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

[Agile]

You done seem to have told us the make and model of boiler but it seems that its a Vaillant.

This boiler cycling is likely to be as a result of the heat output being set too high and the anti cycle for too long.

There are settable in software but I dont know if you would be able to manage them. Most installers cannot!

Tony

 

[D_Hailsham]

You don't seem to have told us the make and model of boiler but it seems that it's a Vaillant.

Yes she did, back in last November. It's an Alpha CB28X.

This boiler cycling is likely to be as a result of the heat output being set too high and the anti cycle for too long.

That's what I said last November!

Whole House calculator (last November!) showed that a 12kW boiler is needed, including 2kW for a HW cylinder (I know the CB28X is a combi.), which means the heating requirement is 10kW. Her rad total is about 13kW.

The CB28X modulates between 11.2kW and 28KW. So it is massively oversized for the heating requirement and will be short cyling all the time.

 

[fluff29]

Hi D_Hailsham

Thanks for your thoughts again.

What I would ask of everyone else is if your thoughts on the boiler not being able to modulate down low enough would give these symptoms if I did only need 10 kw heat in my house and with 13kw of radiators.

If anyone can concure, hopefully agile, then I may have the answer and will just get a new boiler.

No other plumbers that have come to look at the boiler have agreed with this, including the alpha engineer.

Thanks

fluff

 

[Agile]

This is a massive powered boiler to be fitted to a microbore system.

However, it should be able to work, but will need setting to MINIMUM power output and minimum anticycle delay time.

I dont have time to look up the settings and these would be best changed by an engineer.

Independent service agents are not all very good as they are people short of better paid work who have to do those visits for less than £45 including all travelling in most cases.

Paul Barker is an ex-service agent for Alpha and if he is about should be able to say what resetting is possible and if he thinks its suitable for a DIYer.

The danger is that inexperienced tampering with engineer settings can make the boiler unusuable. In some cases only the manufacturers staff engineer has the codes to reset them.

Pity its not a Vaillant as their settings are reasonably useable by a good DIYer and many know the important ones by heart.

Tony

 

[fluff29]

Hi Agile

So do you think this could this be our problem all along?

Would you say it is not worthwhile spending money on powerflushes, fitting new pipework, etc if the boiler is too powerful.

If it is too overpowered, would I just be better getting a new boiler fitted?

If I left the boiler as it is, and got 15mm pipe fitted to our big rad, would we get more heat going into it with the boiler that we have, or will it not make much difference?

Can't believe the plumber fitted this size of boiler to this house, if it wasn't necessary.

Just going to go back and look at what D-hailsam said originally about the oversized boiler.

Thanks fluff

 

[fluff29]

Hi all

I have decided that I am a bit thick.

I still don't get how the boiler is oversized and what modulating down means.

The water heats up really quick, no question about this, but I don't understand why the water gets so hot that it cuts out the stat when:-

1. None of the rads are up to temperature.

2. Why does it cut out when the return temperature is still cold? Or is the heat not governed by the return temperature?

Is it because the pump cannot circulate the hot water around the circuit quick enough?

Sorry for stupid question.

Fluff

 

[D_Hailsham]

I still don't get how the boiler is oversized and what modulating down means.

Let's start with modulation. Basically this means that the boiler output is automatically adjusted to meet the immediate requirement. A bit like the way a car accelerator adjusts the power of the engine. Before modulating boilers were invented, the only way of controlling a boiler was by the boiler thermostat. The boiler ran until the thermostat reached the set temperature (about 80C). It then turned off, but the water continued circulating. When the water temperature had dropped to about 70C the boiler would relight and the cycle restart. This is known as on-off working.

A modulating boiler automatically adjusts the output to meet the demand. So, if you have a 20kW boiler but the heating demand is only 15kW, the boiler will modulate down, by adjusting the rate at which it burns gas, to supply 15kW.

I don't understand why the water gets so hot that it cuts out the stat when:-

1. None of the rads are up to temperature.

Every time the boiler start up it runs at maximum output (28kW). As this is over twice the total rad output, the water temperature rises extremely quickly. The boiler will try to modulate down. When it has dropped to minimum output and the temperature continues rising, all it can do is turn off and wait for the water to cool down.

2. Why does it cut out when the return temperature is still cold? Or is the heat not governed by the return temperature?

Some boilers monitor the flow temperature, some the return temperature and others both flow and return temperatures. The CB28X has only one temperature sensor which monitors the flow temperature, which is set by the dial on the boiler.

Is it because the pump cannot circulate the hot water around the circuit quick enough?

That's part of the problem, but nothing can be done about it as the pump is an integral part of the boiler and is already set to max speed.

One thing you need to understand is that the heating requirement in your house is not a constant 10kW. It varies with the outside temperature. The 10kW figure only applies when the outside temperature is -1degC. I the outside temperature is 10C, you will only need 5kW of heating. As the number of days a year that the temperature is below 5degC is very few, the required boiler output is normally much less than 10kW. THis means your boiler will be running in on-off mode virtually all the time.

