Wireless thermostat and Combi Boiler query

[Simon Anderson]

Sorry to trouble you with what is probably a naive question but I am a complete Combi bolier virgin. Since moving out of my parents home 17 years ago I have had a traditional oil bolier with a hot water tank and in our new home we have taken the plunge when we needed to re-plumb and installed a combi boiler.

So far the boiler has been installed (Baxi Duo Tec 40 HE), I have TRV's on all rads, new pipes and new rads throughout the house. I am about to get a wireless thermostat installed (Honeywell 927). The boiler controls are obviously pretty simple (on/off, water temp contol and heating temp control) but I am keen to understand how the system actually works.

At the moment as it is mild (and we have not actually moved in) I have set the boiler temp to about 25 degrees and have set the TRV's in the various rooms to 1 or 2 throughout the house, hot water temp is right down to just above luke warm as we are not really needing hot water. It is keeping the house from getting cold but if you were sitting watching TV you would want it warmer (but as I am frantically decorating it is grand).

Query:

1. The heating temp range on the boiler goes from 25 to 80 degree C according to the manual but what level should I set the boiler to in order to get it right when we actually move in?

2. If the boiler range is 25 - 80 degree C does the boiler kick in if the temp it reads at the boiler is less than 25 degrees and as I dont have a wireless stat to override does that mean that probably at the moment my boiler (which as mentioned is positioned in the garage) is going to be kicking in all the time as the external air temp is about 11 degrees currently.

3. Is the way I have the system currently set up correct in order to keep the energy costs down at the moment but prevent the house from getting very cold (house temp appears fine at moment).

4. If I turn the boiler temp right down to lowest setting will it kick in at 5 degrees and if so is this due to a frost stat in the actual boiler identifying the surrounding temperature? Given the fact my boiler is in the garage which will be colder than the inside of the house is this likely to kick in a lot over winter & if so does it just take the bolier to +5 degrees or to the temp set on the boiler?

5. Is about 19 degrees for the heat and about 50 degrees for the hot water a reasonable boiler setting when we are moved in and use the TRV's & wireless room stat to control rooms.

After installing the wireless thermostat I have a couple of queries:

1. Say I want the temp of a our living room to be set at 20 degrees what temp setting to I set the boiler to in order to ensure that it achieves that or does it not matter and I can set it at the lowest temp as the wireless stat will override it anyhow?

2. Has anyone a view on whether hot water should be set to its highest temp the boiler can go or the highest temp that you actually use the water at e.g the temperature that we would normally use for a hot bath or hot water for dishes. Is it just a waste to run the water warmer than that and does it affect flow rate?

Sorry for the barrage of questions and I hope some kind soul can take pity on me and answer them. Many thanks in advance.

 

[PaulKnappett]

Hi Simon

I'm not a professional, but I installed a Honeywell programmable thermostat only yesterday evening! I have a similar setup to yours, with a combi boiler in the kitchen and TRV's on radiators.

Your first 5 queries need to be answered by those with much more knowledge, but here's my go at the last 2!

1. If you set the boiler temp very low, the radiators won't get hot enough to get the room up to temperature at all, especially if it's cold weather, and the stat will just keep the boiler on continuously. The higher you set the boiler (and so the radiator) temp, the quicker the room will heat up and the stat will cut off. I have mine set between one-third and half, so the radiators are bearable to touch for a couple of seconds and not red hot (sorry, no temp markings on my boiler)!

2. If you set the water too hot, you then have to use cold water to cool it down most of the time. I set mine so water is hot enough for washing dishes, bathing, etc. and if I need some really hot water, e.g. mopping kitchen floor, I boil a kettle - it's cheaper!

I think general guideline for heating and hot water is to experiment and use the lowest temperature that works for you in each case. It's probably kinder to the boiler and most economical on fuel.

Hope this helps.

 

[D_Hailsham]

I have TRV's on all rads, new pipes and new rads throughout the house. I am about to get a wireless thermostat installed (Honeywell 927).

You should not have a TRV on the rad where the CM927 (good choice) is located.

The heating temp range on the boiler goes from 25 to 80 degree C according to the manual but what level should I set the boiler to in order to get it right when we actually move in?

Set it to max; the CM927 will control everything.

When the heating first comes on the boiler will run continuously, so the temperature will rise to the maximum, meaning the house will heat up quicker. But when the required temperature is nearly reached the CM927 will start cycling the boiler, which will effectively reduce the water temperature until it is giving off just enough heat to maintain the required room temperature.

If the boiler range is 25 - 80 degree C does the boiler kick in if the temp it reads at the boiler is less than 25 degrees and as I dont have a wireless stat to override does that mean that probably at the moment my boiler (which as mentioned is positioned in the garage) is going to be kicking in all the time as the external air temp is about 11 degrees currently.

The boiler thermostat controls the temperature of the water leaving the boiler. It has nothing to do with the air temperature. It would be a good idea to get the CM927 installed asap. This has an adjustable frost setting (default 5C), so the boiler will only come on if the house temperature drops that low.

Is the way I have the system currently set up correct in order to keep the energy costs down at the moment but prevent the house from getting very cold (house temp appears fine at moment).

See above.

If I turn the boiler temp right down to lowest setting will it kick in at 5 degrees and if so is this due to a frost stat in the actual boiler identifying the surrounding temperature? Given the fact my boiler is in the garage which will be colder than the inside of the house is this likely to kick in a lot over winter & if so does it just take the boiler to +5 degrees or to the temp set on the boiler?

