Central Heating boiler safety cut out

Page 1 of 2  
Hi,
I'm having a problem with my central heating boiler, the safety cutoff keeps 'popping' out when ever the room thermostat is *not* calling for heat. Looking at my system I can see the flow feed from the boiler passing through the pump (high speed setting), through two 'T' piece's, and then the motorised valve (and onto the rads). The two 'T' pieces are for (I think) a) a loop circuit and b) to feed the hot water storage tank. There are also two gate valves on these two pipes.
Can anyone give me ideas on why my system keeps cutting out like this?
How open should the two valves be? Should the loop valve be more closed than the others?
Thanks in advance
Danny
PS: I've just flushed the system completely, used cleanser for a week, flushed twice, re-filled and treated with inhibiter. Boiler does not cut out when the room thermostat is calling for heat
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

When everything *stops* calling for heat, and the boiler burner shuts down, the water needs to continue circulating for a little while. If this does not happen, the residual heat in the metal parts of the water get transferred to the static water inside - causing the temperature to rise to a point where the safety cutout operates.
In order to prevent this, two conditions must be satisfied: 1. The pump must be controlled by the boiler - so that it goes on pumping until the boiler has cooled down sufficiently 2. The water must have somewhere to go! If you have individual motorised valves for central heating and hot water, there *must* be a by-pass circuit to allow the water to circulate when both of these are closed. [If you have a single 3-port valve, this is not usually a problem - since it can never close both of its outlets at the same time].
The first thing to check is that the pump is wired correctly, so as to be controlled by boiler, as described above.
Then - if you have separate motorised valves for CH and HW - identify the by-pass circuit, which should short-circuit the boiler flow and return pipes without going through any motorised valves or through the CH or HW circuits. The by-pass may well have a gate valve to stop *all* the flow from going straight back to the boiler without going through the CH or HW circuits. If this gate valve is fully closed, you ain't got a working by-pass!
Start by opening it fully, and check that it cures the problem. Then close it progressively in order to find the position where it is open as little as possible without causing the boiler to trip.
HTH, Roger
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hi Roger,
Thanks for the info. The pump runs for approx 10 minutes after the boiler shuts down, I guess this means its wired correctly.
The two valves (one for HW tank, and the other for by-pass) were only slightly cranked open. My concern for opening the by-pass valve fully was that the water would simply travel the easiest path, thus reducing the efficiency of the CH when the motorised valve is actually open. However I take your point about opening it fully and workng backwards to find the 'best' position.
With regards to the valve that regulates the water supply to the HW tank, do you recommend that this be fully open also? Can adjusting this vavle help to stop my boiler from short-cycling? (I think thats the correct terminology!)
Regards,
Danny

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The valve on the HW circuit is there to balance the flow when the motorised valves for CH and HW are both on - i.e. to make sure that most of the water goes to the radiators. You need to find the right position for this, so that the HW gets hot in a reasonable time without stopping the radiators heating up quickly. I would experiment with positions around half open - or maybe a bit less.
Assuming that the boiler only trips when both motorised valves are closed (i.e. when CH and HW demands are both satisfied) the position of the manual valve on the HW circuit won't affect this. It only comes into its own when both motorised valves are open. [If the boiler trips when the HW is still being heated, this valve definitely needs to be opened more].
Incidentally, "short-cycling" is something different - and not what I believe you've got. Short cycling is when the boiler runs for a few seconds at a time and then go off. And then comes on a bit later, and does it all over again. AIUI, your problem is that the overheat stat trips - and has to be re-set manually before the boiler will light again. Is this right?
Roger
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Again Roger, thanks for all of your help.
Thats right, it only trips when the CH motorised valve is closed. I fully opened the by-pass valve, with the CH circuit off, I noticed that the water wasn't passing through the by-pass valve, massive temperature difference on either side of the valve. I'm thinking that this valve may be faulty and is stuck closed (it's not motorised), should I replace this or is there another method of fix?
Regards

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Sounds like you need to replace the valve. You might consider using an automatic by-pass valve - which only opens when a certain (adjustable) pressure is reached, so that nothing goes through it when either or both CH and HW circuits are operating. Screwfix do a suitable valve for about 25.
Roger
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Sounds knackered, just like most gate valves ten minutes after installation. When you replace it, use an "Automatic Bypass Valve" instead of a gate valve. This stays closed when the zone valves are open, but when they close, it detects the pressure differential from the pump and opens fully. This way, you get an effective bypass and no short circuit when the system is actually working.
Christian.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Thanks for the advice. I will give the automatic by-pass valve a go and let you know how I get on!
Again, thanks for Roger, Chrisian for all of your help
Regards,
Danny

