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 Post subject: Thermostat advice
PostPosted: Tue Jan 22, 2019 10:59 am 
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Hi folks, I am a novice, could somebody please offer me some advice on the following.

I recently had my 25 year-old G rated boiler swapped out for a new Ideal Logic 35KW C Combi. It seems to be working well except I am having real problems with the temperature control. I have got a wireless "CENTER EHE0200361 RF Programmable Room Thermostat" which I have tried in several rooms in the house.

If I leave the boiler temperature set to Eco (around 68 deg) then the house very seldom reaches the target temp of 21 deg. If I turn the boiler up to full (around 80 deg) then is reaches the target at around midday. The real problem is that once it switches off then it stays off for up to 2 hours until the temp drops by a couple of degrees, by then the rads are all cold and the house is cold. The only way I can regulate it is to operate the thermostat manually.

The engineer has changed the thermostat and the wireless connection is showing as excellent.

All the settings are factory default.

We are considering fitting a wired thermostat (as my old my old one always worked well) but I have looked at the specs and it seem to me that wired would work in exactly the same way as wireless.

I would really appreciate some advice.

Thanks,


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 Post subject: Re: Thermostat advice
PostPosted: Tue Jan 22, 2019 7:38 pm 
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Some modern stats have a hysteresis, you may need to adjust this setting.

In short it means that once a stat gets to a set temperature it will switch off (as it should) but as the temperature falls the hysteresis option comes in and the stat will not turn on until it is say 5 degrees lower than the switch off temperature.

The figure of 5 degrees, I just made up, it may be more, it may be less on yours.

If it helps, I don't like wireless stats, the battery will fail at 3 O'clock in the morning so when you get up the house will be cold and you have not got a spare battery. :cb

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 Post subject: Re: Thermostat advice
PostPosted: Tue Jan 22, 2019 8:00 pm 
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:welcomeuhm:

:withstupid: it does sound like it's not clicking back on soon enough.

Additionally, may not be relevant, do you know if the radiators have been balanced through the property, or, if you have TRV's, are the lockshield valves all fully open?


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 Post subject: Re: Thermostat advice
PostPosted: Tue Jan 22, 2019 11:01 pm 
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Hi guys your advice is invaluable. I had already managed to research hysteresis today and it is a concept I understand, so thanks for that.

1. I'm pretty sure that the rads were not balanced.
2. I asked for TRV's to be be fitted throughout but I suspect they are very poor quality as 2 do not work properly and I leave the others on full as they only have about a quarter of an inch of operation.

Can you please advise if I can buy a thermostat where I can change the hysteresis value?

Thanks.


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 Post subject: Re: Thermostat advice
PostPosted: Tue Jan 22, 2019 11:05 pm 
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I would look at your setting first of all.

You say that the programmer is set to default settings

If that is the case then Mon to Fri at midday the boiler setpoint should be at a temp of 18 degC according to the manual. Sat and Sun the setpoint should be 21 degC at midday

What day is the boiler not reaching 21 degC


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 Post subject: Re: Thermostat advice
PostPosted: Fri Jan 25, 2019 11:51 am 
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I think the Ideal Logic 35KW C Combi is OpenTherm enabled although not sure, so the CENTER EHE0200361 RF Programmable Room Thermostat is not opentherm so possibly the wrong type of thermostat for the boiler.

However the problem is likely to be settings of the lock shield valves on the radiators and nothing to do with boiler or thermostats.

Unless using a modulating thermostat, modern boilers modulating boilers use the return water temperature to control the output, there may be a thermostat to turn off the boiler in warm weather so it does not cycle, but room temperature is controlled by the thermostatic radiator valve (TRV) with the valve fully open there should be around 12 degs C drop in temperature across the radiator input to output with the TRV on input and Lock Shield valve on output, as each TRV closes more water is forced through the remaining radiators until finally the by-pass valve opens. As the water returning gets hotter and hotter so the boiler slowly reduces output until at minimum at which point it turns off/on and cycles using a mark/space ratio to further reduce output.

So if one radiator has the lock shield valve wide open, then the return water is far too hot, so the boiler turns down prematurely, same if by-pass valve stuck open.

So starting closest to boiler upstairs first as heat raises turn off radiator completely with lock shield valve, allow pipes to cool, then 1/4 turn at a time with a few minutes between each adjustment open the valve again counting turns until you feel hot water return, feel the pipes, and stop as soon as you feel a little warmth.

Now shut every other radiator lock shield valve and reopen the same amount, this should mean all radiators heat up even rather than first one then the next, and also mean the water has time to cool in the radiators so boiler will now give full output, until the by-pass valve start to open.

You may then want to open some valves a little more, specially those with furthest run, but you have a basic adjustment and from that you can improve the balance.

TRV's take time to open and close, the lock shield valve slows water flow, giving the TRV the time it requires so the temperature does not over shoot.

The thermostat should be in a ground floor room with no outside door, no alternative heating, and normally kept cool, it has one job, that is to stop the boiler cycling in warm weather, in cold weather it should always be on, the TRV controls room temperatures not the wall thermostat.


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 Post subject: Re: Thermostat advice
PostPosted: Fri Jan 25, 2019 10:27 pm 
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1. What make/model/kW was your old G rated boiler?

2. Is there one room without a TRV (s there should be)?

For info, the Center Thermostat is a re-badged Honeywell CM727 which is a very good product and unlikely to be the cause of your problem.

An Opentherm boiler will work with a non-Opentherm thermostat, it just doesn't have the means of adjusting the water temperature.

Rads with TRVs need to be balanced, which means adjusting the lockshield valve. TRVs are not self-balancing, despite what some say.


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