Flow rate in central heating system

[D_Hailsham]

Cure for this is simple,down rate boiler so it matches your pump performance(you cant you need 36KW),increase pump speed(you cant its on full),increase pump size.

Your analysis of the problem is spot on, but there is another solution.

The cause of the problem is the very high loss through the boiler's heat exchanger, 4.05 metres, which does not leave much available for the heating circuit if a 5.5 metre pump is installed. Assuming a total head of, say 6 metres, the Grundfos recommendation is a Magna 25-60, which costs about £450, plus installation say £600.

The alternative is to separate the boiler from the heating circuit using a low loss header. The existing pump would be suitable for the primary feed from boiler to header and a much smaller pump e.g an Alpha 2 could be used for the secondary from header to radiators etc. Whether it would be a cheaper solution is another matter.

 

[Biggles..]

Dont fit a magna or an alpha variable speed pump this will cause the same problems due to the pump modulating down for the load,the boiler wiil see this as an increase through the HEX and trigger S53.

Fixed speed pump UPS25-80.

Lee

 

[Agile]

Put into simple terms the system has such a large volume that when starting up there is so much cool water returning to the boiler that the differential remains high for longer than the boiler is programmed to expact so it thinks there is a fault.

There is a simple work around to your problem.

As its a large system it should be devided into several zones and the UFH is a typical example.

All you need to do is to seperately time part of the system so that it does not all start at vthe same time ( or put a few minutes delay into part ).

In any case the UFH should be timed to start an hour or more before the rest of the system as it takes longer to warm up.

Tony

 

[Biggles..]

That's a poor excuse when the answer is so simple.It's not the volume bring high it's a high kpa within the boiler causing the issue.

I wouldn't fit a boiler then tell the customer to get it to work you can't have it all on together,might as well have fitted a smaller boiler sized for part of the system if running each bit separate.

 

[heatingman]

have a decent quality 28mm auto bypass installed and have it adjusted and set to maintain the correct flow across boiler.
The system is not balanced or set up correctly for the boiler.

 

[Biggles..]

the op has stated the heating is zoned,it has a bypass and its not a balancing issue.

The overriding problem with S53 on these products is pump performance see the graphs and do the maths.

the way i see it by altering the bypass to raise the return temp at the boiler all your doing is tricking the boiler, the boiler will modulate down and not give the full output thats required giving a longer warm up period and longer firing cycles than is required.

 

[Agile]

have a decent quality 28mm auto bypass installed and have it adjusted and set to maintain the correct flow across boiler.

At startup the whole system is cold and the TRVs all open thus giving the lowest flow resistance.

To set an ABV to be passing at that setting would mean the full boiler output would never be obtained.

The "fault" is within the programmed operating parameters of the boiler. We see this often on large systems.

My solution is free or cheap and in any case starting the UFH first would be part of the normal setting procedures for a mixed system.

Tony

 

[Biggles..]

Tony, Im not saying your idea isnt a working solution it is however a work round and compramise on the intended system design.

Maybe you and your customers would be happy with staggered heating times making the stats and programmers slightly more complex than need be, maybe the op will be quite happy running his heating,ufh and dhw at differing times.Personally i find that inadequate to have something installed that cannot be used in the worse case of needing all 3 to run together.

Having fiited lots of the 4series vaillants,there is another solution but again involves alteration.

I fiited a 438 onto a 37KW load(30 radiator,3 storey office conversion),boiler ran no problems on a 15/60 by pumped return.Never fired a S53.

 

[JohnD]

Tony, Im not saying your idea isnt a working solution it is however a work round and compramise on the intended system design.

if it works, why not just do it?

 

[numpty1]

if it works, why not just do it?

Because you are not repairing the fault just masking it.

 

[Biggles..]

john you know lots about electrics,it's like saying you can't have your upstairs lights on at the same time as down because the designer used the wrong sized Cable and the mcb blows everytime,so if you want to have upstairs lghting turn off down and vice versa.

I know they'll more than likely be split buts its an anology.the system shouldn't have to work that way to be correct.

 

[JohnD]

buit it would let edb49 warm his house while he saves up?

 

[Agile]

The "fault" is that this design perameter of the boiler cannot be changed ( as far as I am aware ).

Had I specified the system, I would have included it as part of the quote that the system would need to be started at different times.

Its normal to start the UFH early so with a correctly programmed system this "error" is unlikely to arise. In a 38 kW system it should have several zones so a few minutes stagering of start ups should not be any problem.

That would give the cheapest installation which is all my customers ever seem interested in.

Merely fitting a larger capacity pump will create too low a temperature diffferential once the system has warmed up.

Tony

 

[Biggles..]

It would but like I said it's a work round

I presume anyone that finds the time or the information to calculate what he has wants to know the real reason he's getting what he is.what he does about it is up to him.

 

[Biggles..]

The pump needs to match the load and be sized to give the right delta t the auto bypass and boiler modulation avheive the rest.

1.6m3/hr for 38kw @ delta t of 20degc