CH System Behaviour - Is It Normal?

27 Jan 2019
Reaction score
United Kingdom
Hi Guys, I wonder if you could help me understand my central heating system a little better. Here's the setup.

New Vaillant Ecotec Plus 832 combi boiler connected to older system of 7 flat panel radiators in 3 bed semi.

Boiler fires up and burner and pump are at low settings for a few minutes. Burner and pump then increase and heat the water in the system to the target flow temperature. After reaching target flow temperature the boiler and pump modulate to the boiler's lowest output (5.2 Kw) and slowest pump speed (15%). The drop across flow and return is 12 to 15 degrees C to start with. It falls to 4 or 5 degrees C when system water is up to target flow temperature. This is the same irrespective of the LSV settings (even when fully open). All radiators get hot and have been bled of air.

Before the target temperature on the room thermostat is reached the system water temperature rises to 4 or 5 degrees above its target temperature (sometimes before upstairs TRVs shut down and definitely after they do). In other words, the return water is then at the target flow temperature. The boiler then goes into anti-cycling mode.

When the anticycling time has lapsed the boiler fires up, gradually raises the system temperature to 5 degrees above target and anti-cycles again. It continually repeats this process. Consequently, the target room temperature is never reached or is reached very slowly. The system water temperature and anti-cycle period are controlling the heating not the room thermostat. It's as if the boiler cannot modulate low enough to maintain the system flow temperature and/or the radiators cannot emit enough heat to stop the system return water temperature rising above flow target (i.e. to balance heat input and heat loss).

Is this normal? I would have thought the goal is for the boiler to modulate to maintain the target flow temperature until the target room temperature is reached. Then the room stat will switch it off and along with the anti-cycling period, keep the room at target temperature within half a degree or so.
Sponsored Links
D.00 setting makes no difference to the anti-cycling behaviour. It just takes the boiler longer to reach (and them exceed) the target flow temperature as it limits the max output. The minimum output modulates down to 5.2Kw whatever the D.00 setting.
Sponsored Links
I think you may have identified the key issue Jeff. Radiators are,

Flat panel double
1400mm x 400mm
1280mm x 400mm
1120mm x 400mm
1000mm x 400mm

Flat panel single
2200mm x 600mm
630mm x 700mm

Single with convector fins (replacement for original)
1300mm x 600mm

They were originally connected to a 1970s back boiler which ran at 80/70 flow/return temperatures from a non modulating boiler. The installer estimated the heat output to be 7Kw to 8Kw and was probably anticipating a temperature differential of 10 degrees.

I've re-read the Vaillant manual which gives the minimum heat output as 5.4Kw at 80/60 and 5.7Kw at 50/30. They obviously anticipate a system with 20 degree differential, presumably with larger output radiators taking more heat away from the system. Is there any way I could get a greater differential?

Grumpy Gasman, I've tried setting the pump to constant 100% both at 'D.00 auto' and other settings but it didn't make any difference. Isn't comfort setting just pre-heating some DHW?
Last edited:
I've re-read the Vaillant manual which gives the minimum heat output as 5.4Kw at 80/60 and 5.7Kw at 50/30. They obviously anticipate a system with 20 degree differential, presumably with larger output radiators taking more heat away from the system. Is there any way I could get a greater differential?
Possibly the reason why the appliance has a range rating facility (d.00).
d.071 might also help.
I do apologise for not saying thanks, I really am grateful for all the responses. As you can see, I haven't mastered the art of brief responses and I guess just stating facts was my attempt at this.

Are you suggesting setting D.00 low and D.071 high Grumpy? I can certainly play around with these. What would be the correct setting for the integral bypass?
Over the weekend I've tried D.00 settings at 5, 10, 15 and 24 each with flow settings of 50, 60, 70 & 75. This alters the maximum output and slows the time taken to reach (and exceed) flow target. However, doesn't affect the flow/return differential and boiler cycles when return reaches the target flow temperature. The boiler always modulates to minimum burner and pump when target flow is reached. On none of the settings is room target temperature reached before boiler cycles.

The house is now up to target room temperature so room stat is kicking in but I have set up as follows to see what happens tomorrow from cold.
D.000 to 14
D.014 to 5
D. 018 to 1
Flow target to 75
Last edited:
Here's what happened today.

Overnight house cooled to 16.5 degrees
Room stat set to 19 degrees. It's on the landing and when it hits 19 all rooms are about 20 which I find comfortable.
Target flow set to 75
Pump 100%
Comfort (constant pump) on
Boiler Load 14 Kw

8:14 I switched boiler on. Burner went to minimum (5.4 Kw)
System water temperature was at 25 degrees
8:15 burner went to 14 Kw. Temperature differential 9 degrees
System temperature rose to 75 flow, 68 return by 8:36
8:45 Boiler modulated gradually down to 5.4 Kw 75/70
8:52 room temperature 17 degrees
9:06 room temperature hit 17.5 degrees. Flow/return varying from 75/71 to 74/70
9:45 room temperature at 18 degrees. Boiler still at 5.4 Kw 75/70
10:00 room temperature 18.5 degrees. Boiler 5.4 Kw 75/70
Lovely sunny day, TRVs in south facing rooms shut down. System temp rises to 78/74. Boiler output still 5.4 Kw
10:45 room temperature 19 degrees. System still at 78/74
11:00 Boiler cycled but thermostat still calling for heat.
Boiler continued to cycle (3 cycles per hour, cycle = off to off). Sometimes this was due to thermostat switching off, sometimes not.

So, house got to target room temperature without boiler cycling. I guess it made little difference whether cycling was due to thermostat or not at this point. However, had target temperature been 20 degrees and boiler cycled at 19 it would have delayed house getting up to temperature a little.

Thanks for the advice Grumpy Gasman. Could you explain what was happening with these changes to the settings please? I'm guessing the two pump settings increased the flow rate but the temperature differential remained pretty much as before. Input/output seemed nicely balanced until TRVs shut down. Was more heat being put out by the rads even though house did not heat up at lightening speed? What effect does setting D.000 to 14 Kw and raising the target flow temperature have?

Is the fundemental issue that the old radiators and new condensing boiler are not an efficient match (rads not emitting enough heat especially at current outdoor temperatures and boiler not modulating low enough to achieve balance)?

I'd love to understand this better.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links