Help with central heating problem from a chilly Edinburgh

[hootsmun]

HI,

Wondering if anyone can ofer some advice on the following.

I have a 14 month old system which is causing a bit of grief at the moment. it is a sealed system, unvented hot water tank, greenstar 40cdi boiler.

I have three zone valves and radiators are al fitted with TRVs, no room stats and a couple of towel rails, one of which I fitted at the weekend.

The programmer, zone vales and circulating pump all seem to be working fine and boiler fires up, runs up to to the set temperature and then cuts out. Hot water is being fed to the right zones but the return loop is at best lukewarm when the boiler cuts out. As the boiler cools down, the feed loop also cools considerably to almost the same temp as the return loop.

Any ideas what this might be ? Airlock ? something up with the boiler?

Any speedy response greatly appreciated - freezing my **** off here

Cheers
Hoostmun

 

[Rob]

Circulation problem.

If the pump is external (not inside the boiler) then suspect this.

when did the problem occurr? When was it last working correctly?

 

[hootsmun]

Pump is external,

Grundfos 15-50.

Seemed to be working fine prior to me fitting the towel rail (oops).

Boiler just cycles on and off but return loop isn't getting hot. I vented the pump (no air) and have bled all the rads several times.

Could this be caused by an airlock in the pipes between rads? if so, how do I fix it?

Thanks for your promt response

 

[D_Hailsham]

I have three zone valves and radiators are al fitted with TRVs, no room stats and a couple of towel rails, one of which I fitted at the weekend.

What is controlling the zone valves?

 

[hootsmun]

I have a programmer with 3 seperate zone controls

 

[D_Hailsham]

I have a programmer with 3 separate zone controls

But that only controls the times when the zones are being heated. How do you ensure that the boiler does not continue running after the rooms have reached temperature and the TRVs close the radiators? Or are you relying purely on the boiler thermostat?

boiler fires up, runs up to to the set temperature and then cuts out.

 

[hootsmun]

As far as i can see there is no other control/thermo stat to the boiler and it is purely running off the boiler thermostat.

I assumed that the boiler thermostat would rely on the return loop temp reaching that set by the control on the front of the boiler before shutting off. Is this correct? or will it just run up to the temp control set on the front of the boiler and keep cycling without a continued call from a room thermo?

Does this sound unusual, i.e should i get a room thermo fitted to control boiler, otherwsie it will continue to cycle as is?

Many thanks for your help on this

Cheers

 

[D_Hailsham]

As far as i can see there is no other control/thermo stat to the boiler and it is purely running off the boiler thermostat.

As your system was only installed 14 months ago it should have complied with the Scottish Building Standards. Technical Handbook Section 6 says:
For gas and oil fired boilers, sufficient system controls should be installed in an arrangement which switches off the boiler when there is no demand for heat to the radiators and hot water storage vessels (where they are installed). This is commonly referred to as a boiler interlock.

I am quoting from the 2006 edition, but the 2007 and 2008 editions all say the same thing.

I assumed that the boiler thermostat would rely on the return loop temp reaching that set by the control on the front of the boiler before shutting off. Is this correct? or will it just run up to the temp control set on the front of the boiler and keep cycling without a continued call from a room thermo?

The boiler thermostat setting determines the flow temperature of the water leaving the boiler. When all the TRVs have closed down the water will continue to be pumped through the bypass, or the HW cylinder coil, until it has cooled sufficiently for the boiler thermostat to relight the boiler. This could continue indefinitely, wasting gas and electricity (pump), until the time switch turned the boiler off.

should i get a room thermo fitted to control boiler, otherwsie it will continue to cycle as is?

Yes, you need a room thermostat for each zone, or the boiler will cycle. The radiator in the same location as the thermostat should, ideally, not have a TRV. But you can get over this by setting the TRV to max.

How is your hot water heated? Or is that one of your zones?

 

[hootsmun]

Big thanks for your replies, I suspected the proper controls hadn't been fitted during the installation but we didn't move back into the house unitl the spring and so have never really used the system in anger until now.

The hot water is one of the three zones, yes. How do I ensure that the boiler will switch off if only the hot water zone is on? Do I need to get a water temp thermo fitted to the unvented cylinder?

So it looks like I need to get a thermo for the upstairs and one for the downstairs.

Thanks again

 

[Mr Smyth]

Hootsmun,
Are your radiators heating sufficiently? From your first post I suspected a circulation problem.

Having a system without room thermostats will work but will not be a efficient as with room stats. The building regs quoted are about efficiency.

 

[hootsmun]

Hi,

No rads not heating properly and hot water no where near what it was.

It is taking ages for the return pipework to heat up and the rads are taking a long time too.

I installed a towel rail upstairs at the weekend and thought this may have introduced an airlock, but all rads are heating, just very slowly. I vented the pump, which is running quietly and have bled all the rads several times.

I am assuming that if I install the room stats the whole system will be much more efficient, as it stands the boiler just cycles on and off whilst the programmer times are on - can't imagine this being the most efficient use of gas !

Thanks for your reply

 

[D_Hailsham]

The hot water is one of the three zones, yes. How do I ensure that the boiler will switch off if only the hot water zone is on? Do I need to get a water temp thermo fitted to the unvented cylinder?

Yes, you need a stat on the HW cylinder

So it looks like I need to get a thermo for the upstairs and one for the downstairs.

That is correct.

As for your original problem - rads not heating properly and return pipe not very hot:
1. What speed is the pump set to?
2. Have you changed it?
3. How many radiators and towel rails do you have?
4. Did you rebalance the system after you installed the towel rails?

 

[hootsmun]

Problem fixed - it was an airlock/blockage in the system.

Increased pressure to 2.5 bar on the expansion tank, then drew off a gallon at the boiler - did this serval times, whilst keeping the system charged - bingo. boiler operation retruned to normal and we are all toasty again - yipee !

Thanks to all who replied.

One final thing though - goes back to the room stat question and efficiency, I dont have any but could someone eplain what they control?. Do they control the zone valves or actually go into the boiler.. Dumb question I knwo but if you can point me in the direction of a how this works be much appreciated.

Thanks Again

Hoots

I

 

[D_Hailsham]

One final thing though - goes back to the room stat question and efficiency, I dont have any but could someone eplain what they control?. Do they control the zone valves or actually go into the boiler

Good to hear you solved the main problem

The room and HW stats directly control their zone valves zone valves. These have auxiliary switches in them which then control the boiler.

Here is a typical wiring diagram (just one CH and one HW zone):

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