i installed (well, a corgi friend did anyway) a new boiler earlier this year. It was great at first, in the bath there was a serious fast flow of exceedingly hot water. Even when we started using the central heating at the same time it kept up ok. Then i noticed the water wasnt quite as hot as it used to be, so i started turning the CH off, and then it would be back to full heat on full flow. Now however its a little worse again, you cant have the tap on full flow or it gets a little less hot (not cold, just not hot enough for a super hot bath). So now im stuck running baths on half flow! Not a problem in itself, but soon i'll have the shower screen up and i need full flow and full heat for the shower.

The boiler is a biasi 28kw condensing combi boiler, new about 8 months ago. It feeds 5 radiators in a two bed house, and the hot tap in the bath is fed via a 2 metre length of 22mm pipe.

I have two theories which i would you input on.

Number one, i suspect my mate didnt both putting any preservatives etc into the radiators. Is it possible that limescale might be building up on the element, if there is one (im not sure exactly how these boilers work)

Number two, i believe it is fed gas via a 15mm pipe, which isnt to the regs. However, the pipe wasnt accessible to change it at the time we installed it so we left it as is. Is a potential lack of gas going to cause these symptoms? Im guessing that this is less likely as the situation shouldnt be getting worse.

Any ideas chaps?

28Kw in your house should be more than adequate, as indeed it was initially.
I have a 23.4Kw boiler in a bungalow with 3 full size double bedrooms, so my roof is masssive. I live by the sea and currently it's blowing a gale. My boiler is on it's lowest setting (ie No1) and every room is like toast and I never have a problem with hot water.
I would suggest that you get your mate back to look at it pronto.
My boiler is also fed via a 15mm gas pipe.
As yours is gas fired it will not have a heating element for limescale to build up on.

[Edited on 19/11/06 by Confused but excited.]

hmmmm sounds like the ballistic reciptifier is up the creek!

Just a note but with combi boilers, you do get slightly cooler water if the taps on full flow. You have to have it half flow to give it time to heat up if you want it really hot. Is it possible that you're just becoming more accustomed to it and therefore just noticing this?

i'm corgi registered, no way would i install a 28 kw combi on a 15mm gas supply. Unless it is a very very short run it will not be able to carry enough gas to burn to produce the 28kw out that you payed for the boiler to be capable of. When in heating mode, the boiler more than likely will not burn anything like the same amount of gas but when in hotwater mode the full 28kw will be needed. Think of an electric shower you need a big cable10mm to carry the required current, think of a powerfull engine with a big carb, no use having a piddly fuel pump or undersized fuel pipes. the reason that your hot water is getting worse, is that the incoming cold water is colder now than in the summer. I'm not familiar with your model but as an educated estimate i would say most 28kw combis should be capable of a tempreture rise of 35 degrees c at a flow rate of 12l/min. the easiest way to tell if your appliance is undergassed is to open the hot tap (usually on the bath and time how long it takes to burn a cubic foot of gas on the gas meter test dial, i can then work the actual heat input of the boiler.

Sounds like the mains water into the boiler might need throttleing down to sac you having to do it at the tap now its winter and the incomings waters colder.

I Wouldn't have thought it would scale up that bad in such a short period of time, unless you've had to keep topping it up?

Once the original water has been in a while and heated up its not so corrosive to the system anymore... but if it keeps leeking and being topped up then the changing water causes more scale and silt as more impurities have been through the system.

On the gas pipe theory: It does sound a little bit naughty if the mfr says it should be 22mm.
In reality if the boiler maintains 20mb of gas pressure at the inlet at appliance's maximum rating (which is usually when its heating the water) then the gas supply is sufficient.
When heating water the appliance is at it maximum rating i.e all the heat is going into the hot water, if you have the heating on as well it has to share this heat.

your boiler is rated to heat a set flow rate( probably about 11.5 to 12 litres per minute) by 35 degree celsius.
A. a 15mm supply will not be sufficient for a 28kw boiler to operate to its full potential.
B. a 15 mm supply will fail its commision tests because it wont provide a maximum 1 mbar drop between the meter & boiler during boiler operation.
C. as your incoming water at this time of year will be cooler than in the summer some performance decrease will be inevitable.

The bare basics are get it installed to the manufacturers instructions! Then start looking for faults when you know its set up to its proper settings. Because until then you dont stand a chance of getting it running to its full potential. Remember the manufactuer does'nt provide installation guides! They give instuctions to provide all the neccesarry conditions for the appliance to operate at the ratings it was designed for.

was it just a new boiler or a new full system?

turning the CH off makes no difference, combis always give priority to HW.

Run a hot tap and feel the CH flow pipe, if it gets hot the diverter valve is sticking, which will give you reduced heat to the DHW plate heat exchanger and so less heat at the taps.

P.S is that why you phoned me the other day?
i did phone you back but it went to 121.
I'll check it out for you if you want.

it was a new boiler into existing pipework.

I rang you the other day about the pipe work at marks house, but worked it out. Bloody plastic pipes

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Run a hot tap and feel the CH flow pipe, if it gets hot the diverter valve is sticking, which will give you reduced heat to the DHW plate heat exchanger and so less heat at the taps

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