NEW Boiler / NEW Pipework - Low Hot Water pressure !!!

[louis]

Evening all!

I've made an attempt at using the search function but have only found posts about "low hot water pressure" where only the boiler has been changed in the house or new pipework has been laid down & still using the old boiler.


Whereas my circumstances are as follows:

* 4 Bed semi-detached, 2 Bathrooms
* old pipes, old boiler gutted out fully
* New Worcester 42CDI installed
* New copper pipes laid out throughout property
* mains water pipe has also been upgraded from the street to a blue 25mm PVC pipe so shouldnt be any problems there
* typical combi boiler setup was used for the piping (please refer to link below)


[DLMURL]http://www.sosplumbingandheating.co.uk/assets/images/combi.gif[/DLMURL]


Cold water pressure off the mains directly is very very good at all outlets throughout the property.

Hot water pressure is OK if one bathroom is used at a time, however if the other bathroom is in use as well, it drops for both bathrooms.

The Worcester 42CDI boiler supplies 17.1 litres/minute which should be enough for 2 bathrooms running at the same time.

My installer is at a loss as to why this is happening, hence the reason why I am here today hoping for someone to shed some light on the matter.


Thank you in advance,

Louis

 

[stevejordan]

is your hotwater pipe 22mm / 28mm or is it just 15mm ????

 

[Blackcatgas]

17 litre per minute hot water split between 2 showers, then the cold water from the mains to the 2 showers, so in effect 4 outlets off the mains, then somone goes and flushes the toilet etc, take the temp of the cold water, then take the temp of the water coming out of the tap run a 20 litre bucket under the hot tap for a minute, if it raises the water temp by 35 degrees and 17 litres a minute then your boiler is doing what it says on the tin.

 

[Centralheatking]

In order for all on here to get the right info from you :-

Has anybody actually tested the pressure at various points on the system and given you values ?
ie at the main, at the boiler input & exit for the hot and at the points of use.

It is important to distiguish between pressure and volume esp using a combi boiler. It is easy
to look at the flow and think pressure and volume 'are ok' - I will check my nominal flow figures
for appliances and see if anything is amiss.

All the basic installation principles seem to have been met

centralheatking

 

[Blackcatgas]

In order for all on here to get the right info from you :-

Has anybody actually tested the pressure at various points on the system and given you values ?
ie at the main, at the boiler input & exit for the hot and at the points of use.

It is important to distiguish between pressure and volume esp using a combi boiler. It is easy
to look at the flow and think pressure and volume 'are ok' - I will check my nominal flow figures
for appliances and see if anything is amiss.

All the basic installation principles seem to have been met

centralheatking

I was just trying to explain gently in laymans terms that theres probably nothing wrong with the install and that his expectations are probably too high.

 

[Billy Bob Bob]

no combi can feed 2 outlets at once thats asking way to much

 

[Centralheatking]

As above - the demands on the water supply and the boiler are at the limits of the boilers capacity to cope with the hot water side and when additional cold water demands are made the supply cannot meet demands. The boiler is most likely ample for the c/h but not suitable regarding hot water for the size of house and lifestyle requirements.

Wait until the summertime when pressure and volume drop due to area demand you might be lucky
to get one hot tap going properly if you live in some areas.

centralheatking

 

[ecowarm]

Combi boiler??????? New water main ........... should have been a system boiler and unvented cylinder

 

[stevejordan]

Combi boiler??????? New water main ........... should have been a system boiler and unvented cylinder

everytime unvented hotwater cylinder , never fails

 

[Avatar]

All this is true plus a 40kw boiler is huge for domestic property - have you got about 40 rads or something??????

 

[Toddyplumb]

Come on guys , we all know that some people just cant afford the unvented system and a Combi is their choice. It does not matter how many radiators you have the 40kw has obviously been chosen for its hot water flow rate .
Get your pressures and flows checked and then if needed fit flow restrictors to give an acceptabe performance.

 

[ecowarm]

Come on guys , we all know that some people just cant afford the unvented system and a Combi is their choice. It does not matter how many radiators you have the 40kw has obviously been chosen for its hot water flow rate .
Get your pressures and flows checked and then if needed fit flow restrictors to give an acceptabe performance.

W/B 42CDI is not the choice of somebody on a tight budget! A correctly sized system boiler and Unvented cylinder would have cost not a lot more including installation and would have no problems with flow and also be far more energy efficient. The extra outly would be recovered with lower gas bills🙂

 

[Toddyplumb]

What with system boilers the price they are !! Plus cylinder +zone valves + wiring etc
Dont get the energy efficient bit

 

[moony]

to be honest a 2 bathroom - 4 bedder - a big combi is on border line of coping - border yes but should cope none the less. i would be checking the flow rate at various points as someone mentioned earlier - its not going to give an amazing output with 2 bathrooms going at same time but it should be acceptable none the less - an unvented system sometimes only gives off as little as 20 litres a minute.

