New Combi boiler performance - when using hot and cold

[macadonald]

Hi Guys,

Just had a new combi installed last week in the loft. previously had an old back burner and tanks etc. Central heating is great, hot water seems fine, but when switching on the cold when the hot is running, the hot turns into a trickle. Clearly related to water pressure?

I new that you can't run 2 hots with a combi and keep your pressure, but surely a hot and a cold should work? What's goign to happen when I instal the new bathroom and mixer taps?

Sorry, a bit unsure of the details of a combi, but my neighbour doesn't get the same results. Will ask the plumber tomorrow.

On a seperate note, is it possible to run 2 seperate supplies off of the street mains in any way and still have them metred. As I'm completely gutting and plan on replacing the mains pipe anyway, I just wondered. It would surely mean that you could run one cold for the hot water supply and one for the cold, without ever comprimising the pressure.

Cheers guys.

 

[D_Hailsham]

Just had a new combi installed last week in the loft. previously had an old back burner and tanks etc. Central heating is great, hot water seems fine, but when switching on the cold when the hot is running, the hot turns into a trickle. Clearly related to water pressure?

I new that you can't run 2 hots with a combi and keep your pressure, but surely a hot and a cold should work? What's going to happen when I install the new bathroom and mixer taps?

Did the installer not explain to you that all water outlets are supplied from the one incoming main? So when you turn a second outlet on it will inevitably reduce the flow from the first.

Did the installer measure the flow rate and pressure?

 

[Ahab36]

Hi Guys,

Just had a new combi installed last week in the loft. previously had an old back burner and tanks etc. Central heating is great, hot water seems fine, but when switching on the cold when the hot is running, the hot turns into a trickle. Clearly related to water pressure?

I new that you can't run 2 hots with a combi and keep your pressure, but surely a hot and a cold should work? What's goign to happen when I instal the new bathroom and mixer taps?

Sorry, a bit unsure of the details of a combi, but my neighbour doesn't get the same results. Will ask the plumber tomorrow.

On a seperate note, is it possible to run 2 seperate supplies off of the street mains in any way and still have them metred. As I'm completely gutting and plan on replacing the mains pipe anyway, I just wondered. It would surely mean that you could run one cold for the hot water supply and one for the cold, without ever comprimising the pressure.

Cheers guys.

don't know if this is going to be of help to you? as you will obviously get some advise from your plumber tomorrow. but, you have already answered most of you problem in your question. water mains?? if you plan on changing your main does that mean you have a lead main at the moment and if so why didnt you have this changed before your combi was fitted. did your plumber check mains pressure before installation or not? the incoming mains is directly connected to the boiler so wot ever cold water connections you have connected to your system,ie toilet bath basin. these will reduce the out going pressure on the hot water when used at the same time? so the right size incoming watermain is very important. so is having fitted a quality A rated boiler, if you have??? then wot Kilowatt out put is it? the higher level the greater liters per min supply of hot water you can get? quite often people choose to go with a boiler around the 24kw because of price, when a 30kw boiler or higher would have been better to deal with your problem. location of the boiler is also a key factor here...you've said you have placed yours in the loft but how close to the supplies is it?? the further away from the taps/shower the more likely the lose of pressure per meter?? also if your plumber hasn't
removed all the 22mm pipe work running for the hot water [presuming you had a cylinder before hand] this to will effect the pressure??

I've never heard of two water mains suppling a property but this could be an option [very expensive way thou] but you would have to get permmision from your local waterboard as they normally only supply one shut off tap per property?? i personnally don't think you would gain much from this, but sticking in a 25mm or 32mm watermain to your property in the blue alkethene [not sure on spelling] pipe you can get from most plumbers merchants would help and most water suppliers or plumbers could tell you wot incoming pressure you've got before you decide on wot route you go down......

hope this was of some use to you....

 

[macadonald]

Thanks for the advice, you've pretty much confirmed what I thought. BTW, I have a 28kw Vailant pro. At first I was talked into going with a 24kw, but glad I increased this.

Out of interest, you siad "also if your plumber hasn't
removed all the 22mm pipe work running for the hot water [presuming you had a cylinder before hand] this to will effect the pressure"?

The plumber has not removed all of it, and in fact I have had a lot more 22mm pipe installed to connect to the new taps? Is there a problem with 22mm pipework? I was not aware of 22mm bein ga problem and thought it was the norm.

 

[Agile]

I am not at all impressed by your installer.

Indeed it makes me wonder if he is even gas registered!

22mm HW pipework gives a long delay before it gets hot at the tap and wastes water.

Tony

 

[macadonald]

Thanks Tony,

You've made me worry now. Is it just the delay that will be a problem with 22mm HW pipe or will it also make a difference to the original pressure loss I talked about in my first question when using hot and cold together?

Would you and all other plumbers always run 15mm HW piping from the combi to all taps? Is that a standard norm?

Thank you very much. I haven't paid in full, so want to get this sorted asap. My concern is whether this is a must do or just a best practice?

 

[macadonald]

In addition to the above, assuming I replaced the mains connection outside and hence increased the flow rate, will 15mm HW pipe still be preferable over 22mm?

 

[Ahab36]

In addition to the above, assuming I replaced the mains connection outside and hence increased the flow rate, will 15mm HW pipe still be preferable over 22mm?

Hi, regarding your question about the 22mm pipe the answer is yes?
all hot water supplies should be in 15mm unless stated by the boiler manufactures as some combi/store units can supply hot water using 22mm pipe. you could increase the cold incoming main to 22mm around the house as this well help with the demand problem..but unless you have a good enough pressure coming into the property then this is all in vain.

have you had your cold water mains tested for wot pressure you are getting yet???