 

[fluff29]

Hi again

Had another company around tonight. The guy that came around is a gas-safe engineer.

He was very much trying to sell us lots of different types of work which may need done and I told him what has happened to date.

He asked if all radiators except for problem rad get hot. We said yes. Also, told him about draining off water. He doesn't think that we have a blockage in the pipes especially when there is not alot of water discolouration.

He said that the pipework may not be piped right, ie piped upstairs to come back down to feed a rad. I don't think this is the case, looks like a manifold upstairs and downstairs with flow and return pipe running up inside of cupboards to the upstairs.

He then said that the modulating thing was rubbish and that he was the one that needed to worry about the boiler modulating. He explained that the boiler runs on a high flame for DHW and a low flame for the CH.

We originally asked them in to provide a quote for the repiping of the rad and drain valves.

He said that he would recommend the following and will quote for these:-

1. repipe the big rad in 15mm plastic pipe, with copper sticking out the floor (so that we don't need to take up too many floorboards).

2. Fit the drain valves(my request).

3. Provide a quote for a new boiler.

He thinks that the heat exchanger may have went on the boiler - but he had no basis for this. If this has happened, he said that there would not be much difference labour wise between fitting the boiler or fitting a new heat exchanger (4hrs + £400 for the exchanger or 8hrs to fit a new boiler). We would therefore have the additional expense only on top of this for the new boiler. Boiler will be a WB 28i junior condensing around £1800 with hot flush and change to pipework to boiler.

We then said that we would probably get the rad repiped and the drain valves fitted first, and if this didn't improve matter, possibly get the boiler replaced.

He said that this would be an awful expense as we would be paying for him to drain the system down twice. He thought we should get the repipe and boiler done at the same time. I didn't think much time was involved in draining the system.

I asked if we would need to get a powerflush done for the warranty on the new boiler (it will be a worcester bosch condensing boiler). He said not necessary and they would do a hot flush with some cleaner in the system.

I was not any further forward from the conversation other than the guy trying to get me to spend a few thousand on a boiler that he could not provide didn't work.

I then phoned the Alpha technical department.

The guy told me the following:

1. He thinks the boiler is working correctly.
2. He said that the boiler works at 80% efficiency and the hot water needs to be taken away from the boiler quickly enough. He said that the microbore is probably not up to the job and recommended that it was probably better to repipe the whole house in 15mm and this will allow the boiler to work better.
3. As a minimum, he said that I should first try repiping the problem rad to see if this takes the water away quick enough.

At the moment I have the option of fit a new boiler or repipe the system but the boiler may still not work properly.

The alpha guy also said that I will probably need to repipe the whole system in 15mm anyway as condensing boilers work at a lower temperature and the pump needs to pump quicker to get the hot water to the rads quicker. So I may need a complete new system.

So fed up now. At least my husband experience first hand the conflicting reports for the plumbers and we are non the wiser.

My boss thought it may be a faulty diverter valve. He asked me to turn on the hot tap with the heating off to see how much water flows before the boiler cuts in and then turn it back until the boiler goes off, but water still coming out of hot tap. Then turn the heating on and see if the heating comes on and the hot water tap gets hot. When I did this, there was water going down the CH flow pipe and the water from the tap heated up abit at the same time. The plumber that came this afternoon said that this proves nothing and that the diverter valve is not faulty.

Is what he said correct?

fluff

 

[tamz]

I then phoned the Alpha technical department.

The guy told me the following:

1. He thinks the boiler is working correctly.
2. He said that the boiler works at 80% efficiency and the hot water needs to be taken away from the boiler quickly enough. He said that the microbore is probably not up to the job and recommended that it was probably better to repipe the whole house in 15mm and this will allow the boiler to work better.
3. As a minimum, he said that I should first try repiping the problem rad to see if this takes the water away quick enough.

This is about the best advice you have had.

 

[D_Hailsham]

Had another company around tonight.

He said that the pipework may not be piped right, ie piped upstairs to come back down to feed a rad. I don't think this is the case, looks like a manifold upstairs and downstairs with flow and return pipe running up inside of cupboards to the upstairs.

He's just clutching at straws.

He then said that the modulating thing was rubbish and that he was the one that needed to worry about the boiler modulating. He explained that the boiler runs on a high flame for DHW and a low flame for the CH.

He's the one talking rubbish. Yes, the boiler will run at max output for hot water, but for CH the flame will vary depending on the heating requrement. The problem is that your boiler is so oversized that low flame still provides more heat than the rads can dissipate. So the water temperature quickly rises and the boiler cuts out.

He thinks that the heat exchanger may have went on the boiler - but he had no basis for this. If this has happened, he said that there would not be much difference labour wise between fitting the boiler or fitting a new heat exchanger (4hrs + £400 for the exchanger or 8hrs to fit a new boiler). We would therefore have the additional expense only on top of this for the new boiler. Boiler will be a WB 28i junior condensing around £1800 with hot flush and change to pipework to boiler.

