Biasi 24S boiler...only tepid D H W flow?

[Hunkamunka]

Hi all ..I would like to start by saying that I am an industrial AC engineer so have skills but they are not related to central heating.Just over two years ago I had a new boiler fitted..a Biasi Riva compact 24S....I have been very pleased with it until a few weeks ago when the upstairs shower and bathroom taps suddenly dropped in temperature to lukewarm..it starts hot for a couple of seconds and then the temperature falls away.
I have had the installer(corgi registered) back within the last few days to carry out an annual service and he says that he can find nothing wrong with the boiler functions??..but that there is a heat loss of about 10 degrees between tha kitchen sink tap (right next to the boiler and also much cooler than previous) and the bathroom taps and shower etc (upstairs)..he has said that maybe the DHW piping in the house(original and quite a long run)..is too long and giving too much heat loss between floors especially now it is winter and entering water is down in temperature..the original piping is still lagged with hessian tube and not foam...however I have pointed out to him that the boiler functioned with piping hot water all over last winter with no problems...DHW stat now set to maximum by me to try to help.
I note that when in the shower if the cold tap on the bathroom wash basin is left running that the water gets a little hotter but nowhere up to normal...also if the basin tap is run on its own but throttled back to about half flow the water is at a higher temperature but not as normal either....there does not seem to be any loss of normal water pressure anywhere in the house.
I have had a scan of the paperwork left on installation and can see that the DHW circuit is dependant on a threeway valve...I am suspicious of this...can anyone out there help me with some experienced information...I do not intend to carry out any repairs to this boiler myself I just want to point my installer in the right direction...he seems very competent but everyone has off days so I am giving him the benefit of the doubt at the moment....by the way ...the gauges for pressure and temperature read 1.1 bar and 60 to 65 degrees respectively.

 

[Agile]

Your boiler is fine!

You have cold water feeding back into the hot taps!

You can prove my diagnosis to yourself by turning the cold into your boiler off by turning the blue tap clockwise 90 degrees.

Then NO water should come out of any hot tap anywhere.

The most common cause of this problem is a faulty shower or joystick tap and you can often hear the crossflow using a stethescope or listening stick on each tap in turn.

You can try dismantling the faulty component and will hopefully find dirt on some kind of valve seating.

Fitting non return valves in the hot feed to any shower or mixer tap will eliminate this problem.

Your CORGI installer may "seem very competent" but obviously he is not competent enough!

Tony

 

[Hunkamunka]

Thanks Tony...I really appreciate your reply....I have tried what you suggested and it appears to stop all flow at any of the hot taps...still at least its one thing eliminated. Just prior to reading your posted reply I carried out a test of my own...
The hot water leaves the boiler and then goes into a 'T'...one branch of the 'T' goes downwards and travels for about five feet and then comes out of the kitchen sink tap......the other branch of the 'T' goes upwards and disappears at ceiling level to feed the upstairs bathroom....using the tools of my own trade I fitted a thermocouple around the down branch of the pipe and found the temperature to read 50 degrees.....when measuring the water actually coming out of the tap I got 52 degrees which is fair enough allowing that the first measurement was taken "expipe" with just a bit of tape holding the thermocouple on.(Comark themometer which I use at work all the time and it is calibrated regularly).
Next I taped the same thermocouple to the rising pipe just before it disappeared into the kitchen ceiling and when the bathroom sink hot tap was run I only managed to record 38 degrees....when I checked the water actually coming out of the tap it was at 40 degrees which tied up differential wise with my first measurement???....but 12 degrees lower.
At first I would start to think that I had a blockage on the "up" run but the full bore pressure seems just as good upstairs as it is downstairs and anyway if the flow was less the water should be hotter I guess????.....have you any more suggestions that I can try please....

regards....John.
PS DHW stat still set to maximum.....boiler stat reading about 60'ish.

 

[Gasguru]

Have you measured the flowrate at the kitchen sink (ie time taken to fill a saucepan) and then set the bathroom tap to the same flowrate and measured the temperature?

 

[Hunkamunka]

Thanks Gasguru...now why didnt I think of that test??...presenile dimentia I guess...only a few years from retirement now.
Anyway...when I set the taps to fill a large saucepan with water at eighteen seconds each tap the water leaving each tap is the same....52 degrees....so please tell me where to go from here??

regards.... John.

 

[Hunkamunka]

Also why has everything worked A1 for over two years at full bore with the thermostat set lower on the boiler and with hotter water????....could the mains pressure have changed??....if so why only effect on the upstairs????

 

[Hunkamunka]

Also...further info is that I had a shower a short time ago and turned both the hot AND cold taps on full in the bathroom basin....and I enjoyed a piping hot shower for the first time in weeks?????

