plumbing - no water pressure in loft

[leopold]

hello.
i notice that if i run a tap in my loft floor - hot or cold - and then run a tap on another floor then the top floor water stops. in other words if i am having a shower in the loft bathroom and someone runs the sink downstairs in the kitchen then the shower water stops. it comes on again when they stop running that tap.
we recently had a combi installed and i can't say if this was happening before the combi or not. the plumber that installed the combi said it could be due to low mains pressure coming in although he tested it before installation and said it was a very good flow.
i have a house with three floors - 5 bedrooms, 2 bathrooms. the plumber installed a 28kw combi. i wondered if it isnt powerful enough but he says its more than adequate and the problem may just be something we have to put up with to have a combi.

is there any test i can run or anything else to suggest?

any advice appreciated

[gucciman]

a combi no matter how powerful should always be regarded as a 1 tap appliance, i would presume that the lower floor taps take precedence due to gravity or may be down to the way the hot water system has been piped.

saying that i have a traditional hot water system with boiler, cylinder and tank which has an unbelievable flow at the hot taps but if im running 1 tap and turn another on then i also lose most of the flow.

where is the combi in the house by the way?

28 kw is not all that a powerful boiler for a house of your size to be honest, i would have been looking at something in the mid to high 30kws personally.

you can call an independant heating engineer out to take a look at your system if you feel its really a problem. if you look in the paperwork for your boiler you should find some performance tables which shows the temperature rise and flow rate that your boiler is meant to deliver. the heating engineer can then use a weir guage to check the flow rates on your taps and a thermometer to see how high a temperature rise the boiler is achieving at that flow rate. if its not meeting the spec. then he can check the gas rate to see if the boiler is burning the correct amount of gas and if the installer has used the correct size of gas pipe .

i would be very surprised if your boiler isn't doing what its meant to, most people are dissapointed when they swop over to a combi for the first time. if you want a fast flow of hot water then the more powerful the combi the better.

hope that helps, let me know if u need further advice

[leopold]

thanks for this advice.

the boiler is in the kitchen. i suppose my concern is partly that this happens on hot or cold taps - is that down to gravity? would the combi affect the cold pressure?
i'm also concerned that i have been given too small a boiler. the plumber had originally said it would be a 33kw and then put in a 28. he said because the flow in was so good that it would be more than adequate. on the other hand he also suggested the water stopping in the loft may be down to low flow coming in to the house so i don't have a definitive answer. the flow rate out of the taps when only one tap is on seems to be good at least when the temperature is not too high. i suppose i am questioning whether if i had a 33kw boiler would the fall in pressure be less when two taps are open? and in addition for example when i turn the heat down on my shower mixer the pressure increases out of the head. would i get better hot pressure out of the head with a more powerful boiler? i had read that a combi gives better pressure than conventional shower. when the water is hot, it's no better.
i read a which report on my boiler which described it as a great choice for flats and small houses which is a concern. is it just down to the judgement of the plumber or is there a way to determine the power output you need?
apologies for this rambling reply but a bit more insight would be much appreciated.

[terambulan]

You may need another pump inserted into the central heating system to pump the water to the upper floor.

[gucciman]

i really cant make sense of your plumbers reasoning that he put a less powerful boiler in because the incoming water pressure is so good. if you have a less powerful boiler it cant handle high volumes of water passing through it so the boiler makers put in a flow restrictor to slow down the flow of water thru the boiler to a level where the boiler can still heat it up to the required temperature.

to be honest im struggling to see that a more powerful boiler would actually stop the problem of you losing water pressure upstairs when more than one tap is used. a 28 kw boiler may give a flow rate of say 11 lts a min but a 37kw boiler may only give you a flow of 15 lts a min so theres not a tremendous difference. as an experiment you could try timing how long it takes to fill a 5 lts bucket at each of your cold taps and then report back on here, may help.

i only have just over 2 years experience as a heating engineer and i dont know of any calculation where you can work out what power output of boiler you need, all i know is that if you want a fast flow of hot water you buy the most powerful boiler you can afford (bearing in mind it will cost more to run as well)

when your installer quoted for your installation did he base the price on a 33kw boiler and when he changed it to a 28kw boiler did the reduce the price?

you will often find that installers fit a boiler of there choice because the plumbers merchants they use are offering a good deal or incentive on that particular boiler.

[gucciman]

the only other reason i can think of him putting a less powerful boiler in is that if he had of put a very powerful one in then he would probably have had to run a new gas supply direct from the meter to the boiler and in 22mm or 28mm copper. this would add a lot of extra money to the price of the job so make sure he hasn't quoted you for a new gas supply and then not done it because of the less powerful boiler he's put in.

as for the mains cold losing pressure the same as the hot, i'm not really a plumber so cant say much. thinking logically though the boiler runs off the cold mains so when the boiler is being used to heat the hot water and you open a cold tap you aren't going to get as much pressure at that cold tap compared to if the boiler wasn't being used , and your also going to get less pressure at the hot tap if you've got another cold tap running dependant on which tap is being opened i.e. our old friend gravity again.

to be honest, from my experience of installers i really think you will stuggle to get any joy from yours, they usually don't want to know once they've been paid.

your only other option i know of would be to get your system converted to a mains pressure hot water system by having a hot water storeage cylinder plumbed in, water would then be heated by the combi and stored in the cylinder, if the cylinder was in the loft it would give you a brilliant flow of hot water, im no expert on these though has i haven't done any unvented hot water training yet.
i hope that makes sense, there are at least 2 other more experienced heating engineers who regulary use the site, gasman and stevegb, hopefully they will give you an answer.

[gucciman]

sorry i missed the question you asked when you said would a more powerful combi mean you lose less pressure if more than 1 hot tap was open, the answer is no, a combi will always be a 1 tap appliance. this is something you have to put up with. just tell your family not to use any taps until you've finished your shower!

[leopold]

many thanks for this long and thorough reply. i tend to agree that the loft issue is insoluble without a lot of cost and upheaval.
could i ask you to clarify a couple of things?
if a combi is only really good one tap at a time why is a more powerful boiler recommended for a bigger house or a house with 2 bathrooms?
if i have a good flow in, would a more powerful boiler (ie the extra 4 or so litres a minute) make a marked difference to the hot pressure in a high up shower? would it be approx 25% more powerful?
finally, can i use my first mains tap (the kitchen mains) to re-measure my flow rate in?
thanks again

[gucciman]

like i said im no plumber but i would say that the best measure of the incoming flow rate is going to be the tap nearest to the stopcock where the mains enters the house. you could then also measure it at a cold tap on each floor to see what pressure loss your getting.

if you have a low power boiler, you can get a situation where to get hot enough water you have to turn the flow of the tap right down, but then it takes that long to fill a bath the water in the bath goes cold before its full. a more powerful combi simply gives you a faster flow of hot water.

i guess any increase in flow rate is going to give a better shower. another option may be to rip out the combi powered shower and have a power shower with a pump installed instead, dont know a lot about them though.