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I noticed that when I removed the gate valve, the pipe was completely full of sludge - I would have thought the Fornox sludge remover would have fixed this! Anyway I've clean the pipe as best as I could then ran the pump on full. My system now gets to 65 degrees, a 15 degree improvement!!!
Thanks for the advice! snipped-for-privacy@blueyonder.co.uk (Danny) wrote in message

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 22 Oct 2003 06:15:38 -0700, snipped-for-privacy@blueyonder.co.uk (Danny) wrote:

If the system is as sludged as this, then a chemical sludge remover won't make too much impact. There is, however, likely to be a fairly low flow rate through the bypass, so it can be a collection point for grot.
When you have some more time, you might want to try a more thorough clean of the system by flushing the radiators separately - I've posted on how to do this recently. If you can get rid of most of the sludge mechanically - i.e. by flushing through with a hose, and then treat the system with chemical sludge remover, you will get much better results
Also, if you have gate valves, apart from the bypass which is best with an automatic valve, it is worth replacing them with lever ball valves. These shut off properly and easily and are much less likely to seep. .andy
To email, substitute .nospam with .gl
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
one problem after another! Now I'm back to my orginal problem, CH valve shuts off, boiler cuts out! Interestingly enough though, when I fully open the gate valve into the HW storage tank, turning the pump on full speed, you can hear water rushing through, air being expelled, and then gushing water into the FE tank? I've heard about over-pumping (?) but why would this cause water to be expelled though the FE tank this way? I would have thought that considering the pump is sited after the pipe work leading to the FE tank (ie water is being sucked rather than pumped) that the water would only flow up this pipe if there were a blockage?
(Danny)

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 25 Oct 2003 10:54:56 -0700, snipped-for-privacy@blueyonder.co.uk (Danny) wrote:

If the feed pipe and the vent pipe are on opposite sides of the pump, pumping over or sucking down of air will tend to occur depending on which way round they are. This would in any case be a fault in the design.
However, you can get the same effect if they are both on one side but that there is a pressure differential between them - it doesn't have to be very much. This can happen if they are connected to points on the circuit that are too far apart or if there is a flow restriction. Winding up the pump speed will exacerbate this.
Based on what you've found at the bypass, I suspect that other parts of the system are full of sludge as well......
I would suggest you drain the system and if you can undo a fitting near the points that the feed and vent pipes join the system see if there is more sludge, or take a look at a nearby radiator.
I'd be willing to bet that there is a lot of grot in the system.
.andy
To email, substitute .nospam with .gl
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
(Danny)

Or it could be a leak from the coil in the HW tank.
--
www.basecuritysystems.no-ip.com

Outgoing mail is certified Virus Free.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sat, 25 Oct 2003 20:01:56 GMT, "BigWallop"

True, although generally those seem to start as small seeping with the FE tank filling slowly and water through its overflow.........
.andy
To email, substitute .nospam with .gl
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

That's the bit that's getting to me too. That's why I first thought of an over heating problem, but turning the cylinder 'stat down didn't seem to help. Others have also brought up an over speed on the pump, but adjusting this also didn't help. Now it's just still a mystery. :-))
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hi Andy, I've followed your suggestions from other postings regardin flushing the system, here's what I've done so far:
1. Drained system, noticed water was clean 2. Removed each rad and flushed through with hose pipe 3. Ruined a carpet in the process! 4. Removed pump, inspected, no sludge build-up 5. Checked by-pass circuit, no sludge build-up 6. Half filled system 7. Added heavy duty central heating restorer (sluge, scale remover) 8. Continued to fill, bleed system
Three days later I still have the same problem, the boiler keeps popping off. The rads get hot really quickly so I don't think that there is a circulation problem. The by-pass circuit is 15mm with an isolater valve (b&q 74p!), again no sludge here.
To combat the problem so I can at least have some heat in the house I have to open by-pass fully and open the 22mm gate valve feeding the HW coil to 3/4 - full. but the rads don't get to full temp with this config. I set the HW circuit to approx 1/4 open, rads getting piping hot, room thermostat pops off, boiler safty cut-out soon follows!!
Could it be the boiler thermostat? If so, can I replace just the thermostat, and approx how much are they?
BTW, temp of pipes is approx 65 degrees
Regards,
Danny
(Danny)

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 3 Nov 2003 14:06:05 -0800, snipped-for-privacy@blueyonder.co.uk (Danny) wrote:

It sounds like it could be and normally you can. Did you say which boiler you have? If the thermostat is one of the capillary type then it is pretty easy to replace - push on connectors normally. I replaced one on a Glow Worm about two years ago and IIRC the replacement was about 30.
If you have a boiler with electronic controls, then Geoff at CET is your man - his company reconditions them.

Do you mean flow or return? 65 on a return is within spitting distance but on a flow is too low. If anything, you should be closing the bypass to encourage more water around the heating circuit.
Have you checked the diverter or other motorised valves? Does the situation improve with higher pump speed?
If the boiler has good flow and is cutting out with a flow of 65 degrees then evidence does point to the thermostat or possibly an over temperature sensor.

