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PostPosted: Sat Nov 04, 2017 4:56 pm 
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Hi all

We're in the process of modernising mother-in-law's house.

The house has recently got its first ever gas connection and now has full central heating supplied by a Baxi Duo-Tec 33 combi boiler..

The heating thermostat is a Hive 2.

What I can't get my head around (we don't have a combi at our own house) is the purpose of the central heating temperature dial on the boiler - I understand what the hot water temperature dial is for. If you're using a thermostat (Hive or any type), wouldn't you just set this dial all the way to maximum (80) so the house heats up quicker and then the house thermostat will switch the heating off?

What I also notice happening is that when the heating is on, the boiler is constantly firing to maintain the temperature set by that dial. So, you set it to 60, it gets up to 60 and cuts out, the display then drops to 59 and it fires up again. Is this what you would expect to happen?

As I said - new to combis - we have a system boiler- so apologies if I'm missing something obvious.

Cheers.

Ian


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PostPosted: Sat Nov 04, 2017 7:00 pm 
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Your system boiler has a temperature control as well.

With the duotec if you can, run the temperature around 60-65c.

Yes, if you set it to 80 the rads get hotter and heat the room quicker, but the boiler will hardly be in condensing mode, which is how a modern boiler achieves its extra efficiency.

Although dew point is technically a variable, in heating systems a figure of 56c is generally quoted, the return water temperature needs to be below this to maximise condensing mode.

To achieve this, new complete heating systems will have slightly larger radiators so a lower flow temperature will heat the rooms up just as quick due to the larger heat emitting surface.

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PostPosted: Sat Nov 04, 2017 7:04 pm 
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Thanks for your quick reply Dave - I appreciate it.

So the constant firing on and off should be expected?

Ian


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PostPosted: Sat Nov 04, 2017 7:11 pm 
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The boiler will cycle on and off regularly, just to keep itself up to the set temperature, but this should stop when the room thermostat turns off.

However, most modern boilers have an anti cycling software built in that often limits this to around every 3 minutes.

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PostPosted: Sat Nov 04, 2017 7:47 pm 
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Thanks again Dave. Yes it does stop firing when stat is up to temperature. But when it is on it can be switching on and off every 30 seconds. I'll run it by the installer next week.


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PostPosted: Sun Nov 05, 2017 2:10 pm 
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up_north wrote:
... when it is on it can be switching on and off every 30 seconds. ...

It does that because it cannot modulate low enough to produce the heat required.

Your boiler will modulate between 9kW and 29kW. You may not need more than 15kW to heat your house when it is very cold (-3C) - unless it is very large and old without decent insulation. In the current relatively mild weather (10C), you may only require about 7-8kW to heat the house. But your boiler cannot modulate this low so it has to turn on and off to maintain the required water temperature.

This is one of the disadvantages of a combi boiler. It has to be powerful enough to heat cold water instantly from 10C to 50C or 60C, which means that it is normally over powered for heating the house.



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PostPosted: Sun Nov 05, 2017 3:26 pm 
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Thanks - that's genuinely interesting. But the manual states this:
-----
2. Once main burner ignites the fan speed controls the gas rate to maintain the heating temperature measured by the temperature sensor.
3. When the flow temperature exceeds the setting temperature, a 3 minute delay occurs before the burner relights automatically (anti-cycling). The pump continues to run during this period.
-----
So doesn't this mean it shouldn't fire more often than every three minutes?

I understand that when a combi ignites it can release a bit of gas but becuase ours is igniting so frequently there's a really strong smell of gas near the flu.

Is there anything I can do with the other controls - thermostat, TRVs etc to 'help' the boiler?

Thanks again.

Ian


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PostPosted: Sun Nov 05, 2017 4:12 pm 
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In theory the boiler should start the spark fractionally before opening the gas valve.

The 3 minute anti cycle will be broken if a hot tap is used during the process.

Ignition electrodes were a problem on these, but I thought they had sorted this for the new ones, so get your engineer to check them out.

The 33Kw is very powerful for an average house, but combi's are usually sized for HW use.

These boilers do not have the facility to range rate the heating side as far as I am aware, so are relying on the built in software to keep the burner modulated down. Your boiler should be able to modulate down to around 10-11Kw, but if several of the thermostatic valves have closed, this will still be way too high.

I would suggest you experiment with different settings of the main room thermostat to 'learn' what is a good setting so all the rooms are a comfortable temperature when the main stat turns off.

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PostPosted: Sun Nov 05, 2017 11:03 pm 
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Thanks Dave. Unfortunately no hot tap was in use.

I vidoed it - unfortunately it looks like I can't embed it but the url is:
https://youtu.be/y8eo6o69llA

Dial on boiler was set to 60. Room stat set to 23 - had reached 19 when I filmed this. All TRVs fully open. All rads except towel rail have a TRV. Heating had been on for approx 45 minutes.

Ian



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PostPosted: Mon Nov 06, 2017 8:00 am 
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I have to admit I've never seen one doing that before.

From here I wouldn't have a clue why its doing it that often.

Hopefully your installer can shed some light on it, being there, or possibly put a call in to Baxi.

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PostPosted: Mon Nov 06, 2017 5:25 pm 
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You should not smell any gas is your installer gas safe registered did he issue certificate and tests you can upload certificate and place paper over personal details

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PostPosted: Thu Nov 09, 2017 5:12 pm 
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Multiman - sorry - thought I'd reolied to this. Yes definitley Gas Safe registered.

A bit of an update on this. As I said when the installer came round it worked perfectly. I later noticed something and I can now replicate it. Switch heating on and let boiler come up to temp - then run a hot water tap for at least 30 secods. It seems that running hot water then triggers the constant firing cycle until the heat call is stopped which we do quite quickly because of the fumes from the flu.

Ian


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PostPosted: Thu Nov 09, 2017 5:17 pm 
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Call Baxi as dave (gas4you) suggests. in the meantime spend £20 or so on a carbon monoxide alarm.

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PostPosted: Thu Nov 09, 2017 5:44 pm 
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Something definitely needs professional attention.

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