Do modulating boilers also allow manual control?

[DanR]

Are there any modulating gas condensing boilers where the modulation can be both automatic and also manual?

I believe there are gas condensing boilers where the gas burning rate can be modulated, and the amount of modulation is set by the return water temperature.

Can someone please tell me if I should be able to find any boilers where, in addition to the above, the amount of modulation can also be set by hand? That is, the maximum gas rate can be easily set to a reduced level just by turning a knob, without needing anything like a manometer and screwdriver?

[John Rumm]

Many boilers will feature a maximum water temperature control. This will in effect do something simlar to what you describe - the boiler will auto modulate so as to not exceed the specified upper fixed temperature point that you have set - so lower temp - less power input. If the required heat input falls below the minimum modulation level then the burner will shut of and the boiler will cycle to maintain the average temperature.

[Andrew Gabriel]

Is this so you don't have to replace a length of gas pipe with a larger bore pipe? That thought occured to me when I was putting mine in, but I don't recall seeing that feature on any boiler.

[DanR]

Thank you for your replies.

The question centred on a comfort thing. Some years ago I had a solid fuel central heating boiler. Being solid fuel, you could not turn it off, but just regulate the air supply and the consequent rate of burning. Thus the pump had to be on all the time.

So there were long periods of time when, with the air supply manually turned to a minimum, the radiators were at a lowish temperature, just above blood heat, never getting any hotter. This could be maintained for long periods of the heating cycle. I found this to give a greater feeling of comfort than the off-or-on-high performance of my present gas boiler.

As I need a new boiler, I was wondering if I could buy one where I could over-ride the automatic controls and let the radiators be on a low heat for long periods, like the solid fuel one.

DanR.

[Andrew Gabriel]

OK, there are two things mixed up here...

Modulation is adjusting the burners to maintain the required water temperature in light of varying demands, which I think all condensing boilers do.

I think all condensing boilers also allow you to set the radiator temperature. Having sized my radiators for a condensing boiler, I find I run them at 45C most of the time (Keston Celcius 25 boiler).

Some more expensive boilers also have a mode where the boiler will automatically adjust the radiator temperature based on factors such as outside temperature, called temperature compensation. Basically, they aim to adjust the radiator temperature so as to avoid having to switch on and off as far as possible. In addition to the comfort you mention from a constantly running system, it also allows the system to run at the lowest radiator temperature to be effective, and this gives most economical operation of a condensing boiler.

[Andy Hall]

Yes, you can find the latter functionality from boilers such as

- MHG Procon Plus

- Viessmann

have weather compensation, full pump modulation as well as burner modulation. You can set a max operating temperature, but there is no need because the boiler will drop down to very low output levels automatically ~4kW or so total.

[Doctor Drivel]

There are some set burner rate and rage rated boilers about.

[Andy Hall]

Why do they get cross?

[Doctor Drivel]

Matt, because they know DIYers are screwing them up.

[Ed Sirett]

To get what you want you simply need to turn the boiler to a low level. Modern condensing boilers can be set very low flow temperatures, the modulation is fully automatic (allowing the boiler to operate powerfully when starting from cold).

If you really want to be able to control the boiler directly and absolutely then there is a 'test mode' on a Keston Celsius 25 where the front knob maps directly to burner power rather than to a particular return or flow temperature.

The only problem is that if you have a stored hot water system then you need to be able to set a flow temp of around 65-70C when the unit is doing HW. Some but not all (including the Keston C25) don't have a facility for this. Combis tend to have seperate heating and HW controls any.

[Frank Erskine]

Just a bit of cross-modulation.

[fred]

In article , Ed Sirett writes

Now that would have been useful to know when I was setting up/servicing mine. Don't remember seeing that in the installer manual, care to share the trick of entering test mode (or any other Celsius 25 hidden options)?

[Ed Sirett]

Apparently Andrew Gabriel was told about this by Keston. There is a jumper which you remove on the low front corner of the PCB. When you cycle the power on the front knob the boiler moves between. and then I found out about the variable power setting by experiment.

Normal mode Test mode - Variable power - knob controlled. Min power Max power Normal mode ....

When you replace the jumper then the boiler will work in those modes. Note min power is much lower than ignition power and will flame out unless the gas valve is set near to correct.

[fred]

Thanks Ed, I'll try that out the next time the case is off.