vaillant 418 does not heat flat

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11 Dec 2012
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Location
Edinburgh
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Hi,

I had a Vaillant 418 installed in my two bedroom flat. Three of the five radiators are quite large. The system is vented.

The boiler has a tendency to get up to its maximum flow temperature, and then stop firing, even though the room thermostat is still calling for heat. Once the temperature has dropped, it fires again, and repeats the process. The flat is not getting warm enough. I don't understand why it does not turn itself down, and keeps producing heat until the room thermostat reaches its set temperature.

Any suggestions?
 
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A 418 (18kW) is oversized for a two bed flat! I have an 18kW boiler in my four bed detached, and that's just right when it's -10C outside.

Use Whole House Boiler Size Calculator to find out what size boiler you need.

You now need to set the boiler's maximum output to slightly above this value (to allow for colder weather than the calculator assumes), say 2kW more. You do this as follows:

• Press the “i” and “+” keys simultaneously. The display shows “d.0”.
• Press the “i” key. The display shows the current value (18kW).
• Use the “–” key to change the value (display flashes).
• Save the new value by holding down the “i” key for approx. 5 seconds until the display no longer flashes.
• Press the “i” and “+” keys simultaneously to exit.

You can always change the value in the same way, but you cannot go below 5kW.
 
What idiot specified an 18 kW boiler?

Setting d0 to 8 kW may help a litle but the main problem is the boiler is grossly over powered! One wonders why the installer did not do that.

Tony
 
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I would say that you have a good legal case for getting him to change it for a 412 or whatever the lowest power one is!
 
I would say that you have a good legal case for getting him to change it for a 412 or whatever the lowest power one is!

The lowest one in the 400 series is the 415 & about £50-£60 less than the 418
 
Thanks.

I turned the power down to 10kw, which seems to have made a difference, in that the boiler tends to go on running for longer when it reaches its maximum flow temperature.

The flat is still not warming up properly. The boiler was off for about 5 days, so the fabric of the flat is maybe still warming up. I will also have a go at balancing the radiators, as the large radiators furthest from the boiler get only lukewarm.

It takes about three hours for the temperature in the small, north facing bathroom to be raised by 1 degree (from 19 to 20), which is not satisfactory.

If all else fails, I'll get an engineer out to do some diagnostic pondering.
 
Have the same boiler & did have the same issue & I found that balancing the Rads did help

Check your thermostat isn't near a heat source or a room that get hotter the warm air might be cutting out the stat like was happening with the heat from the kitchen

I also found that having the radiator water temp too high was awful, too much heat, have mine set to 58°C
 
Thanks.

The thermostat is in the coldest room of the flat, so I'm OK there I think.

Will try to balance the radiators, but also thought I would have a look at the bypass valve. I think this should be only open a tiny bit, and I had a plumber in who may have changed that.
 
The problem seems to be solved. A plumber who came to have a look at the boiler turned the manual bypass valve fully open. I have almost closed it, and the flat is heating up nicely.

Any opinions as to how far this valve should be closed?
 
You should have an auto bypass valve.

Manual ones dont work properly, lower efficiency and are not permitted under the current regulations.

Tony
 
Thanks. I'll have the boiler serviced in February, and I'll ask the engineer to put one in.
 

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