Ferroli Combi 77 FF Boiler switches on and off too frequently

[k_d_ch]

I have a Ferroli Combi 77 FF boiler, which switches on and off about every 30 seconds in a comfortable temperature setting, when the temperature knob is close to min setting. When the temperature setting is increased (to about midway between min and max), the boiler switches on and off every 3-5 mins but the radiators get too hot to be comfortable. I have had an engineer come today, who replaced the CH flow temperature sensor, which made no difference, and he says he can't think of another possible cause of the problem. I would very much appreciate any thoughts on this, the way the boiler is working just does not feel right.

 

[tamz]

Get it range rated or try fitting a room stat or trvs and turn the boiler stat up a bit.

 

[k_d_ch]

Thank you very much for your advice, it has prompted me to think a bit more about the way the system is supposed to work.
Actually (I did not mention it in my post) if I somewhat restrict the flow of water into the radiators by turning the radiator usual knob (not TRV), the point on the boiler stat where it starts switching off and on too much moves up so it is closer to the midpoint between the min and the max on the boiler stat. It seems that the effect of TRVs may be similar (please correct me if I am wrong), in which case they do not address the problem I am having.
I am not completely sure how the room stat would interact with the boiler. Can it be programmed for a range of temperatures, say 18 to 21, so that the boiler only switches on when the temperature in the room falls below 18 and switches off when it rises above 21? In this case increasing or decreasing the temperature of the flow out of the boiler should only affect the time that it takes for the boiler to bring the temperature from 18 to 21. If this is so, it may well be a possible solution to my problem, and might even increase the energy efficiency.
Regarding range rating I did not know what it was, so I have read up a bit online: this decreases the burner gas pressure, so the same thermostat setting should probably give a lower temperature on the output (?). Here the possible danger is in making is not warm enough in the largest, coldest room of the house I guess, so I presume some care needs to be taken...?
On account of all the above, a room thermostat seems the best solution. Please correct me if something is wrong in my reasoning, I would much appreciate it. And again many thanks for your comment.

 

[jase158]

do you have a pump? if so try turning it down

 

[jase158]

Are all radiators getting hot, top and bottom?

 

[tamz]

Your boiler has never been set up and you are working your heating wrong.
What is happening is the water flow rate throught the boiler is not enough to keep the boiler firing at the rate it is set at. A bit like trying to drive at 30mph with your foot flat on the floor.

1). You need to get a competent gas man to set the boiler output to suit the heating load (range rate) ie ease off the throttle a bit. This is NOT a diy thing.
When these boilers were made (many moons ago) they came set at maximum output to central heating (77000 btu, 22 and a bit kw) which is too much for your std 3 bed house. Most were never set to suit the systems because most installers didn't know how to do it.

2). You would be better to fit thermostatic valves to at least the bedrooms but prefferably to all rads except 1 (usually the hall) which needs to be left as an open circuit. This will allow you to control the temperature of the rooms and set them individually. Turning wheelhead valves half closed or whatever and turning the boiler stat to minimum does not control the temperature in the rooms effectively or efficiently. It restricts the flow through the system and compounds your problem. It is costing you money.

3). Fit a thermostat on the wall in the hall or wherever you leave the trv off. This gives an overall control to the temperature of the house and can be set at whatever suits you from 10º to 30ºC and will shut the boiler off completely when it is satisfied.

4). You need to have the system balanced to get the correct temperature drop around the system and flow rate through the boiler.

5). A system flush would help as this is an old system and will have sludge in it. This needn't be a powerflush. A flush using chemicals like Sentinel X800 or Fernox F5 will make a vast improvement.

6). Think about a new boiler as this one is about past it's sell by date.

7). Get a better engineer in next time instead of a part changer.

 

[k_d_ch]

There is a built-in pump, whose rate is factory set. It looks as though it is a matter of adjusting the burner gas pressure as tamz explains. Thanks for your post anyway.

 

[tamz]

The pump should be set on its highest setting.

 

[k_d_ch]

Many thanks tamz for your thorough advice. I will make sure I follow all the steps! Could you at all expand a bit on point 4) please? Using what controls is this balancing carried out? Does it have to follow the installation of all thermostats or it can be done independently of 2 and 3)? Thanks very much again!

 

[jase158]

Many thanks tamz for your thorough advice. I will make sure I follow all the steps! Could you at all expand a bit on point 4) please? Using what controls is this balancing carried out? Does it have to follow the installation of all thermostats or it can be done independently of 2 and 3)? Thanks very much again!

do a google search on balancing it can be very difficult to explain. you dont need to do 2 and 3 to do 4

I think tamz has done a brilliant job on explaining this, good luck

 

[k_d_ch]

I am already reading up on this at this moment... I can see it is about adjusting the temperature drop as one moves along the system away from the boiler.

 

[tamz]

You need 2 thermometers or a well tuned pair of hands and a bit of patience. The idea is to get an 11 degree temperature drop across flow and return at each rad and at boiler by adjusting the lockshield valves and bypass (if fitted).

