Vaillant ecoTEC additional expansion vessel sizing

[Tim_R]

Hi,

We've just moved into a house which has a Vaillant ecoTEC 624 plus boiler (24kw).

The house had very old (1950s/60s), inefficient radiators which we've replaced with new Stelrad double-panel/double-convector and single-panel/single-convector rads as appropriate based on heat-loss calculations.

There was a red 8 litre expansion vessel originally attached to the CH return near the boiler, but our installer removed when installing a uniSTOR 260 unvented cylinder it as he said it was no longer necessary as our new rads were lower volume.

As the rads are all new, and being the kind of guy who likes to be sure, I calculated from their spec sheets that the total new rad volume is just under 124 litres (across 15 rads & 3 towel-warmers). I estimated the pipe run volumes as:
25m of 22mm main trunk @ .32 l/m = 8 litres
120m of 15mm runs to rads @ .15 l/m = 18 litres

For a total system volume of around 150 litres.

So it sounds to me like I need an additional EV

It says in the ecoTEC docs that the boilers come with a 10 litre EV built-in which is sufficient for a total system volume up to 100 litres. Above 100 litres it says you should install an additional EV as well and provides a table on sizing guidance.

The table says that for 150 litres total system volume at an initial system pressure of 1.5 bar that a minimum 23.4 litre EV is required.

It's not clear to me if 23.4 litres is the size of additional EV I need, or do I only a need a 13.4 litre additional EV as the boiler comes with a 10 litre one already?

I called Vaillant and they said it means I need a 23.4 minimum volume for the additional expansion vessel. But that doesn't make sense to me as the table would then also be requiring additional expansion vessels for below 100 litres as well which the docs says are taken care of by the 10 litre built-in!

Confused!

Help appreciated.

Picture of table attached.

Untitled

• Tim_R
• 7 Sep 2011
• 1

Many thanks

 

[Gasguru]

Most installers are clueless when it comes to expansion vessel sizing, hence, on larger systems it is very rare to find an additional vessel fitted and the systems often with pressure problems (going to high and opening the safety valve). The problem is compounded by the fact that most boiler manufacturers fit undersized vessels to cut costs.

Anyway, the table appears to be the total vessel volume required. Therefore you can subtract the 10 litre boiler vessel from the required figure. The system pressure is determined by the system height from the vessel to the higest point. For the majority of 2 or 3 story properties the vessel air pressure is set to 0.8 to 1 Bar with a cold system pressure of 1 Bar.

However, I suggest you fit the largest practical vessel. There is little cost between the vessels. Up to 24 litres can normally be wall mounted. A larger vessel than necessary offers an advantage of a much reduced pressure rise when the radiators are up to temperature and a reduced topping up frequency. The only downside of over spec'ing the vessel is that small water leaks are masked since it will take longer for the gauge to drop (but still better than with an open vented system).

 

[The831Bunny]

Gasguru is spot on. Just goes to prove how useless Vaillant tech is now, they can't even understand their own books...

The only other thing to add is that the minimum size vessel I would add is 12 litre, but as Gasguru says fitting a larger one than this causes little problems. I'd also advise an isolation valve and drain off on the vessel side to assist on yearly servicing too.

 

[Tim_R]

Thanks guys!