Vaillant 438 with 3 zones, frequent S53

[Agile]

My original installer too advised me against a system boiler for exactly the reasons of maintenance and parts changeability. So no real regret there. The only time I had wished I had installed a system boiler was when the Vaillant engineer said, users rarely see an S53 on a System boiler, because it modulates it's heat output downwards, if the system cannot cope.

Your 438 should also modulate downwards equally as well!

In practice, as you have discovered the 438s do seem to give more S53 problems.

A boiler should be rated on the heat loss of the house. Your's will not be very high.

You might think that extensions significantly increase the heat loss. But because of the current good insulation requirements most extensions make quite a small difference. In fact loft conversions often actually reduce the heat loss.

 

[ScottishGasMan]

For reference system boilers rarely suffer s.53 because the have a 7.5m head pump in them and good internal auto bypass which will help keep the flow rate through the main heat exchanger up even if the system can't take as much.

Heat only boilers with s.53 are very very often fitted with undersized pipes/pumps and fittings on dirty open vent systems with little thought to required minimum flow rates (which are stated in the manuals and the key to stopping these sort of issues)

 

[ScottishGasMan]

Should state the 7.5m head is on the big system boilers only

 

[bruce_miranda]

The 438 was probably there or thereabout when it was put in, because the house was an old cold 1930s 5 bed room with solid walls and there was no loft insulation. I think it's clear that the pipework sizing was wrong and the pump is too small. The pipework now is slightly better but the pump is still undersized. Over the time since it was installed, the heat demand has actually gone down. As @Agile says, the newer bits to the house are all well insulated, the radiators sizes have gone up and UFH installed. Also Evohome is used but that creates a new problem and that is what got my attention back to this thread. Because on Evohome, every rad can fire the boiler independently, what happens is that often the boiler will fire up just for that single zone. But with the rest of the zones closed the flow rate is quite poor and the small pump them struggles - S53!

 

[Agile]

That is where the auto bypass should come into its own!

 

[fezster]

An auto bypass (honeywell at least) seemed to me to be the most random device to set correctly. I remember I did the required calculations and used the pump curves to set it, only to find it wasn't working as expected. The "turn one quarter close" just before it passes water method seemed to be logical. Some people suggested always set it between 3 and 4, which seemed to assume most people have a standard system of some sort.

 

[bruce_miranda]

And that brings me to the ABV. Yes my installer didn't set the ABV and he just said, leave it at 3! It's a pity these boilers don't seem to have a flow rate reading, it would have been so handy when setting the ABV. I've seen some pumps that have it. They provide a minimum flow rate through the boiler in the manual, but how are you supposed to ensure it's getting that?

 

[Agile]

Flow is difficult to check without a gauge.

But smart pumps are almost incompatible with an ABV!

 

[denso13]

almost incompatible

Is that like "nearly impossible"?

 

[Dan Robinson]

And pretty rare too by all accounts.

 

[denso13]

And pretty rare too by all accounts

Pretty rare though is actually much more not uncommon than rare.

 

[Dan Robinson]

I'd estimate it to be 685000 which is precisely the quantity that determines the boundary between uncommon and incredibly infrequent. Perhaps even a mediocre tradesman could work it out but my education and bus pass preclude being insensitive enough to comment.

 

[denso13]

We don't agree. Especially if that tradesman is a plumber.

 

[Dan Robinson]

How rude.

 

[bruce_miranda]

Flow is difficult to check without a gauge.

But smart pumps are almost incompatible with an ABV!

Yup I learnt the hard way, that the Auto Adapt in my Alpha2 was useless. Now it's set to III all the time. Wasted money on that pump, quite frankly. But it is SO much quieter than the previous normal Grundfos it replaced. When I bought it, I should have bought a 25-80 then instead I listened to my plumber, who said to just replace the 15-60 with another 15-60 Alpha2.

Also if flow rate is just as important, then why isn't a permanent gauge installed, much like the pressure gauge? Seems to me like it is important enough to know that information. The other info can be read off the boiler.