by ericmark »
Fri Feb 28, 2020 4:36 pm
The Part L and Part P and all the other parts have an approved document to try and tell you what is permitted, but that approved document is not the actually law, it is some ones interpretation of the law.
As an electrician more involved with Part P where the actual law says very little.
However hunting for official government documents each county council seems to have different ideas as to what complies, one I read seems to say fitting thermostatic radiators valves (TRV) is enough.
However in real terms what you want is to reduce bills, and the traditional zone valve has a major problem, it is either on or off, no half way control, this is all well and good with an oil boiler, not much good with a modern gas boiler.
So first let me explain the boiler, to gain the latent heat, the return water needs to be under a set temperature, so unlike the oil boilers gas boilers are controlled mainly by return water temperature.
To keep the temperature within limits, the gas boiler can modulate, i.e. turn down the output, so typical output is between 8 kW and 28 kW. There are two ways to control the boiler, one simply return water temperature the other is to use the ebus.
ebus is all well and good, but some makes of boilers will only allow one thermostat connected to ebus, some used OpenTherm which can connect to a hub and from the hub many rooms, but when your forced to use one room as a sample room to tell boiler when to run, it makes life hard.
So some systems are designed to simply let the boiler do all the control, so basic problem is the boiler can turn up/down and once at lowest setting it can cycle on/off but it can't work out when the weather is warm enough to switch off for an extended time.
So some where some how the boiler needs to be told to turn off when weather is warm and on when weather is cold, this can be from many rooms or just one, but turning boiler on/off is a different task to turning boiler up/down.
So easy way to turn boiler down is a programmable TRV head, these start at under £10 (eQ-3) up to around £80, problem with cheap version they can keep room from getting too hot, but they can't tell the boiler when to run.
The more expensive types have a wifi like to a thermostat/hub these can tell boiler to fire up as well.
And the next stage up works out at what point to stop heating so it can heat the room up much faster without over shooting.
But the main thing with gas central heating is nothing switches on/off it all turns up/down, analogue is king with gas central heating it stops a large hysteresis.
Every time the boiler switches off, the heat in the boiler is lost out of the flue, and if you externally turn it off, when it switches back on it switches on at max output, often with no latent heat extracted, so you don't want boiler switching on/off all the time.
In the main the motorised valve means the boiler is switching on/off, which is not what we want, so we need to use the TRV which slowly turns up/down.
With domestic hot water (DHW) there is a case for on/off, also as with this house I have a flat under the house which most of the time does not need heating, so a motorised valve in that case makes some sense.
So now looking at house design, the likes of Drayton Wiser, Honeywell EvoHome and Tado will work A1, however they are expensive, so what most do is use a mixture, the Hive is an odd one out, rest the thermostat/hub can connect to boiler using OpenTherm, hive does not have an option of OpenTherm it keeps boiler running if any TRV head sends a demand for heat. Nest is also an odd one out, it has got OpenTherm but it does not talk to the TRV heads.
Each one has some thing special, the Wiser TRV is designed to heat rooms fast working out (learning) when to turn down so it will not over shoot, the Nest wall thermostat has both geofencing and occupancy detection, the Energenie TRV heads have two sensors one for air and one for water so compensates for heat from radiator, the cheap eQ-3 has open window detection I use on in kitchen so when unloading shopping from car it turns off kitchen heating.
EvoHome is likely the best, been going I think the longest, but who really knows? Once we fit one we are stuck with it, I fitted Nest which was claimed to work with Energenie Mihome, well it does, but not reliably. So all my TRV are set independently, same schedule but not linked, so the cheap eQ-3 would have done same job, I got the bluetooth version of eQ-3 at £15 each, the non bluetooth seen for £10 each.
The programmable TRV allows me to set time and temperature for each room, I have 9 of them, in theory the Energenie Mihome can be set to geofence, however I just use the Nest, if boiler is not running the TRV can't heat up room.
Last house a massive 4.5 kW gas fire in living room, no need to have central heating geofencing it heated up so fast, last house also open plan, so one thermostat controlled all.
Every house is different, so no one size fits all.
