Potterton Boiler Low DHW Flow

[Daviddf]

I have got a Potterton Puma 80 permanent pilot light boiler, its about 7 to 8 years old. For the past couple of weeks i have noticed that the flow out of the hot water taps has been very low, so low in fact that the boiler stops firing whilst the hot tap is open, then after about 10 seconds or so the boiler fires up again. I have measured the flow rate out of the taps and it is 4 litres per min, i think this is too low the manual says that the DHW switch is set to 10 L/min. I thought i would try turning the taps on upstairs whilst the down stairs were on just in case of an air lock, unfortunately absolutely nothing came out of the top taps until i closed the bottom taps. Please help! Is it the DHW flow switch/limit switch? what should i be looking for? or can the boiler have caked up with scale and crud thus reducing the flow of the water? Though this would suprise me as the CH seems to work fine.

Thanks Dave

 

[ChrisR]

Are you in a really hard water area?
What's your cold mains flow - has it recently reduced?

 

[Daviddf]

Hi Chris, i live in kent which is quite hard (about 300 to 400 ppm) white cliffs of dover and all that, but i haven't noticed a drop of in water pressure, the electric shower upstairs is fine and the cold water taps deliver well in excess of 10 litre per min. Do think this is the problem? or is it more likely a boiler component sticking?

 

[Agile]

David, you are confusing different parts of the specifications!

The boiler should switch on with about 2.5 litres flow.

At this point the flow is limited to about 4 li/min until it warms up and a waxstat opens a restriction to allow the full flow.

You have the problem that you dont know if its the cold flow limiter or some other aspect that is restricting the flow.

The cold flow limiter is very reliable and I have never changed one but it is still a possibility.

More likely, in my experience, is a flow limiter within the cold inlet bits underneath. You dont change it but just remove it unless your mains pressure is over 3 Bar.

Check both those and you are likely to solve the problem.

The main heat exchanger is unlikely to have become scaled up unless there is another fault.

Tony

 

[Daviddf]

Hi Agile, thanks 4 reply, i am now a little confused. I agree with you that the boiler does fire up ok but the DHW flow is very low, i dont know what the exact flow was when it was working ok as i never measured it, all i do know is the hot water flow out of the kitchen taps is 4 l/min which is lower than it used to be and the boiler swicthes off as it seems as if the flow rate is too low? I think the water pressure is more than 3 bar but to be honest i have not measured it.

 

[ChrisR]

There's nothing else in the flow. The flow switch is in fact a pressure switch, no water flows through it.
The main heat exchanger can block up with limescale, but it isn't common.
If you look in the manual which is at partsarena.com/baxi you'll see reference to the Automatic flow limiter and the Manual one.

To find the culprit you can, to a point, dismantle and look, or disconnect things and see how much water then flows out (possibly backwards by turning off input, connecting washing machine taps together and opening H and C).
You have to remember that as explained, the auto, wax restrictor is supposed to restrict until the water is hot. Dont confuse the wax restrictor with the other wax device, which is in the diverter valve.

 

[Gasguru]

And check the flow restrictor isn't blocked.

 

[Daviddf]

Thanks Chaps, i think i know what i am doing, but just to confirm, where is the automatic flow limiter? where is the manual flow limiter? and where is the flow restrictor? I have ploughed through the part arena pages and there is no reference to these parts, i am guessing they are on the pages just named something slightly different. Please can someone calrify what parts i am looking for? thanks

 

[Gasguru]

Start by checking the flow restrictor, from memory this is located on the outlet connection of the cold water inlet isolating valve.

A pipe runs from here to the inlet manifold - part 98. This part contains the flow adjuster (I normally find these are set fully open).

Inside the manifold is a small drilling - on very low pressure mains this can be too restrictive and may require drilling out.

Part 108 (the thermostatic flow limiter (water saving feature)) is located on the end of the hot water outlet pipework from the heat exchanger. I have never had to replace this item though.

 

[Daviddf]

thanks gasguru, i will look at these items, but in your experience what about items 110 and 111 DHW flow switch (pressure switch) and micro switch?. From a basic diagnosis point of view, i think that i have got low hot water flow when it used to be ok so something must be faulty and i thought it might have something to do with this switch malfunctioning and stopping the water from flowing? Do 110 and 111 fail and if they do what are the symptons?

 

[Gasguru]

The pressure switch assembly has nothing to do with the flowrate.

All this device does is detect differential pressure and activate the microswitch to fire up the boiler when you turn on the hot tap.

This part is unreliable but it won't make any difference to the flowrate.

 

[Daviddf]

Thanks Gasguru. This is what is happening.
1. Turn on the hot tap, cold water comes out low flow rate, boiler fires up straight away.
2. Water warms up, still low flow rate, boiler still fired.
3. Water gets very hot, low flow rate, boiler stops firing,
4. Water running hot, low flow rate, boiler fires up again
5. Water running hot, low flow rate, boiler stops firing

and so on .... until i turn the tap off.

Thats why i thought it was a swicth problem, because the boiler keeps turning on and off.

I remember when i first moved into the house and i had no experience with my combi boiler, i turned the hot tap on and cold water came out at a very low flow rate. It was because the mains gas shut off valve was switched off, once i turned the gas on i turned on the tap and the boiler fired up and the water flow rate increased as the water got hotter, i assumed this to be the wax automatic flow limiter.

This is what makes me this its either an actual bloackage as you suggest or a problems with the wax auto flow limiter or the DHW flow pressure switch.

thanks

Dave

 

[Agile]

David, if you want to fix it or get it fixed then the time has come for the parts to be looked at!

Its irrelevant really what you think is wrong with it, we fix boilers every day and we have told you what you need to check first.

I would suggest you start with the flow rate limiter in the cold feed inlet. When you have removed and checked that I think there is a gauze filter around that to check as well if the flow limiter was not the problem ( which it probably is! ).

Last would come the waxstat cold flow limiter.

Tony

 

[doitall]

Some taps have an aerator on the end of spout which gets blocked as well, unscrew an check, if you have one.

 

[Daviddf]

Hi Agile, I have removed flow restrictor from water inlet manifold, there was nothing to clean as such, every thing looked clear. Turned the cold water inlet valve on and water was running freely, Have not checked the wax flow limiter yet as that is going to take a little more time (weekend job) The cold water rund to the diverter valve, haven't checked that yet either, does that get stuck in the CH position? Dave