He's just looking for an excuse for selling you a new boiler.

I didn't think much time was involved in draining the system.

There isn't. In any case there are always jobs he can do while the system is draining.

I asked if we would need to get a powerflush done for the warranty on the new boiler (it will be a worcester bosch condensing boiler). He said not necessary and they would do a hot flush with some cleaner in the system.

He's talking sense for once!

I then phoned the Alpha technical department.

1. He thinks the boiler is working correctly.

What does he mean by that? It may be working as is supposed to do, but that is not suitable for your particular installation.

2. He said that the boiler works at 80% efficiency and the hot water needs to be taken away from the boiler quickly enough. He said that the microbore is probably not up to the job and recommended that it was probably better to repipe the whole house in 15mm and this will allow the boiler to work better.

It's not the flow rate of the water which is the problem. It's the fact that your boiler is giving out more heat than the radiators can dissipate. You could repipe the whole lot in 15mm but if you have a boiler giving out between 11 and 28 kW into rads which can only handle 12KW you will have problems.

The alpha guy also said that I will probably need to repipe the whole system in 15mm anyway as condensing boilers work at a lower temperature and the pump needs to pump quicker to get the hot water to the rads quicker.

Where do Alpha get there tech support staff from - the queue outside the JobCentre??

The flow rate depends on the temperature differential, not the actual temperature. The greater the differential, the slower the flow rate. Your existing boiler works at a 17C to 20C differential and most condensing boilers are designed for the same differential. So the flow rate will be the same.

As for repiping in 15mm, this should not be necessary. The more heat a pipe carries, the greater the flow rate (litres/sec). The smaller the diameter of the pipe, the higher the speed (metres/sec) of the water though the pipe. There are recommended maximum and minimum speeds. If the flow rate is below the minimum, there is a danger that sludge will settle in horizontal pipe runs. If the maximum is exceeded, the flow of water may be audible.

Most installers seem to ignore the minimum flow requirement and set the maximum to about 2/3rds of the recommended maximum.

Assuming this, and using a 20C differential the capacity of different sized pipes are:

8mm: 0.9kW min; 3.0kW max
10mm: 1.45kW min; 4.8kW max
15mm: 3.63kW min; 12.1kW max

Your largest rad (dining room) has a quoted output of 2845W. This is with flow temp of 75C and return of 65C. As you boiler runs with a 20C differential the actual output is lower - 2429W for flow of 75 and return of 55. This means that both 8mm and 10mm are suitable and 15mm is actually too large as the heat flow is below the recommended minimum.

 

[fluff29]

Hi all

Could this be the answer

My boss wanted me to do a test on the diverter valve. He thought that was the problem. He may be right.

I opened the hot water tap to a trickle without starting the boiler up. Then switched the boiler on for CH and the results were as follows:-

1st time boiler fired for 19.30 minutes.
2nd time boiler fired for 5.30 minutes
3rd time boiler fired for 5.24 minutes
4th time boiler fired for 5.11 minutes

All rads in the house heated up lovely except for the problem rad, however we got more heat in it than we have every had before and the heat did go further down the rad. Think this may just need repiped.

I then turned the hot water tap off completely.

1st time boiler fired for 35 secs
2nd time boiler fired for 3.10 minutes
3rd time boiler fired for 3.05 minutes

Boiler stayed off for around 3 minutes between each of the times above.

Then put the tap on a little again.

Boiler fired for 9.50 minutes.

We are getting heat around the system, so no pipe problems, except the possible repiped big rad.

The boiler has never stayed on for 20 minutes. Could the diverter valve be the problem and is this enough proof to show the Alpha engineer when he comes to prove that the valve is broken?

Thanks

Fluff

 

[transam]

I have not read all the pages on this post , but Tamz posted on I think page 5 ? ? repipe the radiator as he stated , problem solved !

8mm is a waste of time on any system , most boilers are over sized , at some time or another like during the summer months when only the hot water is on ??
repipe 15mm as said by tamz , sorted !!

 

[fluff29]

Hi Transam

thanks for the advice. Agree with repiping the rad and this will definatley be getting done but I don't think this will solve the problem of the boiler cycling but leaving the hot water tap only slightly does.

fluff

 

[Agile]

My previous post seems to be invisible to everyone else apart from me!

I love the way that everyone thinks that spending YOUR money is a good idea.

Whatever I say I know that I am not getting paid anything so I can be totally objective.

If you cant be bothered to set or have the boiler set correctly first at no or little cost then obviously thats up to you.

Pearls before ..............

 

[transam]

If it is possible to alter the parameters of the boiler as per agiles post than you would be advised to do so ??

We have installed several (premix) modulating condensers on microbore systems 8mm ect , most of which have caused problems (customers were pre warned) generally they will complain that the radiators do not get as hot as they used to on there old fixed rate boiler ! re piped quite a few !