If the system temperature falls below 5°C then the boiler will fire on its minimum setting until a flow temperature of 30°C is reached.

Is about 19 degrees for the heat and about 50 degrees for the hot water a reasonable boiler setting when we are moved in

Room temperature is a personal preference. 50C is too hot for the DHW (apart from washing-up when you are wearing rubber gloves) as you will have to mix it with cold water for the bath shower.

Say I want the temp of a our living room to be set at 20 degrees what temp setting do I set the boiler to in order to ensure that it achieves that or does it not matter and I can set it at the lowest temp as the wireless stat will override it anyhow?

See above.

Has anyone a view on whether hot water should be set to its highest temp the boiler can go or the highest temp that you actually use the water at e.g the temperature that we would normally use for a hot bath or hot water for dishes. Is it just a waste to run the water warmer than that and does it affect flow rate?

There is no point heating the water to a higher temperature than you can use. 50C may be OK for washing up if you are wearing rubber gloves, but baths and showers need a temperature about 40C or less. If you have hot water at 50C you will have to mix it with cold, but turning the cold tap on immediately reduces the hot flow rate. So you might as well run the hot at a lower temperature to begin with.

 

[DP]

DH wrote

Set it to max; the CM927 will control everything.

Do not do that unless it is baltic brass monkey weather outside- keep the boiler ticking over around 60. Yours is a condensing boiler, Running it at max means it will be operating outside the 'condensing' opertonal window unless WC is fitted.

 

[D_Hailsham]

DH wrote

Set it to max; the CM927 will control everything.

Do not do that unless it is baltic brass monkey weather outside- keep the boiler ticking over around 60. Yours is a condensing boiler, Running it at max means it will be operating outside the 'condensing' operational window unless WC is fitted.

It can only reach max temperature if the CM927 allows the boiler to run for long enough, which will only happen during initial warm up. Once the room temperature is within the TPI proportional band, the water temperature will drop because the boiler is not running for long enough to bring the water temperature back to max.

If you keep altering the boiler temperature, the Optimization on the CM927 will be confused as the heat up times will not be consistent.

I have been using a CM927 for nearly five years and the only time I can get the water temperature anywhere near the maximum (82C - old non-condensing boiler) is if I turn the CM927 up to about 30C. Most of the time the water temp is about 60C, even in brass monkey weather and the house sits at a steady 21C.

 

[DP]

DH, I think you need to conduct a few experiments and then review the results.

Having read your reply I just did that and your figures do not seem to add up. Hall temperature was 19 degrees. I asked for the hall temperature to be 21 degrees and set the boiler to max (75 for me). Hall radiator differential was 60 and 75 but the room temperature had not moved from 19 degrees C.

My boiler is set at 12kw. I am not aware Duotec can be range rated. It will be firing at max shade under 30kw. I fail to see how one can speculate that CM will exercise control. I can forsee boiler overshooting and going to sleep and TPI function being redundant. When boiler wakes up 28kw output chucks the heat at the radiator to raise the return above 55 degrees thus working out of condensing scope.

Temperature in the hall is now at 21 and boiler is showing 80 degrees C on the display.

 

[Simon Anderson]

Thanks for all the replies guys. I have a much better understanding of how it all works now. I will watch with interest the debate around what temp to set the boiler to

PS totally typical the spark fitted the CM 927 and the pesky screen on the wirekess thermostat appears to be banjaxed! The digital screen goes blank as soon as you touch it and you can hardly set any controls on it for this reason. Looks like a new one is required!!

 

[D_Hailsham]

DH, I think you need to conduct a few experiments and then review the results.

What experiments would you suggest?

Having read your reply I just did that and your figures do not seem to add up. Hall temperature was 19 degrees. I asked for the hall temperature to be 21 degrees and set the boiler to max (75 for me). Hall radiator differential was 60 and 75 but the room temperature had not moved from 19 degrees C. My boiler is set at 12kw.

Are you saying that the hall temperature never increased to 21C with the boiler temperature set to 75? That's not surprising if your system was not designed to work at that temperature. The rads would be undersized to begin with. A quick calculation told me that your rads would be able to maintain a temperature of 19C when it is -1C outside with a flow of 75C and return of 60C

If you are experiencing the same mild weather as we are in the south and you need the boiler temp set to max, what will it be like for you when it gets really cold?

I am not aware Duotec can be range rated. It will be firing at max shade under 30kw.

When boiler wakes up 28kw output chucks the heat at the radiator to raise the return above 55 degrees thus working out of condensing scope.

So the boiler is cycling? That tells me it is not able to modulate low enough.

I thought that most boilers start off at a low-medium output and then modulate up or down to give the required output to maintain the set flow temperature. In any case I would have thought the control system of a modern boiler was sophisticated enough to be able to monitor flow and return temperatures all the time and calculate the correct output to maintain the temperature.

I fail to see how one can speculate that CM will exercise control. I can foresee boiler overshooting and going to sleep and TPI function being redundant.

How can the CM not exercise control? The boiler can only run if and when the CM tells it to; and if the CM only wants the boiler to run for one minute in ten, the boiler will not be able to produce enough heat to raise the temperature up to the maximum set on the boiler thermostat. If the room temperature rises above the set temperature then you need to increase the Proportional Band.

Temperature in the hall is now at 21 and boiler is showing 80 degrees C on the display.

What's the outside temperature?

If you want to have the boiler condensing, whatever the outside temperature, then the system has to be designed with that in mind.