problems with Viessman boiler

[pw21]

Hi there,
A friend has had a Viessmann Vitodens 100 boiler installed.
(open vented CH with pump & mid position valve & hwc in airing cupboard)

When CH is switched on the boiler fires up ok and hot water gets pumped via the 3 way valve into the CH circuit. - When the display on the front of the boiler reads between 51 and 55 degrees the boiler stops heating the water leaving cool water being pumped round the CH circuit.
5 minutes later when the displayed temp has dropped to about 29 degrees the boiler fires up again. - This continues to happen and the rads never get warm enough.

One boiler engineer that looked at it said the CH circuit needed balancing as the temp difference between flow & return pipes was too great, but the boiler does the same thing when only HW is selected so I'm not sure if he is right.

Does anyone have any ideas ?

thanks

 

[JPC]

is this a new problem or has it been like this since install

 

[pw21]

Its been a problem since installed.

Company A installed boiler
Copmpany B installed additional radiators in new extension

Company A say problem is the rads & pipework
Company B say problem is the boiler

 

[JPC]

lovely !!

I cant guess what the problem might be from here, but having installed a couple of viessmann, they are not always the easiest to set up.

The hot water requires temp setting as does the c/h. the last one we inostalled needed re-setting from underfloor heating to radiator heating etc...so might be worth a call to viessmann tech line, who are very helpful

 

[holty]

have you tried turning the boiler temp control right up

 

[simond]

The eco setting on the Viessmann compact is too low, you need to turn it up as per the post above, that should do it.

 

[JohnD]

my Vitodens 100 compact used to short cycle on HW. The manufacturers "normal" boiler temp setting is only 60 degrees. I turned it up to 70 degrees and turned up the pump speed a bit and now it is OK.

I have also balanced my rads and it runs fine on CH.

Mine stops firing at 71 degrees and restarts when it hits 47 degrees, so its operating range is about 50 to 70. If your friend's is stopping firing at 51 or 55 degrees it is set too low.

If I turn the knob to max it runs up to 80 degrees. The instructions say you can turn it up in exceptionally cold weather if you need more heat.

Whatever the temp setting, it modulates its power down from five bars to four to three to two as it get closer to the set temp, and you can hear the flame getting smaller and quieter.

As an amateur, you can tell if the rads are reasonably balanced by holding the Flow and the Return pipes at each rad in your hand. As a rule of thumb, the Flow should be "too hot to hold" and the return should be "too hot to hold for long". The F & R pipes coming out of the top of the boiler should also be like that.

Or you can buy pipe thermometers, like a pro.

If you have had two heating companies in, I hope they have tested F&R temps with thermometers and are not just guessing.

BTW Viessmann tell me it does not need a Bypass in my installation (I particularly queried this) because of the way it runs the pump on overrun with the 3-port valve open for 12 minutes after it stops calling for heat. If your friends system has been installed with a bypass that might be relevant.

Some Viessmann installers seem to be more familiar with them than others. I don't think there are very many Vitodens 100 Compacts installed in my area.

I did not phone Viessmann Tech Desk about the HW short cycling, I puzzled it out with help on here. I now realise it would probably have been quicker if I had asked Viessmann.

I am just a householder.

 

[micky p]

If the boiler is cycling a lot then sounds like to me that you have a flow problem. They can be quite hard work to vent the primary heat exchanger on a open vent condensing boiler.
I had a simular problem with the same boiler as yours i installed abour 12 months ago. turned out the pump was not powerful enought. So i upgraded it to a 15 /60 pump and it worked a treat. Might be air in the heat ex considering there are several loops in the heat exchanger.

 

[JohnD]

my pump is one of those self-regulating ones, I had to turn it up a bit as part of my tinkering to stop the short cycling.

But the flow through those coiled stainless tubes is surely very easy flowing and fast, as they are quite small diameter?

 

[Agile]

The Viessmann has to be commissioned by a competent engineer as there are many internal engineer settings.

Even filling the boiler heat exchanger has to be done carefully in a special way to expell all the air.

Thats why Viessmann want all their boilers installed by people who have been trained by them and then give a longer warrantee.

If you look on the Viessmann web site it lists their approved installers. I expect the Installer A is not listed?

If not then I suggest you advise the installer than you dont think the boiler has been properly commissioned and call an approved firm and charge the cost to installer A.

Did installer A fill in the Benchmark and advise CORGI who sent a blue certificate to the owner?

Tony

 

[micky p]

The Viessmann has to be commissioned by a competent engineer as there are many internal engineer settings.

Even filling the boiler heat exchanger has to be done carefully in a special way to expell all the air.

Thats why Viessmann want all their boilers installed by people who have been trained by them and then give a longer warrantee.

If you look on the Viessmann web site it lists their approved installers. I expect the Installer A is not listed?

If not then I suggest you advise the installer than you dont think the boiler has been properly commissioned and call an approved firm and charge the cost to installer A.

Did installer A fill in the Benchmark and advise CORGI who sent a blue certificate to the owner?

Tony

thats right I am a listed installer but unfortunatly not in your area.

As for air flow throught the heat ex. Viessmann are one of the better heat exchangers as there water ways are far larger diameter than the commonly used Giaonni. There for it is easy for air to get trapped.
There is a specific proceedure to follow when filling the primary heat ex on the boiler but like i said open vent are a pain to do due to the head pressure not being great.
sealed systems are easy to do as you shut 1 valve at 1 side then fill the heat ex 1 way forcing the air out. but it tells you all this in the commissioning guide.
Does sound like air in the heat exchanger to me but its hard to say without looking at it.

 

[pw21]

Thanks for all the replies.

JohnD, Simond & Holty you were spot on !
Boiler temp control has been turned up and everything seems to be working great.

(Tech guy at Viessman was very helpful as well).

Interesting how the company who installed the boiler missed this?

Once again many thanks.
Pete