Potterton Performa 24 Intermittant hot water

[markthegas]

Brothers boiler above has started playing up. Opens tap and boiler calls for heat. Goes thru cycle and boiler starts, then cuts in and out every 5 - 10 secs or so. As still going thru an extremely steep learining curve when it comes to fault finding you lads are invaluable!
I'm thinking the DHW heat exchanger is blocked and needs cleaning - boiler only in use since last April but a real cowboy installed it (not me) and would not be surprised if no inhibitor added (or system cleaned).
Am I barking up the wrong tree or in going in the right direction?
Any help again - much appreciated.

 

[Vic Spanner]

I'm thinking the DHW heat exchanger is blocked and needs cleaning

Answered your own question..Take it out and have a look,Very easy task

 

[markthegas]

Thanx poxi - trying to think for myself, but nice to bounce thoughts off others who are more experienced.
Is this ther sardine can at the back of the boiler?

 

[GasAid]

LMAO - Yes; it is the sardine can at the back matey.... Also known as a plate heat exchanger

 

[markthegas]

LMAO?

 

[corgiman]

laughing my arse off = LMAO

 

[ChrisR]

Don't do it Mark!! Not yet anyway. Use friends and rellies' boilers for learning on.
Get a surface thermometer to look at the temperature drop across the heat exchangers, and watch the gas valve modulating. See how much higher the temperatures go after the gas pressure has dropped.
See what the thermistors are doing - try disconnecting them for a start. How near is the high limit stat to cutting out?
Write it in a book. You'll lose the book but the act of writing it down will help you remember!

Different boilers are a bit different in the way they handle a blocked dhwhe, but once you've seen about 3, that's most of them.
Then tip the grot out of the hex, and get some descalers and see what dissolves it.
Then when it's all better, measure and record the same temperatures.

Then you'll be able to assess a boiler to see if it's getting blocked up, etc etc. Have fun!

 

[markthegas]

Cheers ChrisR,

just trying to understand the whys and wherefores. Not questioning your reply but trying to fully understand whatgoes on and why.

Looking at temp drop across HEX and gas valve modulates - I thought if temp increased this would cause gas valve to modulate down and less gas to pass thru cos temp is satisfied? Why would water temp increase after gas pressure dropped?

Still do not fully understand what thermisters do - is this to do with resistance and a certain resistance will cause gas valve to open x value?

The high limit stat will have a value in the MI to where it cuts out or is the value the same for all HL stats?

Confused and learning curve still looks like Everest!!!!

 

[baxpoti]

A thermistor is a type of resistor used to measure temperature changes, relying on the change in its resistance with changing temperature. Thermistor is a combination of the words thermal and resistor

If we assume that the relationship between resistance and temperature is linear (i.e. we make a first-order approximation), then we can say that:

Av R = k Av T
where

R = change in resistance
T = change in temperature
k = first-order temperature coefficient of resistance

Thermistors can be classified into two types depending on the sign of k.
If k is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor, Posistor.
If k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor. Used by most (all) manufacturers, not trying to teach you suck eggs!

 

[ChrisR]

I thought if temp increased this would cause gas valve to modulate down and less gas to pass thru cos temp is satisfied? Why would water temp increase after gas pressure dropped?

Time delays, and thermal masses.

The hottest bit is the main heat exchanger. The gas goes off, but it takes a while for the heat to get round to the thermistor, which therefore gets a bit hotter, and has to cool that bit plus the normal "hysteresis" (look that up) before the boiler comes on again. Then the temperature swings about.
A layer of sludge on the sensor slows down its noticing of the temperature change, and scale on the inside of the main hex slows the transfer of the heat from the metal to the water.

So instead of the boiler moduating the gas pressure gently until its stable, it keeps overshooting.


ADDED to that is the inability of the system to control itself with the LOW rate of removal of energy from the secondary heat exchanger. The boiler can modulate within a range. Outside of that it has to turn on and off.

You'll proably find gfood graphs if yo look up Damping in Wikipedia, here's a couple of quick doodles. Vertical axis is temp, horizontal is time. Top is what should happen. Bottom is inaccurate, but you get the idea

 

[markthegas]

are these DWHEX easy enough to remove and clean?

 

[corgiman]

are these DWHEX easy enough to remove and clean?

easier to replace

its a piece of urine to do

 

[Agile]

Whilst its easier to replace that means having a spare. Thats OK for the single make staff engineers but not for those working on all makes.

It also depends on your workload and charging basis. Since we do most jobs on a fixed price basis its quicker for us to clean them on site if we dont have a clean spare.

Another problem is customers agreement to pay. We seem to attract customers who want an economic repair. Even at our slightly lower fixed prices we are still too expensive for about 20% of callers, just last week for a virtually unknown model we were too expensive!

Charging by the hour to go away to get a part makes the job expensive.

Friend went to boiler last week, found water from builders scaffolding had got into boiler and spoilt gas valve, charge £256 excluding replacment valve. Now he's had another call to say builders had not moved cause of water ingress and needs another visit and a replacement PCB, that will come to another £350 !!! Total £606 !!!

Tony

 

[baxpoti]

Friend went to boiler last week, found water from builders scaffolding had got into boiler and spoilt gas valve, charge £256 excluding replacment valve. Now he's had another call to say builders had not moved cause of water ingress and needs another visit and a replacement PCB, that will come to another £350 !!! Total £606 !!!
Tony

My god makes Heateam look cheap!
Fixed price and 12 months guarantee incl all parts and labour

 

[ChrisR]

Even at our slightly lower fixed prices we are still too expensive for about 20% of callers

Apart from Pimlico Plumbers, your rate for a callout is about as high as any I know, Tony. (If it's the £84 you have quoted here)
People are ever optimistic and hope that their problem is "something simple".
If someone else will turn up for £60, they'll get the job, even if they charge another 60 to go off and get a part and another 60 to fit it.

I came across a Keston specialist charging £120 for first 2 hours, in Bayswater London, but he only gets called to boilers nobody else wants to look at!