Newbie Central Heating Questions

[Grantsteve]

Hi all - first time poster. I'll apologise now for my 'newbie' questions.

I've just bought my first property and am trying to get to grips with the CH system. Its a Potterton Puma 80e

I don't use the timer on the system, I just get the heating to come on and off by adjusting the thermostat.

I have a few questions that hopefully you guys will be able to help with:

1. When I crank up the stat to get the heating to come on it does for about 2-3 mins then it goes off again, then a minute or so later it comes on again and so on. It never seems to appear like its brought the room up to temp and switch off completely until it needs to come on again. Always on then off then on ... It heats the rads and water fine. Is this behaviour normal?!

2. The hot water seems to be very cloudy. Limescale? What can I do?

3. I have a normal 'turn the dial' stat in my main room (lounge / kitchen) which I find slips a lot so the dial will spin without actually moving the bits inside to turn the heating on. Can I swap this out for a digital stat? What should I look for and is it easy to fit?

Apologies for the long post. Thanks in advance for any help

 

[ChrisR]

Boiler comes on to make rads hot, then it has to go out until they cool. Room is warmed by the rads, eventually, then the stat should stop the cycle.

Consider replacing the stat with a programmable stat - much more useful than the inbuilt timer. Yes, easy to do.

 

[Grantsteve]

Consider replacing the stat with a programmable stat - much more useful than the inbuilt timer. Yes, easy to do.

Thanks for the reply.

Do you have a linky to the sort of thing I should be looking for?

 

[ChrisR]

Honeywell CM67, just replaced, is a good one, and there's a Centaur at Screwfix which is cheaper. Danfoss, Siemens, and others also OK . Look carefully at features like extending times for an hour.
I don't like the Drayton one , old or new.

Try to get a look at manual (online). Some are very peculiar!

 

[Grantsteve]

Cheers, after a quick look around I went for the Honeywell CM907.

No doubt I'll be back when it arrives for help fitting it!

 

[Grantsteve]

2. The hot water seems to be very cloudy. Limescale? What can I do?

Anyone have any ideas about my cloudy hot water? Is it a limescale problem as I suspect?

 

[Agile]

No its just the water expanding rapidly when it leaves the tap at say two atmospheres pressure and letting dissolved air out. It will clear in a glass in a minute or two.

The Ch problem is probably because the boiler is turned up to maximum 24 kW and your system only needs much less power.

If you want to delve into the inner workings and can do so safely then look at the main PCB and there is a black knob towards the bottom.

The knob is probably max clockwise. If so turn it anti clockwise then up about 20° and start timing the result. It should then stay on most of the time until the room temperature set on the room stat is reached.

You might have a bit of a problem with the rad outputs as they may be undersized for the coldest weather.

Tony

 

[bster]

Could the constant on-off-on-off situation be resolved by using a 3- wire stat with the anticipator (think thats what it is called) smoothing out the on-off-on-off cycles?

 

[ChrisR]

Not what it's for. That actually increases on-off cycling, by making the thermostat warm up when the boiler's on, more than it would from the air alone.
Would you like me to draw you a time/temp graph or can you see it?

If you increase the time between boiler firings, you slow down the response time of the heating.
There are lots of optimising methods - 3 term (= PID) control, "Chrono proportional" methods, etc.

 

[Grantsteve]

No its just the water expanding rapidly when it leaves the tap at say two atmospheres pressure and letting dissolved air out. It will clear in a glass in a minute or two.

The Ch problem is probably because the boiler is turned up to maximum 24 kW and your system only needs much less power.

If you want to delve into the inner workings and can do so safely then look at the main PCB and there is a black knob towards the bottom.

The knob is probably max clockwise. If so turn it anti clockwise then up about 20° and start timing the result. It should then stay on most of the time until the room temperature set on the room stat is reached.

You might have a bit of a problem with the rad outputs as they may be undersized for the coldest weather.

Tony

I have two double rads to heat a room approx 5m by 5m, and i'm only setting the stat to around 18.

What do you mean by the main PCB? Is it just the bit under the flap on the front of the boiler?

 

[Grantsteve]

Any ideas peeps? I've tried turning back the control for the CH on the front of the boiler but AFAIK that just controls the temp of the water leaving the system.

I'd like to try Agile's suggestion but I need to be able to identify the PCB first!

 

[Agile]

If you are unable to identify it then I am reluctant to give you detailed information because it indicates that you may not be competent to work safely on electronic circuits.

A printed circuit board is a flat sheet of insulating material with copper tracks on one side and components on the other. Thats the simple old ones anyway. latest are multi layer and surface mount components on both sides.

When you can do that I expect it will solve the problem.

Tony

 

[Grantsteve]

If its a simple job then I am confident enough to do it. I just need to know where to find the PCB. I will know what it is when I see it!

 

[Grantsteve]

OK, have given up and intend on getting an engineer in to tweak the boiler and fit my new stat.

Can anyone tell me what the black knob on the PCB is called so I know if the engineer is trying to pull a fast one?

Thanks

 

[Grantsteve]

If you are unable to identify it then I am reluctant to give you detailed information because it indicates that you may not be competent to work safely on electronic circuits.

A printed circuit board is a flat sheet of insulating material with copper tracks on one side and components on the other. Thats the simple old ones anyway. latest are multi layer and surface mount components on both sides.

When you can do that I expect it will solve the problem.

Tony

I had a read at the installation manual and worked out how to get to the PCB. I have tweaked the black knob in the way you suggested but if anything this has made the on off cycle quicker, its probably halved the time the boiler is on and then off.

What now??!