There is plenty info on the net on how it is done if you google it.

If trv's are fitted they need to be opened up full when this is being done.

 

[jase158]

I am already reading up on this at this moment... I can see it is about adjusting the temperature drop as one moves along the system away from the boiler ... Will obviously need to be done when the thermostats are installed.

you don't need thermostats installed, however you do need a working boiler

to balance you need to find the pipe that gets hot first on each radiator, this is your flow, the other side is your return.

turn down the return side until you have a 11 degrees difference between each pipe.

 

[k_d_ch]

Yes thanks jase. I have already realised it's the lockshield valves need to be adjusted in this process, so the TRVs are not necessary. With two thermometers I could do this tomorrow!

 

[k_d_ch]

Sorry tamz to bother you, but another plumber came today and checked the gas pressure and it was at the bottom of the range given in the boiler manual: 3.5-4 mbar, which corresponds to about 25000 BTU. I calculated the heating requirement for the house and it comes at 53000 max. The guy said the boiler was too small for the house and that while the flow of water through the boiler was OK (pump at maximum), it was not powerful enough to heat the house properly. He said that a clear indication of this was the high difference in the temperatures between the inlet and outlet CH water pipes at the boiler. He is arguing that the CH flow temperature sensor is on the return pipe, which makes the on-off cycle very short: I have to say I do not completely understand his argument. He says that in fact an adequate boiler would have the power capacity of up to something like 40 kW, and that it would then efficiently warm up the house and switch off for a longer period of time. He also said that balancing the system under the circumstances would make little difference.
I would very much enjoy and appreciate getting your point of view on this.

 

[k_d_ch]

Sorry tamz to bother you, but another plumber came today and checked the gas pressure and it was at the bottom of the range given in the boiler manual: 3.5-4 mbar, which corresponds to about 25000 BTU. I calculated the heating requirement for the house and it comes at 53000 max. The guy said the boiler was too small for the house and that while the flow of water through the boiler was OK (pump at maximum), it was not powerful enough to heat the house properly. He said that a clear indication of this was the high difference in the temperatures between the inlet and outlet CH water pipes at the boiler. He is arguing that the CH flow temperature sensor is on the return pipe, which makes the on-off cycle very short: I have to say I do not completely understand his argument. He says that in fact an adequate boiler would have the power capacity of up to something like 40 kW, and that it would then efficiently warm up the house and switch off for a longer period of time. He also said that balancing the system under the circumstances would make little difference.
I would very much enjoy and appreciate getting your point of view on this.

I have just realised when he was checking the pressure the boiler stat was not at the max setting!
Where does one find competent people?

 

[tamz]

Where does one find competent people?

Ha. You might have more luck tacking down a Yeti. Try to go with personal recommendation if you can.

The heating guy you had out today was talking mince. How can a 77000btu boiler be too small for a 53000 (estimated) load? Yes a 40kw boiler would heat the house (and the one next door too) and be massively over sized. How can a temperature sensor be fitted to the return pipe? A lot of good that would do. Set the boiler to high (82deg) and itr would be running like a steam train before it switched off. He is a muppet!

You need to start at the beginning here. I'll assume the guy got one thing right, the water is circulating ok although circulation may infact be an issue.
How i understand it is.
Your house heats up ok but as you have no external thermostats or trv's, you try to control the heat by turning the boiler stat to minimum which makes the boiler short cycle.
You are also trying to turn the heat down in individual rooms by closing the wheel head valve until it restricts the flow through the rad and drops the temperature.

Have i got that bit right?
What size house do you have and how old (well insulated) is it? I know it is more than 25 years old.

 

[RoryD]

you shouldn't be controlling the heat temperature from the boiler itself, that knob should be slightly shy of max to prevent cycling (which is what you have), this is costly and inefficient. As said have a room stat fitted, A wireless stat is less invasive.

 

[jase158]

Sorry tamz to bother you, but another plumber came today and checked the gas pressure and it was at the bottom of the range given in the boiler manual: 3.5-4 mbar, which corresponds to about 25000 BTU. I calculated the heating requirement for the house and it comes at 53000 max. The guy said the boiler was too small for the house and that while the flow of water through the boiler was OK (pump at maximum), it was not powerful enough to heat the house properly. He said that a clear indication of this was the high difference in the temperatures between the inlet and outlet CH water pipes at the boiler. He is arguing that the CH flow temperature sensor is on the return pipe, which makes the on-off cycle very short: I have to say I do not completely understand his argument. He says that in fact an adequate boiler would have the power capacity of up to something like 40 kW, and that it would then efficiently warm up the house and switch off for a longer period of time. He also said that balancing the system under the circumstances would make little difference.
I would very much enjoy and appreciate getting your point of view on this.

did he recommend a new boiler?