I think if youve got the both bathrooms going in morning and then at night your main worry will be heating the house - as your combi will be concentrating on the hot water. but still none the less you should find it acceptable as its a good boiler and its a big bugger.

 

[Centralheatking]

Avatar Its a common mistake that some installers will go for the highest hot water output combi
to avoid a system or conventional system - this usually results in big boilers and a poorly performing hot water outcome as above.

centralheatking

 

[louis]

Thank you all for the prompt replies.

I will try and clarify a few more details:

* The hot water is on 15mm pipe. It leaves the boiler on 15mm like it says in manual and continues on 15mm pipe to outlets. (Before it left on 15mm but continued on 22mm to outlets - our first port of call before I actually came to the website for answers, but changing did not improve the situation).


* Yes I do realise that a 'system boiler + cylinder' would have been the ideal option, unfortunately having reached the end of the build the already overstretched budget did not allow for the extra cost to be incurred (i.e. cylinder cost, valves, increased installation fee charged by the plumber etc.)


* And most importantly I took your advice and have measured the hot water flow rate at all taps (both bathrooms and kitchen), and instead of the quoted 17.1 litres/min from the boiler specifications I get 13.5 litres/min.


What is your opinion on this matter? What could cause a loss of 3.6 litres/min?


Thank you in advance,


Louis

 

[ecowarm]

Its a flow of 17.1l/m with a temp rise of 35c fine if the incomming main is 15c but if its 3c you have to slow the flow to get a usable temp at the taps

 

[Toddyplumb]

As eco says it is at 35 temp rise !. Fit some flow restrictors that is the only option for me .
Im sure Worcester once tried one with a guaranteed temp output but the flow shut down to a trickle in winter

 

[Blackcatgas]

Its a flow of 17.1l/m with a temp rise of 35c fine if the incomming main is 15c but if its 3c you have to slow the flow to get a usable temp at the taps

She/he doesnt seem to be getting 17l/m regardless of temp rise.? How many l/m are you getting from the cold tap in the kitchen?. how many l/m are you getting at the cold/hot ta of the bath?.

 

[PuraTherm]

I agree. Sounds like the installer just threw the biggest thing he could find on the wall. Unvented with a 24i or max 30 CDi system boiler. Flow rates are affected by pressure. Should check the pressure is good enough for demands.

 

[Avatar]

In truth i have fitted boilers in properties like this - and would go for a 30kw.

Having said that, have only done that after checking the usage in the second bathroom. I would not recommend a combi for 2 bathrooms, if both were in heavy use every day (ie being used at the same time).

I probably would reccommend something else if the customer was using 2 showers at the same time . . .

If you check however, it is fairly rare that people with 2 bedroom properties are going to be using all outlets at the sametime.

Having said this, there is definately something suspect about this installation - check all variables including mains pressure. If this is too high, your boiler will struggle to compensate!

 

[louis]

Having said this, there is definately something suspect about this installation - check all variables including mains pressure. If this is too high, your boiler will struggle to compensate!

Can someone explain this further?! I dont quite understand how too high a pressure can cause the boiler to struggle.


I have also measured the hot water pressure at the tap and it is 2.7 - 2.8 bar.

 

[Toddyplumb]

Too high a pressure can give problems because if you open the tap too far the cold water goes through the heat exchanger that fast it does not have enough contact time to heat up properly.
What you need to set it up is a flow cup and thermometer
Remember on a Combi 10mm is fine for basins

 

[Avatar]

2.7 bar on the hot is a bit bleeding high - an optimium input pressure is bettween 2-3 bar on the mains.
You might be on a commerical water supply (which can be 5 bar and combis will really struggle with)

You can restrict the input pressure somewhat by closing the mains stopcock - although a pressure reducing valve is the way to go in the long term.

Also as a sign that this might be the problem, open your hot taps half way only - is the water hotter like this, or when it is open fully?

If the answer is yes (hotter when 1/2 open) - get someone in to test and properly limit the incoming mains pressure.

Hopefully this will solve your problem (have seen this fool even time served plumber occasionally!)

Good luck - and let us know the result!

 

[miketheplumber]

From memory and thats not what it used to be the 30,37 and 42 CDI are all only 30KW output to heating the higher rating is to get a better flow rate on hot water even so when you have a combi on multiple bathrooms your always going to get poor performance with more than one tap open
Mike