Weather compensation on CH boiler

[Mrs Pingu]

By which I mean one with a sensor installed outside on a north facing wall, and not a smart controller reading the weather off the internet.

Anyone got one care to feedback? Seems like a good idea...

[Kim]

I've incorporated the reading from an outdoor temperature sensor^[1] in my heating control algorithm^[3], and it does seem to help when it's properly cold (and by reducing the heating to appropriately homeopathic levels when it's actually quite warm outside but the interior of the house hasn't caught up yet).  It's all fudge-factors though, so not something I can usefully generalise.


_{[1] A waterproofed DS18B20 poked out through a hole in a west-facing^[2] window frame that used to accommodate some prehistoric TV aerial coax.  It over-reads on sunny afternoons.  That's not actually too much of a problem for these purposes.
[2] Terraced house, so north facing isn't an option.
[3] Roughly "run the heating for (difference between average temperature in occupied rooms and preset threshold * some constant) - (difference between outside temperature and 10C * other constant) + yet another constant" seconds, wait 5 minutes, calculate again", with special handling of things like us being in bed, the front door being opened, the alarm clock being due to go off and whether someone's setting cooking-related timers.}

[Quisling]

Yes, weather compensation is pretty standard now. In the old days when people were still stupid boilers just turned on or off in response to temperature sensor inputs. Usually this is controlled by a room thermostat or a sensor detecting the return temperature into the boiler.  Then we learned to modulate the boiler firing rate.

Condensing boilers only go into condensing (highest efficiency) mode when the retun temperature drops to a certain level. Using weather compensation, the boiler controls can vary the flow temperature and theoretically keep the boiler running steadily and efficiently in response to the theoretical heat loss of the house.

The algorithms involved can be fairly involved but typically are based on a heating curve which is set to deliver say 70C flow at -10C outside and 50C at plus 10C. The heating curve on our boiler is user variable which is handy and helps keep the house fairly close to the set point temperature.

[Kim]

I was just wondering how compensation works in a typical domestic thermostat setup where your demand-for-heat signal is a binary on/off affair.  I'm modulating ours in the time domain, because there isn't any other option.  Presumably being able to do clever things with the flow temperature is a boiler-specific feature that you aren't going to find on landlord-quality boilers.

[Mrs Pingu]

I thought they had modulating boilers. Otherwise how do you control the returning water temperature and thus keep it below the dew point.

[Kim]

Sure, but (on our one at least) it modulates to maintain a flow temperature set by a Mk 1 turny knob.

[Mrs Pingu]

As does ours. I'm assuming (perhaps wrongly) that a new all singing, all dancing, planet saving condensing boiler ought to be modulating....

[Quisling]

Gas boilers modulate well and typically have a turndown ratio of around 5:1, that is to say they can reduce their firing output a good deal. This isn’t quite as possible with oil, and certainly less so with my wood pellet boiler. Nonetheless, all can modulate their output.  However, they tend to only output to hit a single target flow or return temperature via the Mk1 Turny Knob.

Variable temperature circuits use a blending valve to blend the output from the boiler progressively with the heating circuit.  This allows a sort of double modulation, using both combustion control and hydraulic circuit control. Obviously not all boilers are set up for this.

Blending valves are important now to help prevent the shock of cold return water entering a small and very hot heat exchanger, as the valve can progressively open as the central heating circuit temperature rises.   This prevents condensation (and corrosion) in the boiler heat exchanger whilst improving control of space heating.  Using smart algorithms the controls can look beyond the immediate outside air temperature signal, and consider time of day, target temperature, trend direction of recent outside air temperature measurements etc. They can learn how swiftly your house responds to heat input (eg how quick does it get up to temperature in the morning), and start to figure out exactly how much kW input is needed for a given situation. For example, if the outside air temperature is rising, the boiler may hold off firing up if it thinks it will overshoot the set point target temperature or indeed continue firing for longer if it sees the temperature dropping.

This all helps prevent the boiler from cycling on and off repeatedly which can impact on both longevity and efficiency.

Q

[Tim Hall]

We had such in the house I grew up in, some several of years ago. I shall ask the AP how it worked tomorrow.

[Kim]

We had such in the house I grew up in, some several of years ago. I shall ask the AP how it worked tomorrow.

Thinking about it, so did I.  Though it might have been a frost stat.

[Aunt Maud]

We have one fitted to our boiler. It went ballistic and nearly melted the radiators when I accidentally cut the cable, so I guess it works. The boiler controls are very complicated with all sorts of adjustment for outside air temperature, heating curve a n d other stuff.

I set it to the manufacturers settings and left it be, as I don't feel the desire to try and understand it. So I'm wondering if these things are better pre set and don't have all the nobs and dials for uneducated types like me to play with.

[Mrs Pingu]

I think the whole point is that you don't need to fiddle with it, you just set the room stat to the temperature you want and let it do all the clever stuff involved in learning how long it takes to heat up your room to temperature x if the outside temperature is y.
Welcome to the future...

Anyhoo, I peered at a Worcester Bosch boiler download this morning and eventually it told me it was modulating.

[ian]

This is where SkyNet starts isn't it? The real one.

Our last central heating system was so complicated that it made the engineers cry like little baby men. Honest, there was an entire room full of pipes and valves, tanks, and little glowy panels and flickering numerals, and things that occasionally beeped for inexplicable reasons in the middle of the night.

The current one is a decade old boiler in an airing cupboard with all the sophistication of a hot brick. I was going to replace it, but it's too complicated. It can stay until it gurgles its last.

(one of the best things was that to supply my office, a mezzanine extension off the side of the house, the managed to route the hot pipework outside the house)

[Aunt Maud]

We don't have a room thermostat coupled to the boiler, so I wonder how ours knows what to do. Maybe it measures the temperature of the return flow from the radiators and does some kind of sums type stuff.

[Quisling]

We don't have a room thermostat coupled to the boiler, so I wonder how ours knows what to do. Maybe it measures the temperature of the return flow from the radiators and does some kind of sums type stuff.

Most likely to be a return temperature sensor which tells the boiler that the lower the return temperature the higher the heat demand.  Coupled with thermostatic radiator valves in the main living areas this is a reasonably effective control methodology.

[Aunt Maud]

That's what the current DIY work load consists of, a new heating system using manifolds to supply the radiators and thermostatic valves on each radiator. 

I've also hooked up a low energy Grundfoss pump on the return side to take some of the load from the boiler pump, which has to pump along nearly 200m of pipe to the radiators when they're all installed.

The new pump uses 9 watts compared to the boiler pump which uses over 50 watts.

Serves me right for buying a long house, but it's working a treat even though I designed and installed it myself.