 

[k_d_ch]

Yes tamz, you have got the situation right, this is exactly what's happening. The house is 100 old (an Edwardian 4 bed), I suppose that the insulation level is poor, apart from the double glazing that had been there when we moved in earlier this year.
I intend to install TRVs on each radiator but one and a wall thermostat in the hall as you suggested in you earlier message, and perhaps flush, or even powerflush, the system: I think you are right it must have quite a bit of sludge in it. One of the messages in the thread, by RoryD, seems to suggest as well that TRVs could work towards addressing the issue.

 

[k_d_ch]

Thanks RoryD, just to clarify: adjusting the temperature on the boiler stat knob is inefficient, but the same stat at max and regulating the room temperature with TRVs and/or room stat should be better, is this right?

 

[k_d_ch]

He said go to the Bosch website, enter the details of the kind of house you have, and it will suggest you appropriate boilers, which will be around 40kW.

 

[doitmyself]

He said go to the Bosch website, enter the details of the kind of house you have, and it will suggest you appropriate boilers, which will be around 40kW.

Far better to use the Boiler Size Calculator.

 

[tamz]

doitmyself is correct. Better to use the whole house method. The guy you had out is ...well....maybe needing more training to put it mildly.
Regardless of that, you have an existing boiler which you want to give you a good level of comfort.

As a minimum i would suggest you should firstly fit a room stat. Even a basic £10 one would be better than what you have but if you can spend a bit more, an ideal stat is a honeywell cm927.
This is a programmable wireless roomstat that will give you total overall control of your system, give you comfortable temperatures, and, save you money. (you can even move it from room to room with you if you like). Here is a link for a bit reading up CM927 7 Day Wireless Programmable Thermostat - Honeywell UK Heating Controls
It is easy to wire up to the boiler if you have basic diy skills but if you are not confident get someone in to wire it.

After you fit a room thermostat (whichever type you choose) turn the boiler up and let the room stat control the temperature.
You may find after balancing the system, this is all you need as in times past radiators were sized to the heat loss of the room so in theory when one part of the house is a certain temperature the rest of the house should be at the correct temperatures too.
In practice this was not always the case due to other heat gains so that is where TRV's come in. These can be set to control the temperature of each room individually eg most people don't like their bedrooms to get too warm so these can be checked back to suit your needs.

If fitting TRV's i would recommend Danfoss. These are gas filled sensors which mean they react quicker and are more accurate and they are the only truly reversible trv's on the market (fit them on any pipe). Danfoss Ras C are not expensive (about a tenner) and are far superior to the types sold in the diy sheds for around the same money. Ras D are a bit more expensive.
A bit more education for you on TRV's and how they can control the temperature of your home and save you money.
http://danfoss-randall.co.uk/PCMPDF/TRVPt5a.pdf
http://heating.danfoss.com/PCMPDF/TRV_euro.swf
How a Danfoss TRV Works

Look through this lot and learn a bit about controlling your system and get back to me with any questions.

 

[k_d_ch]

Going back to my old post: just using the wireless thermostat has worked well for the overall temperature regime of the house. Thank you again for your help back three years ago.
I have understood a bit more since then how central heating works in a house, and now I would like to improve one more temperature aspect in it. One of the rooms is much colder than the rest of the house, this is a downstairs utility room at the rear of the house, part of an old extension (most houses in our street have one), it can be 12-15 degrees in winter. I would like to bring this room to a normal room temperature, and for this I have installed TRV's on all radiators except the one in this room, moving the thermostat to this coldest room to control the boiler from there. The problem has been solved to some extent: the room is warmer and the rest of the house is at the temperature we want it to be. However, during colder winter nights the boiler needs to work constantly in order to reach 19 degrees in this room. If I lower the wireless thermostat setting to 17.5 degrees, the boiler is not strained any more, but it has a converse effect on warmer days, when the rest of the house may feel cold and I have to manually increase the thermostat setting.
Apart from some basic things to improve insulation (such as filling the holes around the pipes coming out through the wall), my understanding is that the room needs a bigger radiator. At the moment is is a single 600x1200 universal radiator, and I am thinking about getting a similar double one. What is your opinion on this? I appreciate your time and your help.

 

[dancinplumba]

Insulate room or go with a double

 

[PhoenixServices]

As above.

 

[bikeman01]

Hi, I know this is an old thread but google has it 1st place in it's results for this problem so for others I thought I would post how I fixed this issue.

As tamz says above the problem was caused by the boilers flow temp sensor detecting that the water was sufficiently hot with the effect that the boiler cycled on and off every minute.

As there was no way to adjust the boiler temp down I thought about this for a while and deduced that the water just wasn't getting pushed away from the boiler quick enough.

Mine is a micro bore system with an open vent condensing boiler pretty typical of houses built in the last 15 years or so.

Turning the system pump up to maximum did get the water moving away fast enough to fix the issue but I don't like things running at max and it soon caused a leak in the boiler manifold.

I was able to cure the problem by relieving the resistance in the rad system by opening up all the radiator's stop valves by a quarter turn. By adjusting them all by the same amount the rads stayed in balance and warmed up evenly and allowed the water to flow faster through the system and the boiler stopped tripping frequently.

I hope my experience helps someone.