A smarter thermostat

[chris French]

I've been looking to get one of those, but they have announced a new model the Neo, so I'm in an indecisive quandary :-)

>

[Dave Liquorice]

I lied, working from memory. Set point 18.5 C, on at 18.4, off at

18.6 C. Overshoot about the same I don't see any undershoot. So temp held within 0.3 C. Rather than the 1 C plus under/over shoots of that Heatmiser.

[fred]

That's not you cheating and running it in proportional is it?

They only claim 'less than one degree' in the data for on-off which is what I thought you have said you ran it on.

I'm seeing a little overshoot with on-off but probably less than half a degree.

[harryagain]

What happens is you tell it the occupation time and temperature. It starts the heating system up at a time to bring the building to occupation temperature justin time.

The actual time it starts the heating system will depend on inside temperature, outside temperature, thermal massof the building, (maybe) windspeed. Also the recovery time for the boiler and heating system.

So on a cold windy day, it starts the heating sooner. On a warm day, later.

When it is set up properly, the building is at desired temperature just in time for occupation.

If self learning, it needs to operate over the whole range of possible weather scenarios to determine the right set up.

If manually set up, it is a PITA, you will likely never get it just right. I have spent hours f***g about with the things.

The reverse happens on shutdown of the system.

[harryagain]

All these problems are caused by on/off boilers. (Most domestic boilers) Commercial boilers are modulating, this is a huge advantage.

[harryagain]

About covers it.

[harryagain]

It just means in warmer weather , it will turn the system off sooner than in colder weather. (ie before occupation of the building is eneded) Does the calculations taking all the same factors into account.

So, taking both start up and close down into account, you could save 10% compared with simple clock/thermostat.

They used to be expensive electro-mechanical devices subject to drift and calibration problems. I expect things have improved nowadays.

[harryagain]

There are several factors (see previous), to be accurate, all have to be independantly monitored and integrated.

[Andy Burns]

It measures to 1/10th of a degree, you can set the switching differential to 0.5, 1, 2 or 3 degrees. As my boiler is a cast iron dinosaur I don't have it cycling too frequently, personally I don't notice 1 degree.

[John Rumm]

Pretty much all domestic boilers are modulating these days. However that is modulation in response to load, not external temperature.

[Dave Liquorice]

point

overshoot

Nope bang bang mode, don't like the idea of cycling the valve 6 times, or what ever it is, per hour.

300 l thermal store may help as when there is a call for heat the water fed in will be no colder than 60 C. There is no lag whilst the boiler warms itself and the circulating water up.

[harryagain]

If a boiler can be made to modulate in response the external temperature, that is a saving too.

Another saving that might go domestic is continuous combustion gas oxygen analysisng. The air supply is trimmed to suit so getting more efficient combustion. Without it the ratio varies hugely depending on air temperature, pressure and humidity. The usual ploy is extra excess air.

There is a 5-10% save there. However only viable in the past on large boilers due to cost. There can also be associated electricity savings. Also the oxygen sensors were liable to be "poisoned". Especially if the fuel was oil or coal gas.

[John Rumm]

aka Weather Compensation. Many of the higher end models support this out of the box if you select the right controls. Obviously you can retrofit it to any system using blending valves etc, but it does not get you the same efficiency gain as being able to control the actual heat output of the boiler.

Its one of those cases where 5 - 10% is fairly meaningless on domestic scale systems... you start with a basic boiler that is 90%+ efficient out of the box on the SEDBUK annual cycle. Add weather compensation to claw back an extra few of those remaining 10% (and gain an improvement in comfort into the bargain).

At which point you are talking about getting back and additional 10% of the last 5% at best, while at the same time adding complexity to the control system and lowering the overall MTBF.

You will likely get more bang for your buck with a new set of draft excluders.

You could lift much of the technology from automotive applications these days. Most engine management systems will use lambda (O2) sensors to maintain ideal stoichiometric conditions.

[gremlin_95]

I know some of the domestic Viessmann boilers have lamda sensors.

[harryagain]

[harryagain]

All down to the parameters used to determine efficiency. There are several ways.

They always use the one that gives the highest figure. Just a sales ploy to baffle the purchaser.

[John Rumm]

Which is why one uses SEDBUK figures since they are intended to avoid the effect of sales inflation, and perfect lab condition results. They are based on the Gov's SAP 2009 procedures.

Quite the reverse actually, its a fairly respectable attempt to look at real world installed performance:

[John Rumm]

OOI, do you know if they use them to feedback information to the control system, or just for diagnostics and reporting?

[gremlin_95]

Good question, according to the Viessmann the ' Lambda Pro combustion controller automatically adjusts combustion for fluctuations in gas quality, which extends the life of the boiler and service intervals'

I have no idea how much difference it makes but I do know that the cheaper Vitodens 100 model which was made for the UK market doesn't have that technology.

[harryagain]

I can see you haven't grasped this at all.

They use the traditional (to boilers) net calorific value of fuel to work out the efficiency. ie neglecting the latent heat of the water in the combustion gases. This is simply to make them look better.

This means that if all the available energy were got out of the gas, the efficiency would come to around 110%.

ie you were getting out more than was going in. So this is why SEDBUK is actually bollix when it comes to condensing boilers.

So, there is more scope for saving than you think. Our boilers are not as efficient as you think.