Vented System - Reverse Flow - I think ?

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Hi Everyone

I think I have just wasted a few days changing my Central Heating pipework. From what I have read on the Forum I think I have introduced Reverse Flow !!!. Can anyone confirm what changes I now have to make it work correctly. :cry:

Now my central heating only works when two or three zones are open at once. If just one zone valve is open the Vaillant Ecotec 438 boiler shuts down after a short while indicating code S.53 - Appliance is within the waiting period of the operation block function due to water shortage (VL-RL spread too big)

I have attached a schematic of my system (thanks to ChrisR - I changed his original picture)

I have introduced an extra zone (house extension) and connected the return as the last 'T' connection before the boiler (Y) which seems to have messed things up. Previous forum posts I've now read say it should be HW connection last.

However the existing HW return has a 'T' connection directly with the old CH Return (X) and there is even a single radiator 15mm 'T' as the last connection into the 22mm return (W) but it still worked ok.

How come it works with two valves open but not one. Can one of you helpful experts confirm what changes I now have to make so that it works correctly again. :cry:


Cheers

 
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Personally I always connect (common) all the returns back at the HW cylinder.

Perhaps you have wired the valves incorrectly since this change?
 
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all your rad feeds should be joined before joining the common return as said.
 
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Things being teed back in the wrong place wouldn't stop your heating working. It WOULD mean that rads heat up when the HW or the "other" zone is on though.


Perhaps you have air locks so you aren't getting sufficient flow through one of the loops by itself?
If x y and z were all very close together you'd avoid reverse circulation problems


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If just one zone valve is open the Vaillant Ecotec 438 boiler shuts down after a short while indicating code S.53 - Appliance is within the waiting period of the operation block function due to water shortage (VL-RL spread too big)

According to http://www.diynot.com/forums/viewtopic.php?t=102703, "VL-RL spread too big" means that the differential between flow and return temperatures is too great. If you have balanced the system with both CH zone valves open and then shut one valve the pressure seen by the pump will increase and the flow rate decrease. This will cause the differential temperature to increase

Delta T = Kw/(4.2 X litres per sec)

The solution is to put an automatic bypass valve between flow and return, immediately after the pump. When the zone valve closes and the pressure increases, the ABV will open to allow water to circulate back through the boiler, keeping the flow rate high and the differential temperature within range.

Balancing your system will, in any case not be easy. Particularly when you have two branches in parallel, one containing just one radiator.

As others have said, and you have found out, the HW return (X) should ideally be between the boiler and Y.
 
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Yes there should be a bypass but the heating should still work on one circuit with one valve open - assuming some rad valves are open. See boiler MI for flow rate required.

No need to be paranoid about the order of the pipes' connections on the return. All that's important is that they're close together enough that the pressure developed along the pipe between them isn't enough to get water going round a parallel circuit.
 
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Thanks Guys for all the replies. If you have the patience would appreciate you reading through this. I have attached a picture of the zone valve install.

http://www.diynot.com/network/martin2527/albums/1263/5065

I think the introduction of the zone valves for different parts of the house has unearthed this problem as without zone valves the system works normally. But how do I now solve what I think is a flow problem?

When I first switched a single zone on this morning got a couple of code S.53 - Appliance is within the waiting period of the operation block function due to water shortage (VL-RL spread too big).

I tried the following to eliminate any air locks and stop any reverse flow:

Closed down the single rad both ends (W) and closed the gate valves on the flow and return of the HW tank (X). This would then remove any chance of reverse flow as they are both closed off. This left me with two radiator circuits served by individual valves. I opened Zone 1 valve and ran water through each rad individually with pump on full to ensure no air (7 rads). I then opened all rad valves on this circuit and switched the boiler on. Boiler started to heat up but when full burners came in overshot the set output flow temp 65 degrees (by 2 degrees) and went into constant 2 minute recycles showing flow output temperature flow as approx 45 degrees then up to 67 degrees and then back down and so on.

I'm still suspicious of air in the system - is there any other ways of purging. Have unobstructed open vent and have tried the pump through every rad above.

I then did the same with Zone 2 (8 rads) with exactly the same result.

As soon as I open both Zone 1 & 2 valves together the boiler runs normally.

With just the HW zone valve open boiler works ok.

Switched off system. Opened a single rad zone same initial S.53 followed by the recycling.

ChrisR - I checked the manual and the required flow rate is 27l/min. which seems high to other boilers.

D_Hailsham - The original circuit (Zone 2) does have an automatic bypass valve about 3m away due to pipework access constraints...but when it opens it does not make any difference to that zone working on its own. The boiler still goes into recycle mode. If I ever get this working should I have and ABV on both circuits?

I tried to phone Vaillant helpline (60p a min) but was told that as I was just a customer and not a registered installer so I couldn't talk to an engineer but they thought it might be a problem with circulation...thanks a lot Vaillant !!

:cry:

I think it will be call a plumber tomorrow but I hate to be beaten :)

All comments appreciated

Thanks Martin
 
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A simple first thing to always check on these boilers is that the ntc's are fitted correctl on a straight bit of pipe and not the internal curved bit where it will not sense the temps properly. Also that they are connected the correct way round.

Since Glow Worm have been building them, there have been many cases of these ntc's being fitted incorrectly.
 
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D_Hailsham - The original circuit (Zone 2) does have an automatic bypass valve about 3m away due to pipework access constraints...but when it opens it does not make any difference to that zone working on its own. The boiler still goes into recycle mode. If I ever get this working should I have and ABV on both circuits?
You only need one ABV, which must be after the pump and before any zone valves.
 
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Get an by-pass fitted between the pump and valves, an AVDO just under Z2 will do the job.
 
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Thanks again to all those that offered advice.

Corgi plumber hopefully coming tomorrow. I will post results.

Thanks

Martin
 

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