New system - boiler has short cycle time

[Malc C]

Further to my previous posts I've just spent an "enjoyable" weekend ripping out the old rads and pipework and replacing it with new. I have three rads connected, mostly via 22mm with 15mm feeds to the rads. The main feed / return pipes are capped until I have more time to install the remaining rads.

After bleeding the rads I managed to get all three glowing hot, however I notice that the Baxi Solo 60 still tends to cycle quickly, even though the roomstat is calling for heat. By this I mean it lights, runs for 60 - 120 seconds, stops running (boiler on light stays lit), then 2 - 5 mins later runs again.

I've tried all three speeds on the pump, and the fact that the rads are getting hot suggest that its working OK.

Is the boiler working correctly, or should it run continiuosly until the room stat or tank stat no longer calls for heat ? It has been suggested that the boiler PCB could be faulty, or could this be a faulty sensor that sits on the feed pipe sending wrong signals to the pcb ?

PS - you guys were right, the old pipes were sludged up and there was an airlock in the old system which was why I couldn't get the old rads warm ! - new rads are piping hot !

Malc

 

[danlightbulb]

Hi

Sorry to tag on to this, but I have a similar problem with a new Ferroli Optimax combi.

As it was really cold yesterday I bumped the heating up to a 24 deg setpoint. However the best the heating could seem to achieve was 21 to 22 deg. During the day I noticed that my stat would be reading 21 but the rads were only luke warm - hence the boiler should surely have been pumping out at full power to reach that 24 deg setpoint? The rads cycled on and off during the day. It was only in the evening that the house seemed to get red hot and I had to turn it off.

Cheers

 

[chrishutt]

Sorry to tag on to this,

So why do it? Your problem is quite distinct - start a new thread.

 

[D_Hailsham]

I have three rads connected,

If you have only three rads working at the moment and the boiler is expecting to provide heat to six or ten or whatever then, at the moment, the boiler is over specified for the size of the current system. This will lead to short cycling.


See An Oversize boiler is inefficient

 

[tryitandsee]

It appears to me that these replies are a load of old baloney.

The water temperature in the central heating circuit is set by the boiler thermostat, the room stat monitors the air temperature in that room.

If the room stat is set high, the boiler will carry on cycling at whatever temperature the boiler stat is set at and will only eventually turn off, once the room stat temperature is reached.

By turning up the room stat does not mean you are turning up the heat at the boiler.

Some argue that by turning up the boiler temperature, it saves money on your gas bill, by virtue of the fact that the room/domestic hot water reach there temperature more quickly and everything shuts down quicker. Others, me included, keep the boiler temperature down, but have longer periods of the boiler cycling. Someone will explain better than me the best way

 

[danlightbulb]

Sorry to tag on to this,

So why do it? Your problem is quite distinct - start a new thread.

Essentially same problem. Stat calling for heat, boiler not supplying at expected rate.

 

[D_Hailsham]

It appears to me that these replies are a load of old baloney.

Sorry, but you are the one talking baloney

The water temperature in the central heating circuit is set by the boiler thermostat, the room stat monitors the air temperature in that room.

Correct, so far

If the room stat is set high, the boiler will carry on cycling at whatever temperature the boiler stat is set at and will only eventually turn off, once the room stat temperature is reached.

Whatever temperature the room stat is set at, the system should be wired with a boiler interlock so that the boiler does not ignite when the room thermostat is not calling for heat

By turning up the room stat does not mean you are turning up the heat at the boiler.

Who ever said it did?

Some argue that by turning up the boiler temperature, it saves money on your gas bill, by virtue of the fact that the room/domestic hot water reach there temperature more quickly and everything shuts down quicker. Others, me included, keep the boiler temperature down, but have longer periods of the boiler cycling. Someone will explain better than me the best way

It all depends on the design parameters of the system. If the design assumed a flow temperature of 80°C and a return of 60°C, but the boiler runs with a flow of 60°C and a return of 40°C, then the radiators will not be providing their expected heat output. The room will take longer to warm up and may not reach the required temperature in very cold weather.

You have also missed the point that the OP has only three rads working and not the full complement.

Did you read "An Oversize boiler is inefficient" in my previous post?

The Boiler is a Baxi Solo 3 (I learnt this from previous posts). This is a non-condensing range-rated boiler. It does not have a modulating gas valve, so it is either on or off. This means that, when it is on, the gas is flowing at a fixed rate and giving out the maximum heat. This is more than the volume of water flowing through three rads can dissipate, so short cycling ensues.

 

[D_Hailsham]

Essentially same problem. Stat calling for heat, boiler not supplying at expected rate.

It may be a similar problem but certainly not the same problem!

Please read Rule 6 here

 

[danlightbulb]

Essentially same problem. Stat calling for heat, boiler not supplying at expected rate.

It may be a similar problem but certainly not the same problem!

Please read Rule 6 here

Was simply stating a similar problem and adding my information so that someone may have an answer to both in one. How many times do people get slated for asking same questions over and over in new threads? Then when someone tries not to start a brand new thread for something very similar they get told off.

Hijacking is when a second question is too far removed from the OP to be answered concisely in one post. Simply stating that I have observed a similar issue with my boiler is not (imo) hijacking.

 

[tryitandsee]

D_H

Have now read your 'oversized boilers inefficient'.
I was unaware the boiler would activate from cooling air passing up through the boiler casing.
This interlock you mentioned, I assume must be present on all boiler installations in unheated areas such as garages etc.? As the boiler would be firing more than was neccessary. I'm supprised it would have that much effect, when the thermostat (phial) is normally sunk into a heat pocket?

 

[D_Hailsham]

This interlock you mentioned, I assume must be present on all boiler installations in unheated areas such as garages etc.?

A boiler interlock is a requirement under the current building regulations for all boilers, irrespective of where the boiler is located. The purpose of the interlock is very simple: to prevent the boiler running (and the consequent waste of fuel and emission of carbon gases) when the room/hot water is up to temperature.

This is achieved through the wiring of the installation.

You can see a typical wiring diagram here

The C plan is the easiest to follow.

 

[Malc C]

Many thanks for the replies. I'm pleased to say that even with just 3 rads in place the house is becoming nice and warm in a fairly short time. I'm hoping to get the rest of the rads in over the next two weekends, and if the current heat levels are anything to go by then the house will be very cozy indeed.

To anyone who is in a similar position to ours (please read my previous threads) I would suggest doing the same. Not only will we have a nice new system at the end of this, I've also saved myself between £1000 and £1300 by doing it my self.