.andy
To email, substitute .nospam with .gl
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
The temp is 65 degrees on the flow pipe. I've tried closing the by-pass completely and opening the HW circuit fully, hoping that the HW circuit will act as a by-pass.
If I put the pump on full speed then water begins to flow into the FE tank - quite alot too. I have to keep the pump on medium to avoid this and also ensure that by-pass and HW coil are not both fully open (if they are water still comes out of the FE tank, but just a trickle)
How do I check the motorised valve? When the room thermostat calls for heat I can hear the MV opening so I assumed that it works.
Is the boiler thermostat something that I (a novice) could do? Do I have to be corgi registered or something?
Thanks
(Danny)

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 4 Nov 2003 00:20:16 -0800, snipped-for-privacy@blueyonder.co.uk (Danny) wrote:

OK. There are a few possible reasons for pumping over in this way. However, the root cause is the same, which is that there is a large enough pressure differential between the feed pipe at the bottom of the FE tank and the vent pipe over it. The pressure difference doesn't need to be a lot for this to happen - perhaps as little as 30cm of head. THe problem either manifests itself as pumping over or sucking down of air, depending on which way round the differential is. It is bad for this to be happening because it introduces air into the system and that is an enabler for corrosion.
This can happen because:
- The pipes are connected to the system on opposite sides of the pump. This one is a real mistake of installation and should never happen. If the pipework is like that it really needs to be corrected.
- The pipes are on the same side of the pump but there is a pressure differential between them. This could be the boiler or it could simply be that they are connected to points too far apart on the system pipework. It could also be because there is a part obstruction of some kind between the two places.
Could you trace where they are connected and then post back approx distances between the major items of the system - e.g. boiler, pump, motorised valve(s), points where feed pipe and vent pipe are connected. It's useful to use ASCII art for this to make a simple diagram, or perhaps you could sketch one and put it on a web server? It's really to be able to see the layout more than anything else.
As I say, this problem should be fixed but I would deal with it if necessary after resolving the temperature issue. For diagnosis purposes of this, it doesn't matter if there is pumping over for a short while. One solution can be simply to have the pump on the lowest speed, but that may then not be fast enough to run the system properly. The issue is that heat transfer rate from the boiler depends on pump speed. However, if the pump speed is too low for the system requirement, the effect would be that the boiler flow will get hot - up to 80 degrees+, as will the flow side of the radiators, but the return side of the radiators will be relatively cool and the boiler will tend to cycle on and off - simply because you are not getting the heat away fast enough.

OK. You can check its movement by operating the manual lever on the side. You can also take the head off and turn the valve stem underneath by hand to see that it is reasonably free. These valves have either a paddle or a rubber ball inside which is rotated to block and open the ports as required. I did have one once where some crud from the original installation (a piece of wood) had found its way around the system and lodged against the ball, partly blocking the flow. I would just do a simple check for rotation at this point. You can also take the cover off of the valve head and watch the mechanism working. If there is any obstruction or stiffness you will tend to hear it from the motor and their may be the sound of gears jumping. However, I don't think that the valve is likely to be No.1 contender at this point, from what you describe.

The law, in the form of the Gas Safety (Installation and Use) Regulations, 1998, states (section 3) that you have to be "competent" to work on gas fittings. It doesn't define "competent", but does require self employed and employed fitters and businesses (in effect) to be CORGI registered. There is a definition of "fittings" (gas carrying parts) and "appliances". If you read through the document
http://www.hmso.gov.uk/si/si1998/19982451.htm
Part E talks about work on gas appliances.
Changing a thermostat should not involve doing anything with gas carrying parts such as the burner, gas valve, etc. If the boiler is a simple one with a capillary tube and bulb on the heat exchanger or flow pipe, it is a relatively simple job to change it. Normally the control itself, behind the knob, has 4-6 wires with spade tags that push on. The main point here is to label everything........ There are the obvious points about making sure that any parts removed are properly refitted, all seals are in place etc. etc.
The acid test is really whether you are "competent". If you don't feel 100% sure about what you are doing, then it is better to involve a registered fitter.
.andy
To email, substitute .nospam with .gl
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hi Andy,
I've sketched a diagram using word, its not too clevor but I'm sure you'll see how my system is setup.
Visit
http://dognet.no-io.com/ch.jpg
My boiler is just a standard traditional one
I've located the manual lever on the motorised valve and operated this whilst the cover was removed, it sounded free from obstruction, the cogs inside were turning. However when I let go the vavle returned to the starting position and then a small amount of water came out of the FE tank. Is this relevant to your diagnosis?
Regards,
(Danny)

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

    HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.