Boiler cycling when heating hot water cylinder

[NuttySnake]

Hi All,

We have a Vokera Mynute 16e boiler in an s-plan system to give independent control of the hot water and heating.

When the system is calling for heat for the hot water cylinder only we find that the boiler cycles rapidly - 30 seconds burn, couple of minutes rest, over and over again until the cylinder stat is satisfied. Throughout this time the call for heat is constant - so I'm assuming it's because the coil in the cylinder isn't transferring the heat quickly enough to provide a significantly cooler return to the boiler? If you turn on the heating at the same time the cycling reduces greatly - doesn't go away totally but I've not made any attempt to balance the radiators yet.

Tank it's self is, I think, rather small - about 90 litres, so I would have thought it should heat fairly quickly - it takes best part of an hour of this cycling to get us back to 60c in the morning following overnight cooling.

I suspect given the quality of the fit-out of the house in other areas that the cylinder is the cheapest thing that the developers could get their hands on - they didn't even put in proper heating controls - we've done that. They also decided not to fit a drain point in the system too - very helpful!

Anyway, I'm not a gas engineer but I don't think our boiler is capable of modulating it's flame - I think it's all or nothing. Do we need to get a gas engineer in to tame the boilers output a bit, or would a better cylinder with a fast recovery coil be the way to go, or is this the way it's supposed to be and we should just leave it alone?

Cheers

 

[spraggo]

You will probably find that the small head tank in the attic is empty due to a stuck ball valve. Failing this it could be that the coil is air locked which can usually be remedied by releasing the air with the manual air vent situated at the top of the flow pipe going into the cylinder

 

[NuttySnake]

Thanks for the reply spraggo,

The system doesn't have an expansion tank - it fills via a non-return valve that tees off from the loft tank cold feed to the house taps....not sure it's supposed to be like this, but that's how it's connected.

There is an automatic air release valve as you say at the cylinder flow pipe - this is brand new as the old one wasn't sealing (you had to have the cap screwed fully down to stop water spraying out) so I changed it out. Not sure there is a way to manually release air with the valve - but would hope it's doing it automatically anyway - especially as it is new!

 

[spraggo]

No it is definetly not supposed to be installed like that, Hope you have an expansion pipe. Install a small head tank and do the job right.

 

[Agile]

I think that your boiler can modulate. Others may confirm.

However, a boiler can only modulate if its been set up properly! That should be done by a conscientious engineer on a service but not all even know how to do that!

 

[spraggo]

No it is definetly not supposed to be installed like that, Hope you have an expansion pipe. Install a small head tank and do the job right.

Please ignore, misread your post

 

[dcawkwell]

Turn the pump speed down. Give the chance for the water running around
the coil of the cylinder to actually give up some of it's heat into the tank.

 

[NuttySnake]

System is closed, so no expansion pipe - boiler has an expansion vessel in it.
Good idea re turning down the pump, but the pump is integral to the boiler and I don't believe it has a speed control - least I've not found mention of it in the installation manual, and there is no obvious control visible on the pump it's self.

There is a gate valve on the cylinder return, presumably to allow ballancing when heating and water are both calling - it's half open at the moment.

 

[dcawkwell]

Would be very surprised if there wasn't 3 speeds on the pump (little switch on the side of the pump)

With the hot water on turn down the valve on the return from
the cylinder until you see the pressure start to rise a little then open up a little.

That will help reduce the amount of water running through the cylinder.

 

[NuttySnake]

Maybe I missed it - will open the lower cover later and check all round the pump power box for a switch.

Could it be a lack of system pressure? The pressure always reads at zero - as its filled by gravity from the loft, as opposed to a mains pressure filling loop, I dont see how it can ever pressurise.
We did ask the gas service man about the pressure when he serviced it last year but he said it was nothing to worry about. I'm beginning to wonder.....

 

[NuttySnake]

Just checked - def no pump speed switch on the pump

 

[dcawkwell]

Just checked - def no pump speed switch on the pump

Can't quite understand why the manual refers to pump speeds on page 4.
I can only see a 15HE not a 16E model though so confused.

Increase the pressure if it is a closed system you should have 1bar in there.
No point in a header tank feed it straight in from the mains and get the pressure up to 1 bar as there manual says there should be a check valve between the header tank and the central heating system. So the pressure should go up when the boiler is heating the water.
Best get the mains in there and get the pressure up.

 

[NuttySnake]

Yep I couldn't find a manual online for it - although the vokera website has one for the 14e, which is essentially the same:

http://vokera.co.uk/wp-content/uplo...e_installation_and_servicing_instructions.pdf

I found the manuals for ours stuck to the back of the boiler where the installer hadn't bothered to remove them. With all the pipes in the way I had to cut them in half to get them out from behind the unit.

Alas there is no way to increase the pressure - the gate valve on the cold feed above the non-return valve has always been left open, but it's not enough to presurise the system. Prob just as well - the installer hasn't bothered to connect the pressure relief valve to an external pipe....if the thing blows it'll blast hot water all over the kitchen counter!

 

[vulcancontinental]

In some parts of Ireland the boilers are connected like that because of local requirements (Dublin I think mostly) Read the section in the MI that are related to Irish installations specifically. There is a new IE website Vokera have just put up though I haven't looked at it yet.

Talking to guys in IE they say they'll fill the system and bleed it then link out the low pressure switch to get the pump to run and create a standing pressur inside the boiler of over .4bar then reconnect the low pressure switch. As you are disabling a safety feature I'd get an engineer to do that dor you.

Get the PRV sorted of course.

All brochures are available from the website even discontinued models.
If memory serves you have a Grunfos 15/50 on that model so you should have a speed control though leave it at max.

I THINK that model starts up at 75% heat output and should have a 4-5 minute anti cycle delay (if it turns itself off it will not come on again for 4-5 minutes and then only after the temp in the boiler has dropped).

Haven't time to check for you now off out.

 

[NuttySnake]

Sorry I'm not getting the point of bypassing the pressure switch - the pump runs without any problem whenever heat is being called for anyway.