Vaillant 415 Hot Water problems

[MrDuck]

I have a small 3 bed house with Vaillant ecoTec plus 415 boiler feeding a 1980s Y-Plan system with a conventional indirect hot water cylinder and 8mm microbore radiators. There is not (as yet) any control over the water flow rate through the indirect coil (I plan to install a gate valve in the Spring when I change the rads for new ones).

The problem I have is that it takes an age (up to an hour) for the hot water tank to heat up each morning (with rads off). This is with the boiler flow set to 70 deg C and the cylinder stat set to 50 deg C. So this morning I got my watch out and monitored what the boiler got up to. This is what I found:

T=0 Boiler fires up at what sounds like full power. Circulating temperature rapidly rises to 70 deg C.
T=4 Boiler still in S4 mode but with burner running on low maintaining 70 deg C output temperature. Interesting - I didn't realise it could modulate down like that.
T=20 Boiler now in S7 mode (overrun) with burner off and temperature rapidly dropping. Why did it switch out of low power mode?
T=24 Boiler back on at full power for about 1 minute until 70 deg C reached.
T=25 Boiler back to S7 (overrun) and temperature dropping again.
T=30 Boiler back on full power...

The last two states repeat constantly for further 30 minutes or so until eventually the cylinder stat is satisfied. But as the average flow temperature is now so low the cylinder is interminably slow to heat up.

My question is what should I do to correct this? If the boiler can modulate down I would prefer it to stay in that mode but for some reason it switches out and I don't now how to correct this.

Any guidance much appreciated.

 

[Dan Robinson]

New hot water cylinder.

 

[MrDuck]

Thanks Dan, it's a good comment. My current cylinder is labelled as having 0.44m2 of heating area - I don't know whether that's good or bad, but I don't think it can be that old as it's got the thick green insulation on it.

I am hoping for a bit more explanation of the behaviour of my boiler as it's clear that it fails to consistently provide heating flow at around 70 deg C except for the first ten to fifteen minutes. After that the average circulating water temperture is no more than 55 deg C which is not much good! These boilers seem to have a mind of their own!!

 

[MrDuck]

Well, I have experimented a bit in the last few days by reducing the maximum boiler output from 15kW to 10kW via d.0, and magically this has made a huge improvement. My hot water cylinder now heats up in about twenty minutes as compared with around an hour before. The boiler now seems to stay in modulating mode until the cylinder demand is satisfied, which is just what I wanted.

But why does the boiler behave like this? Bizarre!

 

[Agile]

It is surprising how that simple setting can make a lot of difference to performance.

But few people bother much with setting it.

Slight problem though is that when you want to have heating as well as hot water heating more power will be needed.

But as long as you time the HW to be before the CH starts you will be fine!

Tony