Vaillant 438 with 3 zones, frequent S53

[Steelmasons]

If that's a 20C dT per zone you get:

Zone 1: Flow = 75; Return = 55
Zone 2: Flow = 70; Return = 50
Zone 3: Flow = 65; Return = 45
Zone 4: Flow = 60; Return = 40

If it's a 20C deltaT at the boiler then it's a different game. I'm still working on that one.

Obviously I was referring to the primary flow temp after it passes each zone , your still working on it?

75c flow temp.
Total flow rate = 0.95 l/s (80kw)
Zone flow rate = 0.238 l/s (20kw).

1st zone calc.......

(0.72 x 75) + (0.23 x 55)/0.95.

 

[D_Hailsham]

your still working on it?

No. I posted the results in my last post.

 

[fezster]

Well since you asked nicely I will explain
Let us say that the op decides to go for hydraulic separation between the boiler and the system and installs a LLH
He finds that by pure chance the spacing between the flow and return is such that all he has to do is cut out 2 sections of pipe and slot the LLH straight in, happy days!
so he gets himself a suitable pump to fit on the Primary and fits It
He then learns that because of the fact it now doesn't have to pump through the boiler its now suitable as the secondary Pump even more happy days
Now
That's all he has to do mate!
he has got a perfectly working S-Plan system (think of the LLH as a heat because, thats what it is)
no need in his case for two secondary pumps,nor check valves etc
He may need to tweek the auto bypass but thats already installed and it would need tweaking anyway but the boiler or boiler pump dont need it
OP pleased and the Boiler as happy as larry

all a Low loss header is,is a packaged set of "closely spaced tees" with some nice extras thrown in the working principle is the same

That took me bloody ages!
Matt

Some great info in the last 5 pages, but unfortunately I've been distracted with bigger issues.

I do think the LLH route sounds like the most straightforward (though, perhaps not the cheapest). I'm going to ring some local heating engineers to get quotes. If it gets the boiler working fine at full power and heats everything up, that's what I want. I'll stipulate that as a condition maybe !

If anyone has a list of pointers or good questions to ask potential fitters, please do let me know. Reading above, it seems Vaillant sell a LLH which would probably be the most ideal to fit ?

Thanks.

 

[bruce_miranda]

I realise that this thread is really old. But how did @fezster fix this issue in the end? And what was the end result. Were you able to run the boiler at it's full 38Kw?

 

[fezster]

I fitted a 25-80 pump which helped immensely. However, I still cannot run at the full 38. I range rated to 32 I think and everything heats up fine.

I think adding more 28mm pipework may help, but given it's such a large system, I believe a LLH would be the ideal solution. The system is working so I've left as is.

 

[bruce_miranda]

That is good to know. I too have a 438 boiler that was fitted incorrectly by an installer (yes he was Gas Safe registered, before someone asks!). It too suffered from the S53 and it is too big for the house. Initially it was fitted with 22mm tails and we needed to wind the boiler all the way down to 24Kw for it to stay on. Now it's running on 28mm pipework to the first T and that made a lot of difference. Now the boiler is set to 30Kw and works quite well with a normal Alpha2 15-60 pump. I've not even tried to run it any higher than 30Kw simply because it's not needed. A large proportion of the house got converted to UFH and I run Evohome zoning so we certainly don't need a boiler that big.
The reason this thread caught my attention was because recently a Vaillant engineer told me that I should have used a system boiler, if I wanted to avoid all the headache. According to him a system boiler has the ability to gradually ramp up it's heat output and stops at a point when it believes the system cannot cope any more. This sounds remarkably similar to what the revised PCB in the 438 is programmed to achieve. Hence was keen to know which direction was taken in the case above. i.e. new pump or PCB.

 

[fezster]

I had a new PCB fitted and did notice it improved things a little. The boiler ramps up to full power on first start up, and then modulates down. The new PCB (from memory) made it modulate back a little quicker. It certainly seemed to help anyway.

I don't think a system boiler would be any better. Someone more knowledgeable will elaborate, but I believe a system boiler just integrates the pump and expansion vessel. I don't think it would be any better at coping with this scenario.

Also worth me mentioning that my boiler was not oversized. It's an old house and very large. However, it's the combination of size of the system (and therefore the need to push the water a lot further) plus the undersized pipework (relatively), and the two heating zones - which is the problem - hence the recommendation for an LLH by a number of people.

 

[bruce_miranda]

Oh, so you did have the new PCB too, not just the new pump. How did you go about finding a 'new' PCB, if the part number for both were apparently the same?

 

[fezster]

It was a vaillant engineer called out under warranty. He said he knew about the upgraded PCB and had one available to fit.

 

[AGAS]

bruce_miranda what size house do you have ? and how old is your boiler ?

 

[Agile]

I think that an 18 kW would probably suffice and certainly a 24 kW.

Cannot understand why so many installers like to fit oversized boilers!

 

[AGAS]

Cannot understand why so many installers like to fit oversized boilers!

Especially a system boiler as well

 

[Agile]

I suppose there can be good arguments for fitting a system boiler.

Probably mostly as a quicker job for the installer.

But with maintenance in mind I would always prefer a heat only boiler.

 

[bruce_miranda]

My 438 was installed in 2010. The 38KW boiler would have been fine if we had used the original radiators and did all the house extensions we had planned to using radiators. Instead we actually landed up changing all the radiators to bigger ones and 75% of the downstairs got converted to UFH. That removed about 5 existing radiators. So today I doubt we need a 38KW boiler. But as I said, I have never been able to run the boiler at the full 38KW even if I needed it - so thank goodness I don't.

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.

 

[Dan Robinson]

Vaillant's auto adapt feature is pizz poor. Nearly all I come across get range rated to stop strange cycling issues. Seems like the installers in my area use the Spinal Tap sizing method.