 

[macstevie]

i ll say from the outset i am not a boiler installer nor a boiler service engineer.That said as a plumber i have had a fair amount of experience with combi systems.Without a doubt your system has developed a problem,it ran fine last year,now its problematic this year.Being that your boiler is a combi boiler,heating water in a fine mesh heat exchanger,what is the water quality like in your area?Is it hard or soft water?If the water is fairly hard(water that has alot of disolved salts in it)has a water conditioner been fitted on boiler instalation to protect the heat exchanger?Its a fairly common experience to see combi boilers in my area,which is a hard water area,develope reduced hot water flow rate through the boiler as the heat exchanger becomes scaled up in a similar way as kettles scale up.Unchecked it can lead to an eventual failure of the heat exchanger.Another possibility is there is a thermostatic failure on the boiler,but from what you have described,the overall loss of heat seems to indicate heat exchanger performance reduction.Temporarilly until the fault is diagnosed if you have ballofix valve on hot water
upstairs you can close this a little to reduce flow through the hot tap which should increase the temp so at least you can get some consistency.Try the same with the cold supply to the shower if that has a valve on it to reduce the cold flow through the shower as again this may help increase temp.these are 'quarter turn ' valves so adjust them only slightly as a 90 degree turn is on/off.

 

[Gasguru]

It is impossible to set the same flowrate at different taps just by looking at them - timing a flowrate is the only method.

Customers are often amazed at how little they must open bath taps to get the required temperature. Traditional bath mixers with 3/4 valves and a large spout only need to be opened a few degrees of rotation.

A flow restictor would help matters.

Turning on cold taps will reduce the flow of water through the boiler and the hot water temperature will increase.

Street pressure, friction of the incomming main, pipework fittings, boiler flow restrictor, strainers in the taps/shower, height of taps, condition of the plate heat exchanger etc will all have an influence on the hot water delivery performance. Some of these must have changed since last year.

For a start set the flow rate to 9.5 litres a minute. Set the boiler DHW temperature control to maximum and you should achieve a 35 degree rise in temperature. Any less and there's a problem with the boiler.

 

[Hunkamunka]

Thanks Macstevie...this is a "hard" water area and I am used to plate heat exchangers albeit a little larger on some small water chillers...however the loss of heat just appeared so sudden...almost overnight???
In answer to your question there is no softener fitted..well a B&Q wrap around coil type on the incomer but not sure if that is much good??
However it is starting to point as you say to the exchanger...anyone have any idea how much it will cost for me to have it changed??

 

[ChrisR]

Can't explain the loss in the pipe, unless someone snuck in and put a radiator on it....

But you can check that boiler is doing its best. Turning CW tap on in the bathrom could have reduced the HW flow enough to make the HW hotter...

24kW combi should give 35º rise at about 10 litres/min , or pro rata. See what you're getting.
If it isn't up to spec you could check the gas rate. With HW tap(s) on full you should be using 1 cubic foot of gas in 40 seconds, or so.

Also you could check the diverter is doing its thing. With system cold, turn HW on, & the heating FLOW pipe shouldn't get any hotter, see if it does.
SLudge in the heat exchangr(s) can lose you performance, so measure the temperature drops across them.

Then there's sensor(s)..
and the pump..

 

[Hunkamunka]

Thanks again Gasguru...I am starting to think that you are right when you mention the heat exchanger....for the time being I will try to restrict flow a little to increase temperature...however that is only curing the symptoms and not the cause....I see a bill on the horizon....thanks again for your replies....John.

 

[Hunkamunka]

ChrisR...thanks for your reply as well....it is starting to look a little as if it will be a process of elimination....guess I just need to work out where to start....I dont suppose anyone can tell me the expected temperature rise across the DHW exchanger compared to the drop across the CHW exchanger..obviously under best possible conditions...it will give me something to go on???

 

[Hunkamunka]

Hi all....once again thanks to all who replied to my original posting and set me on the right road.....after sitting down and thinking about all your advice I checked the supply to the kitchen tap and found two small bore connections to a mixer tap....all the connections in the bathroom are full 13mm....hence the hotter water on the kitchen tap...it is already restricted.....tried the shower just now by only turning it half on....good water temperature....so it looks like the DHW exchanger is in need of a clean...or replacement.
Last question...we clean our own heat exchangers at work (refrigeration based)...do I need to be corgi registered to remove the exchanger and clean it myself??????
I thought I would descale it and then try a run at full throttle so to speak....if it works and if it is a real pain to get out and in I will have to fit a softener...otherwise it will just have to have a clean every couple of years....thanks again to all....regards....John

 

[Gasguru]

John you have already told us you have a calibrated thermometer.

Measure the cold water temperature, now set the hot water flow rate to 9.5 litres a minute and measure the hot temperature.

You should see a rise of at least 35 centigrade.

There is no need to rip the boiler apart if you can achieve this temperature increase at that particular flowrate.