Replacement radiator is colder at bottom than it is at top.

[ChrisR]

Seems odd they'd say:
This information is based on 17°C and 20°C system design temperature difference.Well, you can't set it, so... eh?

I don't remember seeing anything other than 10/11 degrees for any noncondensing boiler.
I could be wrong..

The MI also say that the boiler has an internal bypass, which could be highly relevant.
But I don't think it has!
The previous model did

 

[D_Hailsham]

The MI also say that the boiler has an internal bypass, which could be highly relevant. But I don't think it has! The previous model did

The CB24 and CB28 do have an internal auto bypass; the CB24X and CB28X do not have an auto bypass.

The OP has a CB28X; so no auto bypass.

 

[DP]

DH wrote

If it is acting as a short circuit, what would the effect be and why?

DHW HE would present a lower resistance loop, therefore less heat to rads.

 

[fluff29]

Thank you to everybody for the various thoughts. I will try to clarify some matters.

1. No internal by pass in boiler as per manual. No external bypass. All
rads have TRV's fitted.
2. Pump replace on 11/8/10 by Alpha.
3. DHW comes on and off as it should, including when CH is on, boiler
will divert to DHW.
4. I do not have any abnormal light sequences. The boiler never locks out. We only ever get either the yellow on intermittently when boiler not on or yellow light on constant when boiler firing.
5. Diverter valve explanation.

The boiler was cycling and only coming of for about 1 1/2 mins at a time.

My boss, who is an engineer, came around and looked at the system, from my explanations of what has been happening, he thought it was a boiler problem. He thought it was the thermostat or the pump, althought he thought at the time that the pump was taking the water away, which it was and then his next thought was the diverter v/v. He asked me to try running the boiler for CH with the tap only open far enough for the boiler not to switch to DHW. I undertook tests with tap open a trickle, so CH came on. These are the last tests result that I showed on my post. I am assuming from him asking me to do this test that the CH should not have stayed on any longer just because the tap was on a trickle, and that this proved something.

I phoned Alpha on thursday night after I did the tests and Alpha told me that I had a blockage and it had nothing to do with the diverter v/v.

My boss disagreed with Alpha, he phoned Alpha technical on friday morning and Alpha agreed that the v/v should be stripped down, cleaned and replaced if necessary. I phoned alpha homeserve (I have a boiler maintenance contract) last night and they are arranging for the local agents to come back.


I said that I would do two clean sets of tests with the boiler from cold. One day with the tap open a trickle, but not enough to fire the boiler on heating and one with the tap closed.

I did the first test tonight with the tap slightly open and I will post the results tomorrow along with the test results that I do tomorrow with the tap closed. I also got a hold of the temperature guage again to check the boiler flow and return temps.

One thing that struck me tonight is that although the boiler is staying on longer with the tap open, I still have bad return flow to the boiler. The flow was 72 and the return was only 40.7.

Which makes me think that there is a blockage.

Does turning the hot water tap on a trickle which allows the boiler to run longer on CH prove that the diverter valve is not functioning properly, or does this prove nothing?

Will post again tomorrow.


thanks all

fluff




[/u]

 

[DP]

Fluff, to check the radiators, do NOT run hot tap.

When CH is selected, ALL the heated water goes to radiators (HW port closed in diverter)
When HW selected ALL the heated water goes through HW heat exchanger as radiator port is closed in the diverter.

If you have a tricle flowing at the hot tap, the diverter likely to be slightly open to allow water flow that should in fact be GOING TO THE rads. Valve could be open such that no water goes to the rads.

If you look at the drawing posted by Chris, you will see two pipe above the pump. With CH demand active, boiler runing, NO water flowing at the hot tap, place hand on each of these pipes after boiler has been running for 5 minutes. Judge if both these pipes are the same temperature. Now check the heat on the two fat pipes below the boiler. Is the temperature same as found on the two pipes in diagram posted by Chris?

Forget what your boss said as pump does not distingush between jhot water and CH- it just pumps the water. If you do the tests asked above, fault location might then just be possible.

 

[fluff29]

Hi DP

I will be running the boiler tommorrow with the hot tap closed. Therefore will feel the pipes at this time. Are there only two pipes above and below the pump?

Also, I don't think the boiler will run continually for 5 minutes with the tap closed. Should I therefore check the pipes after it has been on for 5 minutes in total (which may take about 4 cycles)? The boiler has only ever run for 19 minutes with the hot tap on and that was only the other night. My previous temperatures quoted at the beginning of this thread were when the boiler was cycling and only on for about 1 1/2 mins then off for 3 mins.

