Atag boiler short cycling

Hi all, hoping someone can help me!
I've got an Atag IR24 system (heat only) boiler installed a couple of years ago and I'm having a problem with it short cycling. I'll try to describe my heating system as best as possible: it comprises of 13 radiators of various sizes most of which have trvs turned up to max, with 4 upstairs ones about halfway. Heating flow and return circuits are 22mm pushfit with 10mm legs to all radiators. Hot water is a 170l megaflo cylinder. Central heating and hot water are controlled separately by their respective thermostats and a programmable thermostat which we currently have set on all day for both hot water and heating as we're at home all the time. There is a header tank in the attic with just one feed into the main pipe to top the system up incase any water is lost. There is no expansion pipe into the header tank so I believe this makes my system an unvented system (am I correct??) I've tried to do a bit of diagnosis; all thermostats and programmer seem to work correctly, automatic gate valves are not stuck and both open and close correctly according to demand. Pump seems to run fine and is set to maximum speed. I'll include a pic below of the plumbing near the megaflo where all the valves and pump are located. Problem we're having is even though the central heating stat is set to 30⁰c the atag boiler is short cycling and the house isn't getting warmer than about 18⁰c, we want about 21-22⁰c. This may have been happening for a long time but its only noticeable now when it's much colder outside. Worth noting the boiler does not have an external temp sensor.
Boiler settings are as follows:
Water temperature is set to 80⁰c by original installers. I've taken some data from the boiler when it's been short cycling. Picture shows what the codes correspond to. When it runs for about 5 mins and then cuts off: A0 is around 84, A1 is around 55, and A3 is around 80. So the return temp is not anywhere near the flow temp or the desired 80⁰c. It also sometimes runs for as little as 1 minute, so A0 is around 80, A1 is around 50, A3 is around 75. The strangest thing about this is it sometimes works completely normally, running for 15 ish minutes at a time and the A0 and A3 values are very similar although A1 is usually a bit below.
Anyone have any idea what's going on?? Also should the flow (A0) and return (A1) values be similar at the point the boiler cuts out? Sorry for the long post and thanks I advance for any replies

 

Just to try to explain whats in the pic, the two pipes on the right going upwards with the valve are to flow and return for the megaflo.
The boiler flow comes from the left insulated pipe into the pump and goes out to the right. The magnetic filter is installed on the megaflo return. You can just about make out the feed from the header tank within the wall going into the main flow pipe but there is no expansion pipe going back to the header tank. A lot of the pipes you can see in the background are hit or cold water pipes which is why it looks a bit confusing. I think the white valve is a pressure valve that opens if neither of the motorised valves are open, so if the boiler came on for some reason the water would return through it straight into the return pipework. Any questions please ask, would love to get to the bottom of this as it's really only a problem when it's colder outside- when you actually need it to work!

 

How clean is the system water? Is there water in the f&e tank (header)

Nope, you have just the same as an open vent just with a combined feed and vent/expansion pipe, it should be 22mm up to the F&E tank.

Possible circulation issue though, sounds like the boiler's reaching max cutout before the system can get rid of the hot water. Set a bit high @ 80deg though. Do the radiators heat ok or only just getting warm?

 

The water comes out clear when I bleed the radiators and the system was apparently powerflushed when the boiler was installed a couple of years back. It is a 22mm pipe up to the header tank, and it does have water in it. Upstairs radiators all get hot (other than the ones with their TRVs turned lower) and most downstairs ones do, just a couple that have longer 10mm legs to them take a bit longer to get warm but if the system has worked properly and has been on for a while they do all get hot. However upstairs ones are definitely hotter than downstairs ones. I don't suppose there is much that can be done about that though with the long 10mm legs (just poor design I guess?)

 

Does this short cycling improve if you turn off the hot water generation? If so the valve just behind the ABV is the cylinder balancing lockshield valve. It will typically be 1.5 to 2 turns open from closed to give 15C temperature difference between flow and return temps at the cylinder.
Is the system water clean? Your pump may be partially blocked. Shut its two gate valves and dismantle the pump. Clean it's impellor thoroughly with a piece of copper wire (strip some T&E cable).
The white topped gadget you described is an Automatic bypass valve (ABV) and it works as you suspected.

 

Search 'balancing the system' for the solution to that.

 

80°C is very high for a condensing boiler - it won't condense at that temperature, you're just throwing heat out of the flue. Turn it down to 70.

Your boiler may well be oversized for your system. For 13 radiators I'd expect you to be somewhere around the 12-18kW range, not the 24kW you have. Depending on what thermostat you have, turning it up to 30 might be making the problem worse. It certainly won't be making things any better. Set it to 21 if you want the rooms to be at 21.

As MM says, balancing your system may help

 

Sometimes forcing the hot water to come on (by turning its thermostat to max temporarily) can actually sometimes make the boiler run for a bit longer but generally has little effect, so could easily be coincidence to be honest. It's pretty random and there's 3 main cycle patterns I've noticed, firstly about 1 minute, secondly about 3-5 minutes and of course working correctly! Water seems to be clean. I will try dismantling the pump tomorrow but I think it's probably working OK as it sounds like it runs smooth enough and the pipes either side of it feel like they are at the same temperature.

 

It's certainly a symptom of low circulation, for reasons yet to be found....

 

I'll try turning it down tomorrow and see if that has any effect. The maximum temp it can be set to is 85 by the way. Fair point about the thermostat, I will set it to the temperature I want, to be honest now I think about it there's no point at all turning it to 30 as it's just a standard non electronic stat. It does seem to work correctly, it clicked at about 18⁰c today which agreed with the reading on a separate thermometer. The system should have been balanced when boiler was installed but I will make sure that is the case

Why would the boiler being oversized be a problem? Is it not able to regulate its output according to what is needed? I assumed the installers would choose the correct size.

 

Would you expect the return temp to be similar to the flow temp at the point the boiler turns off?

 

I'm no expert but given the flow is at 80 and the return 55 that's quite a delta which suggests to me either humungous radiators or, more likely, poor circulation and the heat not been drawn away from the boiler quick enough. Perhaps the boiler is sensing this and turning the burner off?

(Just guessing here so those in the know do correct this if it's nonsense)

 

I could probably approximate most radiators to be an average of 600x600mm single panel with two much larger double panel radiators in the kitchen and living room. The pump always seems to be running when the boiler fires up so in theory the heated water should be drawn away - whether or not this is happening is a good question. The intermittent nature of the fault is what is so strange to me, I would have thought that a problem like this would not just go away sometimes and come back.

 

There is circulation but I was just wondering if there's was enough. Have tried changing the pump speed? (Sorry if I missed it)

 

It's set to maximum speed (3rd speed setting) and always has been as far as I'm aware. Just out of interest I tried turning it down the the second setting and it just goes off completely, not sure if that should be happening.