# Forum Home Renovation Heating & Cooling  Vulcan Ducted Heating Problem

## HandyDaddy

Hi all 
I have a Vulcan ducted heating system and have a problem with the heating unit. Its a 12MJ/h (33KW) heater model "Modulating Central Heater Ultra". 
The heating unit has a modulating gas valve. The valve has a diaphram that senses the pressure difference between the two ports (marked positive and negative) connected to the diaphram and adjusts the gas flow depending on the pressure differential.  
The rubber pipe on the "positive" input to the valve goes into a sealed chamber at the top of the heater. This chamber has the impeller of the combustion fan inside it and the pressure pipe just sits in this chamber next to the fan. I guess its detecting the air pressure in this top chamber. 
Therubber pipe on the "negative" input to the valve connects onto a pipe stub coming out of the heat exchanger. I guess that its sensing pressure in the heat exchanger ? 
I've been having problems where the heater wont light up. There is no gas coming through the valve. It I blow into the "positive" pressure input when trying to start the heater, the valve opens and the gas flows. The heater fires up. Alternatively - if the negative pipe is disconnected from the valve, so the negative side is at atmospheric pressure, the heater starts. With both pipes connected as they should be the heater has trouble starting. It may work for a while after playing about with it as above, but its not reliable. 
I've now had 2 service engineers out to look at this but neither of them knew how it worked properly so could not find any fault.  
Can someone help by explaining how this system works and what pressures these pipes are comparing to control the valve. 
Thanks
Mike

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## droog

I am by no means an expert in this field but until someone with better knowledge come along here is my understanding of what you have. 
The pressure switch in central heating units do not vary the amount of gas flow, they are there as a safety lockout to prevent operation of the heater in a fault condition. If conditions are met gas will flow if not there will be no gas flow.
These heaters use a combustion fan, the pressure switch looks for a differential between the heat exchanger compartment and the flue indicating that the combustion fan is running and drawing air correctly through the heater unit, it will also detect if there is excessive back pressure in the flue indicating a blockage.
The amount of pressure measured is only minimal so any leaks can effect operation. 
I would start by checking that there are no holes or seals out of place that could be allowing air leaks, also that there are no obstructions to the flue. Last check that the rubber hose used to connect to the switch is not cracked and that connections to any nipples are firm, it may be required to trim the hose or replace it if the ends are stretched.
While checking the hose ends see if there is any sign of small rubber chunks, if the rubber has started to perish there may be small pieces broken off and blocking flow to and from the switch.

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## HandyDaddy

Hi droog 
Thanks very much for taking the time to reply. 
First of all i had got the wrong idea of what the pressure differential was for, so thanks for the tip off on that. With your enlightenment the problem now seems to become apparent. 
I have one pressure pipe detecting positive pressure from the combustion fan. I can feel the positive pressure in this pipe and indeed can see it by putting some spit on the end of the pipe and watching a bubble form.
On the second pipe (earlier today) I also had positive pressure, or in fact a bubble of spit (sorry no soapy water to hand). I went back and rechecked again after reading your reply. No positive pressure in the second pipe now and heater starts fine. 
After your comment about back pressure from a blocked flue, I started to investigate the flue. I took off the flue pipe and looked inside the heater where the flue connects. Ah - there is the negative pressure pipe poking into the heater exhaust barrel angled in the direction of exhaust flow. OK this makes sense. As the bad air is expelled out through the flue this will cause a negative pressure in the second pressure pipe and along with the positive pressure from the first pipe will cause the pressure switch to turn on the gas.
But the flue is not blocked. Why did I get positive pressure in the flue earlier today? Probably from the wind! 
I went out and took a look at where the flue is. Its about 1m from the apex of the roof and also below the height of the apex. Additionally there is no cowl on the pipe.
I'm guessing that we are getting down-draught causing back pressure in the flue under certain wind conditions. 
The heater used to work fine until winter about three years ago when it became very intermittent. The summer before that winter we had a new roof put on. I'm guessing that the roofers may have removed a cowl or even extension that was on the flue and its never been put back on. 
All just a theory at the moment, but it sounds like a very probably explanation. 
Next stop is Bunnings, then up on the roof. 
Thanks again for your insight.

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## DavidPM

Hi there,
just wondering if you ever got yours working again OK? I have almost the exact same problem by the sounds of it. The first time it tries to start, the ignition 'spark' clicks away, but it doesn't fire up (you can see this through the little glass window). Then after a few mins it tries again. It usually starts the second time and the blus gas flame can be seen, but after about 30 secs when the fan ramps up, the flame goes out and I get a '8 flash' error which means 'Gas interruption'. I'm guessing the pressure goes a bit funny with the fan speed change and the gas switch turns off the gas supply?
Anyway, normally after another reset or so, it finally all works?!? Very frustrating, especially on a cold morning!
So overall, i'm guessing that pressure issues in the positive or negative side are causing the gas to be not flowing for the original ignition fault and the following 'blow out' when the fan speeds up.  
Hoping you worked out how to fix yours and have some tips!  :Redface: ) 
Cheers,
DavidM

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