Efficient settings for Ideal Max Heat only boiler

[fairplay]

This is my first winter of using my new and first condensing boiler - model 18 kW as above. Now I suspect the heating engineer has over sized it a bit to be "on the safe side" as my old Potterton was only 15 kW and fine for my 3 bedroom semi. Generally have the following radiators on - hall double, two in lounge, one a double, small one in study, bathroom. Then two bedrooms, on for just a short time in the evening, spare room off. All TRVs and controlled by the excellent Tado smart thermostat. Insulation average, 60's house.
So I run it on the 'E' setting on the boiler and have my Wilo CH pump on its lowest setting of 6W to minimise electricity cost.
All works extremely well but water in radiators is very hot, almost scalding when on full which is usually only small bursts and return pipes feel very hot as well.
So what's the problem - well none really other than I only seem to only have small reduction in gas useage and wondered if I am running it efficiently like this? Annual consumption with old boiler was 15,000 kWh p.a and we are at home all day. (retired) If 18 kW is overkill can this power be turned down internally, I think I read some can?

 

[muggles]

Almost certainly oversized - my 4 bed semi of similar vintage to yours runs quite happily on 11kW. There's very little you can do though I'm afraid - the Logic Max doesn't have any configuration available, so it is what it is. One thing it does have, however, is the ability to accept an OpenTherm connection. This will vary the flow temperature according to how much heat your house actually needs, rather than running it on the high fixed temperature you have now. Lower is always better with condensing boilers. Tado can output OpenTherm but has probably been connected on/off on your installation. If you can get the OT up and running that will certainly help

 

[ericmark]

@muggles idea of OpenTherm is good, but most modern gas boilers can also be controlled by the return water temperature. This is in turn controlled by the TRV and the lock shield valve.

My TRV exercises at 12 noon every Saturday and it takes around 5 minutes to fully open and fully close and return to setting, so the lock shield valve needs to restrict the flow so it takes longer than that time to heat the radiator or the radiator will get stinking hot before the valve can close.

The word of the day is hysteresis, that is the sine wave of temperature caused if the controls work too rapid, OK some smart TRV's like the Drayton Wiser claim to work out when they need to turn off so there is no over shoot, but unless using geofencing not really required.

So the idea is the TRV slowly opens or closes to maintain the room temperature, this in turn results in the return water getting warmer or cooler, and the boiler turns down the flame height the warms the return water gets, so keeping the flue gases cool enough to gain latent heat, as the TRV's close the by-pass valve opens so the return water gets hotter as the close.

There is however a problem, for the return water to control boiler it must be circulating. So although once the boiler is at minimum output it can start to cycle off/on, it can never switch completely off, as it has no way to turn on again.

So we use a wall thermostat, the idea is it stays on all winter, but stops boiler cycling in Autumn, Spring and summer, and the thermostat is placed some where normally kept cool.

However you can link TRV heads to a thermostat/hub using wifi, so the TRV's tell the thermostat/hub when boiler is required. This is the icing on the cake, but basic idea remains, the TRV is king with a modulating boiler.

So your task is to set the lock shield valve and TRV heads, and I will admit in my mother house this took some time. I don't know how Tado works, but my TRV heads reported to the PC like this so I have current and target temperature displayed, in Winter current should never exceed target, so if it does, turn the lock shield down a tad.

Over the winter living with mother I slowly adjusted the lock shield valves, and by the next year the temperatures in each room were spot on, well one small problem, the anti hysteresis software in the TRV heads was OTT, so in the morning 7 am set to 22°C then at 8 am to 20°C or it would be 10 am before it reached 20°C.

On my mothers death we decided to sell the house, and new owners did not want the electronic TRV heads so all swapped back to mechanical, but now the lock shields were set, and even the mechanical worked spot on.

The problem with mechanical is there is around 4°C between fully on and fully off, so there is no spot on setting, so not calibrated in °C but *123456 and you have to by experiment work out what 20°C is. That is OK if the lock shield is set, but if not the problem is how do you know which to alter?

When I only had 2 electronic TRV heads, I moved them radiator to radiator to set the lock shield valves.

 

[fairplay]

Thanks for the interesting replies. I didn't know about Opentherm and have read up on it now so thank you. As far as I remember it is not wired up for it and I don't want to risk doing it myself although I did the original Tado hard wiring myself. However the Tado Smart thermostat itself does seem to do a remarkable job of keeping the temperature at the set point of 21.3C, rarely going more than 0.2C above or below. The boiler doesn't cycle much either and the Tado has three symbols for what I assume is the heat output of the boiler and it only really stays on three when you make a large change in thermostat setting from say 16C at night to 20.5C day time. The rest of the time it only shows 1 or two bars of heating and radiators aren't fully hot most of the time. It seems to control it very well.
With regard to the boiler setting, given all of the above, will it be best to leave it on the "e" setting and let it work it out itself or should I turn it down to a lower setting?
So I suppose I am a little bit surprised at the amount of gas being used but perhaps it is down to the insulation. Loft is lagged and boarded over but thinner lagging than current regs, would say it is about 10cm. Also Dormer Chalet type so big roof and not sure how well the dormer flat roofs are insulated. Has old cavity wall insulation though and obviously double glazing.
Thanks again for your helpful replies.

 

[fixitflav]

With regard to the boiler setting, given all of the above, will it be best to leave it on the "e" setting and let it work it out itself or should I turn it down to a lower setting?

I would try turning the boiler down below the E setting. If it only comes on in short bursts it sounds like it's heating the house adequately. Lower flow temperature gives longer run times, cooler rads and more condensation, so saving some gas.
Also I'd be surprised if the maximum output is not adjustable, and reducing that would improve things. Somebody here, or the installer, might confirm.