what level to set old conventional boiler?

[andy_nufc]

Hi heating experts and enthusiasts,

With modern boilers there seems to be lots of advice on how to set the controls, in terms of their temperature settings.

But what about an old boiler that just has a single knob (presumably some kind of thermostat for the boiler itself)?

I have a Prima Potterton which is as old as the house itself, so about 25 years. I have HW and CH on Y-plan, open-vented cylinder, and F+E tank. In the first lockdown I also replaced the controller with a Drayton Wiser 2 channel (plus its remote room stat), which I love.

So the boiler can be set between 0 (off) and 6. I have it set at about 3.5. But what is optimal?

Cheers,
Andy

 

[BlueLoo]

Great question.
I have the same and have wondered the same. I've never really got a firm answer tbh.
The manual suggests turning the heating temp to max in winter but doesn't really go into the justification for it.

It isn't a condenser boiler so return temps don't mean much.

The boiler is rated for 11c delta between supply and return but I've never been able to achieve that as my system has too much varience between full and minimum load.

As far as i can gather, boiler temp is analogous to heating reaction time. The hotter the boiler, the quicker the system (zone) heats up but then the hysteresis increases as the rads overshoot.

In the end, and as per my research, I've set my system to 75c outlet.

I used a probe thermometer to do that but you could do linear interpolation on the boiler stat.

Boiler max temp is 87c. boiler stat range is 25c

25/5= 5

So,
0= 57c
1= 62c
2,= 67c,
3=72c
4=78c
5=83c
6=87c

Ish....

I set mine to 75c. Because i wanted a decent delta for heating my HW to 60c.

I also had a HVAC engineer tell me that going above 75c would generate air in the system .

I've yet to find a reasonable answer to the question.

 

[ianmcd]

Quite simple , the lower you set the temperature, the cheaper it is to run, so just keep notching it down a little untill you feel cold then turn it back up a notch so it is in your comfort range, every one is different no two people like their home at the same temp, but a thing to note, the max temp that you set the boiler to , is also the max temp that your hw cylinder will reach, so if you are happy at say 45-50 degrees, then once a week turn it to 65 degrees for an hour or so to kill the possibility of legionella forming in your cylinder, then turn back to your optimal setting

 

[ericmark]

Also interested in replies, what @BlueLoo has said is also my understanding, but we have four controls and the question is how they relate.
1) The boiler maximum temperature.
2) The temperature drop across each radiator.
3) The TRV settings.
4) Wall thermostat settings.
For the TRV to work there are two options, one is to ensure the boiler runs on a regular basis, the other is to link the TRV to the main thermostat, I am using the first option as my TRV heads do not link to the main wall thermostat, so the lower the setting the more often the boiler runs, and the better the TRV heads can control room temperature as long as they can actually reach the target.

However you say Drayton Wiser, so in your case the Drayton Wiser TRV heads link to the wall thermostat and so if boiler needs to run it does, and the Drayton Wiser TRV heads are claimed to have the best algorithms to ensure no over shoot, so although for me a low boiler temperature ensures the TRV can do its job, and also means with a C plan my DHW is not too hot, not sure that applies with your system, as one using Y plan so DHW independent to CH water temperature, and two you have about the best TRV heads made.

But radiator size matters, and looking at recovery time, for me I am retired so home all day, and heating runs all day, so it only maintains the room temperatures, but if we went out to work, then very different as the faster the rooms can reheat then the latter it can be left before the boiler fires up for your return.

Also timed or geofencing? I use Nest and there is very little user control with the geofencing, I can set Eco and Comfort, but have found no way to set what distance from home the heating activates, watching my wife yesterday returning home on google maps it goes in jumps, some times it says just now, other times 20 minutes ago, so at what point it would trigger Nest is any ones guess, well at moment turned off as it started turning off the heating while at home.

What Drayton Wiser uses to see where your phone is I don't know, but think Nest would use google as they are now same firm, so if google maps don't see I am home neither will the central heating.

 

[oilhead]

In short, you set the boiler temperature to your desired comfort level.
75 oC was always considered the optimum flow temp for CH. Higher than that could induce cavitation on the negative side of the pump, lower could involve cool return temperatures which could induce corrosion around the return port of cast/steel boilers from condensation. Even with condensing boilers, the significant measurement is the temperature differential. What you need to ensure is that your flow temperature is sufficient to meet your DHW temperature, so above 60c is needed. It then depends how hot you want your radiators. Thermostat knobs were changed from temperature to reference numbers many years ago, as the thermostats themselves are quite coarse instruments. The advent of electronic control now gives a flow temperature on the panel, which is accurate.

 

[BlueLoo]

In short, you set the boiler temperature to your desired comfort level.
75 oC was always considered the optimum flow temp for CH. Higher than that could induce cavitation on the negative side of the pump, lower could involve cool return temperatures which could induce corrosion around the return port of cast/steel boilers from condensation. Even with condensing boilers, the significant measurement is the temperature differential. What you need to ensure is that your flow temperature is sufficient to meet your DHW temperature, so above 60c is needed. It then depends how hot you want your radiators. Thermostat knobs were changed from temperature to reference numbers many years ago, as the thermostats themselves are quite coarse instruments. The advent of electronic control now gives a flow temperature on the panel, which is accurate.

Great answer.
I now have a decent logic to back up my 75c boiler temp.

 

[ericmark]

My mothers house had a problem, bay windows would catch the sun, and the living room temperature could get silly, the cure was to reduce the radiator temperature, so less energy was stored in the radiator so when the TRV turned the radiator off, it cooled faster.

With her house this was done by allowing the boiler to modulate, we don't have that option, at least I don't, my boiler is simply on or off. However I don't have bay windows either.

But the point is all homes are different, and to try and follow guide lines does not always work.

 

[andy_nufc]

Thanks, this is very informative - especially that by accident, using a setting of 3.5, I will be getting something like 75degC.

I don't currently have the Wiser TRVs but will be looking to get some in the future. Although I'd probably need to get the TRV fittings replaced since lots of them are quite stuck.

Another question if I may!

The upstairs of the house has radiators with the usual lockshield on one end and (Myson) TRVs on the other, from 15mm pipe which emerges through the wooden floor. So I'm assuming that this is the typical two-pipe system.

The downstairs radiators are a bit different, which seem to use an 8mm (microbore?) pipe. I'm assuming they do this so that this pipes are thin enough to come down the drywalls from above (the downstairs floor is concrete). The other weird thing about the downstairs radiators is they basically have the same type of valve on each end - a hand-twistable but non-TRV valve. Am I right in thinking that the downstairs radiators thus work off a manifold that would be in the floor upstairs, near the cylinder, rather than operating on a loop? (Just to be clear, there is only one heating zone, i.e. a single 3-port valve between CH and HW.)

 

[ericmark]

My old house had TRV's upstairs, but not down stairs, reason was simple, heat raises, and the TRV's upstairs stopped the rooms getting too hot, and down stairs was open plan, so one thermostat in the archway between living room and dinning room was enough.

House before that was hot air, with vents in the doors for return air, again one thermostat did all.

I am sure my radiators are too small, as even when house is cool, boiler never runs continuous. Maybe I should try turning up temperature at boiler.

 

[ericmark]

Worked it out, batteries discharged, the TRV (eQ-3) defaults open if batteries are too discharged, so my bedroom radiator wide open, so the other radiators in the house not getting their fair share, clearly also lock shield needs setting a little lower too, but first want to see if a pair of AA batteries cures it.