DIY Doctor UK DIY Forums

Hi,

I had a Worchester Greenstar 30Si compact boiler installed approx 6 years ago. New pipe work and radiators installed at the same time. I had noticed the radiators didn't get piping hot and the flat took longer than expected to heat up, I was told its just the way the system works, old flat, long run between boiler and radiators, yada yada. Flat is 110m2, with 9 radiators. No issues with hot water.
Ive recently been in a neighbours flat, same as mine, same boiler; radiators are piping hot, flat gets warm quickly. Ive studied the boiler / system and can see that the radiator supply from the boiler reduces from 22mm to 15mm after approx 600mm, (before it goes under the floor) Is this the problem? If so, is there any way to rectify other than changing the 15mm to 22mm, Pump or ??

Hi arabella0802,
If you open the flap at the bottom of the boiler you should have two knobs. The right one sets the water temperature and the left one the temperature of the water going round the radiators. Try turning the left one and see if you can increase the temperature, if you can, end by pressing OK.
Regards S

Thanks for the reply Stoney Boy.
The left hand temp dial has been set at Max from day1 - when adjusted the max temp reads 82deg.
When the boiler is running for CH, the temperature fluctuates between 55 - 65 deg, The boiler has been serviced annually, all reports say the boiler is performing as designed, which is leading me to think it's a piping / supply issue.
My understanding is supply pipework is run as far as possible in 22mm, which would usually be several meters, usually over 10 meters. Is this a correct understanding?
If so what would be the implications of reducing the 22mm supply to 15mm after 600mm?
My thinking is the boiler is being 'choked' by restricting the flow, its acting like an orifice, not letting the boiler supply the system, possibly causing back pressure?
Any advice would be greatly appreciated.

Most gas boilers are designed to modulate, i.e. turn down, they do this to keep the return water cool enough to allow the boiler to gain the latent heat from the flue gases.

If the radiators are set up correctly there will be with the TRV wide open about 15ºC difference in the temperature of water going into radiator and the water returning from the radiator, if some one has opened the valve on the other side of any of the radiators called the lock shield valve, then hot water will quickly return to the boiler, which will cause the boiler to modulate.

If all set correct then on switch on radiators will get hot, but it should take around 1/4 hour to reach full heat, as it takes a TRV around 5 minutes to cycle so if the radiator heats up too fast, the room temperature can over shoot.

What should happen is the TRV will regulate the heat in an analogue manor, not on/off, but gradually opening and closing to maintain temperature it is set at, so radiators should cool down once room reaches temperature but not switch fully off, just warm enough to keep room at set temperature.

So once the heating has been running for a time, if radiators go hot then cold then hot etc, there is something wrong, they should just stay warm.

The boiler will adjust output to match the demand from the TRV's once it can't turn down any more then it will start cycling off/on to further reduce output.

There should either be a modulating wall thermostat which turns boiler up/down, or in a room normally kept cold a on/off thermostat to turn off boiler on warm days, under normal winter running the wall thermostat should not turn off, if it does you get a temperature hysteresis, but if the temperature is controlled by the TRV then you don't get the hysteresis.

The problem is mechanical TRV heads are often marked *123456 rather than in degs C, and so it is a bit of guess work setting temperature, electronic types do show degs C and can be set to adjust room temperature according to the time of day.

Hi arabella0802,
If your boiler is only achieving a maximum flow temperature of 62 deg when it is set at 82 deg would indicate there is something wrong with its thermostat.
The symptoms you describe are nothing to do with pipe sizing.
You report that a neighbour with an identical system is roasty toasty so I suggest you get a boiler engineer to check your boiler thermostat.
Ask Worcester to send one of their engineers to sort out this problem, it may cost more than a generic boiler engineer but may well be worth the extra expense.
Regards S

Engineer to me means some one who has learnt their profession by going to a higher education establishment and have qualifications over level 3.

Which means a HNC, HND, or Degree, most tradesmen only train to level 3, but to be frank to work out what is required with a central heating system without paying an arm and a leg for the control system does need some one highly trained, so the label Heating and ventilation engineer is often well deserved.

As to if it were my trade, Would do domestic work after getting a degree? not so sure, my degree is electrical and electronic engineering, and it was rare I worked on Domestic, people did not want to pay me enough.

A person may be trained how to repair a single make of boiler, and I have been told Bosch charge too much for their parts to non dedicated Bosch repair people for it to be worth their while repairing Bosch boilers, bit like Apple Mac with computers, so @stoneyboy is likely right better to use a Bosch guy to repair the boiler.

But if not a boiler fault, then may not be the best option.

I remember when my son had his central heating fitted, the installer did not throttle back a single lock shield valve, so the pump pushed the water around the easiest path, so hot water returned to boiler which in turn caused the boiler to modulate (turn down) the boiler was rated 28 kW but was only putting out 8 kW simply due to lock shield valves not being set correctly, from what you say it could be either the boiler or the lock shield valve settings, to set the lock shield valves one should use a differential thermometer and I can't find mine.

I use the temperature report from the TRV, which shows on my PC both target and current, and if current exceeds target then the lock shield valve needs closing a bit.

ericmark wrote:Most gas boilers are designed to modulate, i.e. turn down, they do this to keep the return water cool enough to allow the boiler to gain the latent heat from the flue gases.

If the radiators are set up correctly there will be with the TRV wide open about 15ºC difference in the temperature of water going into radiator and the water returning from the radiator, if some one has opened the valve on the other side of any of the radiators called the lock shield valve, then hot water will quickly return to the boiler, which will cause the boiler to modulate.

If all set correct then on switch on radiators will get hot, but it should take around 1/4 hour to reach full heat, as it takes a TRV around 5 minutes to cycle so if the radiator heats up too fast, the room temperature can over shoot.

What should happen is the TRV will regulate the heat in an analogue manor, not on/off, but gradually opening and closing to maintain temperature it is set at, so radiators should cool down once room reaches temperature but not switch fully off, just warm enough to keep room at set temperature.

So once the heating has been running for a time, if radiators go hot then cold then hot etc, there is something wrong, they should just stay warm.

The boiler will adjust output to match the demand from the TRV's once it can't turn down any more then it will start cycling off/on to further reduce output.

There should either be a modulating wall thermostat which turns boiler up/down, or in a room normally kept cold a on/off thermostat to turn off boiler on warm days, under normal winter running the wall thermostat should not turn off, if it does you get a temperature hysteresis, but if the temperature is controlled by the TRV then you don't get the hysteresis.

The problem is mechanical TRV heads are often marked *123456 rather than in degs C, and so it is a bit of guess work setting temperature, electronic types do show degs C and can be set to adjust room temperature according to the time of day.

Stoney boy & ericmark Thanks for taking the time to reply, much appreciated. TRV's work well and the rads have been balanced several times, that was my first go to.
I asked both gas engineers to check the stat, both reported no faults with the boiler.
I looked into getting a Worcester engineer out. They just send a local accredited gas engineer, however I think this is my next move. Thanks again for taking the time to reply

Hi arabella0802,
Ask the engineer why when set at 82 the max achieved is 65. Don't be put off by BS.
Regards S

(:

Hi arabella0802.
Just a final thought.
It may be a defective thermostat or, it may be low gas pressure or, it may be your boiler is a lower rated one than your neighbours so just can’t provide enough heat.
Regards S