Central heating stat setup advice
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Central heating stat setup advice

Post by thedoctor9990 » Sat Aug 26, 2023 2:25 pm

About to buy a new build 4 bed property, been told there are 2 room stats, one in the hall controlling the whole house except for bed 2 upstairs over the int garage, this has its own stat in the bedroom which controls bed 2 only
I can't see how this is a good idea, the boiler would fire on and off for bed 2 only if the downstairs room stat is satisfied, plus bed 2 rad could not have a rad stat, as it could be closed when heat is called for
Please help me understand this, unless the sales team has this wrong
Thank you

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Re: Central heating stat setup advice

Post by ericmark » Wed Sep 06, 2023 12:17 pm

The government in their wisdom has required that central heating is split into zones, but it is unclear what they mean by zones, a number of country councils have considered fitting TRV on radiators is counted as splitting into zones, but many central heating fitters seem to think you need a zone valve for it to be counted as a zone.

This has resulted in the reverse to the intended and central heating running in a less efficient way.

With my oil fired central heating it has two states, off and on. However most gas central heating use modulating boilers, this means the boiler can turn up/down rather than just on/off.

There are two ways to control this, one is ebus and a special thermostat OpenTherm for example, the other is with the return water, with the latter the problem is once the boiler goes below minimum output it has to switch off, but there is then no water flow to tell it to switch on again.

So if no ebus then one needs either the TRV's to link to the thermostat, or the thermostat is placed in a room with no outside doors, on the ground floor, with no alternative heating, and normally kept cool. Often there is no such room.

So common is to use the hall, however it has an outside door, but if you fit a TRV to the radiator you can set it to start turning off the radiator before the wall thermostat temperature is reached, allowing fast recovery, but then slows down so rest of house can heat up.

In winter the wall thermostat should never turn off, the TRV's do all the work.

However we tend to want it cool through the night, then slightly warmer through the day, then even warmer in the evening, so either the thermostat needs to be programmable or the TRV's or have them linked.

With 14 heated areas I use programmable TRV's in most rooms, and a programmable wall thermostat for upper two floors, but lower floor is independent as hardly used in winter so just turn off the 4 down steps rooms.

The question is how much do you want to pay? A programmable TRV can be as cheap as £15 each, in 2019 I got 5 x eQ-3 TRV heads with bluetooth for £15 each, since brexit they have gone up in price, the Terrier i30 is similar, but I have moved to Kasa heads (TPLink) as they connect to internet at around £30.

However to link to the wall thermostat with smart technology so you can set temp and time you want to use room and it auto turns on early enough to ensure warm on time, your looking as Drayton Wiser at more like £45 each.

Much also depends on boiler some thermostat will connect to the e-bus, some need an extra bit like Wiser and the Honeywell EvoHome, some it is built in like with Nest (although Nest does not connect to TRVs so bit useless) and some as simple on/off like Hive but still connect to TRV's.

Likely wiser and evohome are the market leaders, but also expensive.

And they all need some setting up of the lock shield valve other side of radiator, if they show target and current, if current exceeds target then the lock shield needs closing a bit, if the radiator heats up too fast, the TRV does not have enough time to close, and other radiators will be starved.

Official method is with differential thermometer, but the TRV reading worked well for me in mothers house.

The reason the boiler modulates is to gain the latent heat, it slowly turns up/down and keeps the return water cool, turning it off/on by external controls means it turns back on at full output, and until the system has settled it uses more gas, so we want to turn it off/on as little as possible using outside controls, if it internally turns off because reached min output it turns back on at min output.

Best is e-bus controls where the wall thermostat turns it up/down, there is the EPH thermostats which can be set as master/slave with up to 10 and can still work OpenTherm, but they don't link to TRV's the Wiser three channel unit can control DHW and two zones and still have OpenTherm.

I have never tried wiring both the on/off to the motorised valves and the OpenTherm to boiler it should work, but not tried with Wiser.

Likely you are now totally confused just like the daft government directive which has resulted in loads of twin zone systems where not required, and has made them less efficient as a result.

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