Vaillant ecoTec plus 837 issue: Radiators slow to heat up

17 Jan 2015
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United Kingdom
System: Vaillant ecoTec plus 837 combi boiler with Honeywell CM900 Programmable Room Thermostat

Problem: I have recently had a Vaillant boiler and Honeywell CM900 installed in our flat. When I turn the heating on via the Thermostat, the boiler fires up as expected. However, the burner is at such a low level that it takes the Radiators about 40 mins to heat up to the 60c setting on the boiler (I can see it it's burning at a low level by watching the graphic on the boiler as well, watching the water temp slowly creep up on the boiler and obviously by feeling the radiators).

If I turn the boiler itself on and off (after waiting some time for the water to return to colder temp) and then repeat the steps above then the boiler fire up properly, the graphic shoots up to about 3/4 of the scale and the radiators heat up very quickly from cold (about 5-10 mins to reach 60c from a starting point of 15-20c). However, later on in the day when the heating comes on again it will return to its slow state and take an age to heat up.

This happens whatever temperature I set the thermostat at (i.e. if the current room temp is 14 and i set the thermostat to 16, 20, 25 etc. then it will still be the same slow speed).

Obviously this is most annoying in the morning when i have to turn the boiler off and on to get quick heat from the radiators.

I've called in the installer who has tested the system but says that nothing is wrong. Can anyone help before I have to call Vaillant?

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Big flat in extremely cold old house (listed building) with no insulation. Any chance you can give me some more detail on how to set 'D.0 down to the heatloss of the flat?'

How big a flat? . I doubt very much it needs anything like 37 kw.

What is the Heatloss of the flat? . You need to calculate that first. . Or work out the output of the radiators.

Covered many times here so have a search.

For immediate testing though count the number of rads and times by 1.25. That is the figure to try D.0 on. . How to change that setting is in the manual.
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D0 is currently set to auto. Doesn't it need to be auto for different weather conditions/different radiators been on and off?
For reference, my dad has a large 4 bed, 450 year old farmhouse.

I installed a 30kw boiler. AND it has plenty of capacity left.

My house is a fair sized 4 bed detached mid 80's Bovis house and the boiler is 15kW, and even this morning with light snow and sub zero temperatures , the compensated heating system only ran at 50 degrees.

People seem to think that bigger is better.

It ain't.
Sorry, that was to the 'nope' comment.

I'm stuck with the 837 (although the actual size is 28 for heating i believe) so can't do much about that. It's more about getting the thing to work properly in the first place. I will persevere.
Explanation of what? . I'm about to go out.

We need to know the heat requirements of the flat.

But an oversized boiler, or one not set up properly will shut down and enter anti-cycling mode. Giving the effect of slow heat up.

Does the boiler spend a lot of time doing something but not actually burning gas?

The s codes are helpful here and are also explained in the manual.

Like I said, 37 kw is insane for a flat. I've worked in some pretty big ones in London and can't think K of one that has needed more than. 12 or so for heating.
Auto mode tries to match the capacity of the boiler to the flat

It is not that effective though.

This is all assuming that there are not other problems of course.

What is the flow temperature of the heating set to?

Have you actually tried any of my suggestions yet?

Without feedback we have no way of knowing if we are on the right track.

Worry about understanding the problem once we've sussed it out.
The flow temperature is currently set to 60c

The total BTU output of the radiators is about 60000 i believe.

Heat loss in the property is terrible - multiple 2.4m single glazed sash windows, high ceilings etc. as well as no insulation
Thanks. I will give that a go.

Can you explain to me the thinking behind the 19 and 70 settings. I'm not disagreeing, just trying to learn and understand to help me in the future.
19 KW to hobble the boiler to something the system can handle. . If the boiler pumps too much heat down a small system it will throw a paddy.

70 degrees to help the rads over come the cold and drafts. . I'm assuming they are the originals and therefore sized for higher temperatures.

Mine are sized for lower temperatures hence capable of heating the house with the boiler running at lower output.

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