For my info, why is it not important that the boiler stays on longer on CH when the hot water is open slightly? I would have thought that this would not be right. The hot tap is not on enough for the boiler kick in on DHW and yet the boiler stays on longer in CH when the tap is slightly open.

Why does the boiler stay on longer when the tap is open slightly?

thanks

fluff

 

[ChrisR]

Why does the boiler stay on longer when the tap is open slightly?

Because the trickle of water is cooling the boiler a bit, so it takes longer to get het up .

This assumes that the diverter valve is opening a bit. But we don't really know whether it is or not, so we can't make dramatic deductions.
It may be (as I suspect) that the DV is "letting by" in normal CH operation , to HW when it should be closed.
If the tap ISN'T running, the HW heat exchanger (blue) would get up to boiler temperature and then not lose any heat, but it WOULD return hot water to the boiler thermostat, so the boiler would short-cycle.

I'll find a pic...

 

[fluff29]

thanks chris

That explains why the boiler is staying on longer.

Could there still be a prob with the diverter v/v?

Don't want to get homeserve out to look at it if there is no problem, no doubt they would charge me?

DP- looking back and one of your other posts, better clarifiy that I do not know if the CH has been like this for a wee while. Only moved in last august, I checked that we got heat to the rads but did not check if they fully heated up.

I then got 6 new rads put in, doubling them all up and replacing the bathroom towel rail with a double panel rad.

When the big problem one was put it, we then noticed that it wasn't fully heating, due to it starting to get cold heading towards winter. Plumber who installed the rads over a 3 month period did not flush and cleanse the system at any time.

hope this may clarify things

 

[ChrisR]

Tap water is green, hot is red, tepid is mauve
The shuttle/piston in the diverter valve (yellow) is shown not quite as far to the right as it should be, so some hot leaks past it through the HW exchanger ( 8 ) and back to the boiler.
The boiler temp primary sensor (5, top right) therefore gets hot and the boiler turns off.

To see if this is happening, feel the pipe arrowed with the fat red arrow, when the heating comes on, from all cold - or the exchanger 8 like I said before.

If it IS getting hot, you need a new diverter valve,
If it's not, it's something else, maybe a modulation problem. Or maybe both

This might not fit your page: if not, if it's open in Internet Explorer when you click the link, clicking the pic should make it shrink and grow
http://img690.imageshack.us/img690/4815/alphacb24xdvletby.jpg

Now I spose I'llhave to find where that pipe is...
(stood in front of the boiler it's all done in half a minute!)

 

[Agile]

To the OP.

Your problem is with the heating.

Touching the hot tap is just confusing the issue and establishing an incorrect set of conditions which have no relevance at all.

The more that I read of the on/off times the more that I want to turn your CH Cap pot down to a minimum!

Could you refresh why turning all the other rads ALMOST to the off position at the LOCKSHIELD valves does not affect the cool rad?

Tony

 

[ChrisR]

There you go:

 

[ChrisR]

Touching the hot tap is just confusing the issue and establishing an incorrect set of conditions which have no relevance at all.

COrrect!

The more that I read of the on/off times the more that I want to turn your CH Cap pot down to a minimum!

It could be.
I've remembered a couple of Alphas of similar vintage on very small systems though, and they don't have a problem.
Alpha diverter problems ARE common though. You can usually fix by removing the internal bits from tthe DHW diaphragm end and cleaning them.
Some of the earlier temperature sensors are also unreliable!

It'll be interesting to see what he solution turns out to be.


Of course a gas bloke would also test how much gas the thing is burning. OP - see FAQs !

 

[Agile]

He needs to do that at full power with temperature at max and a hot tap on full flow.

More relevant to this problem is the second part with the temp turned down to minimum and flow at tap at about 25%. The flow rate may need to be adjusted slightly to be the lowest at which the burner stays on.

Tony

 

[joop1977]

Is there a filter in the return on these boilers?

 

[DP]

He needs to do that at full power with temperature at max and a hot tap on full flow.

What has above got to do with radiators not heating correctly

More relevant to this problem is the second part with the temp turned down to minimum and flow at tap at about 25%. The flow rate may need to be adjusted slightly to be the lowest at which the burner stays on.

Again, your reasoning is makes no sense to me.

Fluff, look at my images. I have coloured the return pipe pink and flow red. If the boiler is shutting down because the diverter is passing, both the pipes will be about same temperature.

If the problem is due to blockage, pink pipe will be a lot cooler than red hot pipe.



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