would say that really to get best flow you should use the largest bore pipe almst everywhere, just taking it down to small bore for the actual taps and room work itself.
Especially long runs.
Size does matter, and bigger is better, and the only downside is expense and the actual physcal fitment.
and the other downsides are .... a) there is a LOT of cold water in the hot tap supply pipe (about double ?) that has to be discarded before hot water reaches the tap - time delay - I replaced our 22mm pipe from the hot tank to the kitchen / utility room sinks (approx 15 metres length) with 15 mm and the time delay was about halved (Gravity) flow rate is just fine...
b) ....and the heat wasted (small but nevertheless present) in the non-heating months.
As IMM is demonstrating the cure for, the fact you measure 3bar stati presssure is meaningless, you need to know the capacity of the supply You would measure 3 bar through a pinhole.
I also agree with the last guy. I would maximise pipe size to the hea source/cylinder, and then use just sufficient to obtain the flow yo require so that you are not waiting too long for hot water and wastin heat and water
Yes, but a lot of the pipework only carries cold water, so I don't care about any heat losses there :-) Also for a sink getting the hot water quickly is more important than the flow rate. For a bath the opposite applies.
Based on my experience and what you have said I would say "very likely".
Go with the 15mm DHW pipework.
The 22 to the first branch is probably the manufacturers covering their corporate behinds. It will eliminate slightly the effect of two draw off points being used together (but in fact the boiler is the limiting factor
The cost of water in modern well insulated homes with condensing boilers is now more than space heating. Wasting a little energy in a secondary circulation loop to give:
No waiting for hot water at the taps
saving 3 litres of water each time you wash your hands.
May be financial beneficial.
All of a secondary circulation draw-off loop has to be lagged.
A perfect illustration of dIMMs lack of joined up thinking. For years he has been claiming that the payback time for replacing any old boiler with an expensive condensing boiler is only a few years (without any qualification by type of house or, indeed type of old boiler). But if his statement above is true then there is no way any saving in running costs of said expensive new boiler would do as much as cover the interest lost on the capital used to fund the new installation.
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