Enough heating output?

[Tipper]

I run my 937 on 'analogue' control using the VRT 360F rather than 'digital' on-off control. This means the circulating water temperature varies according to the heat demand and it should be condensing all the time. It isn't fixed at 80degC or whatever it used to be.

I find that to get the correct temperature downstairs, where the thermostat is, I have to set it to 21degC which is higher than on the old system. It's a 3-storey modern townhouse.

What thermostat are you running?

I also notice that the thermostat actually measures wall temperature not air temperature due to it being mounted on a concrete wall. (Checked with IR temp gauge from Maplin) I intend to insulate it from the wall with some polystyrene before next winter so it better reflects air temperature.

 

[Agile]

Include some reflective foil in the insulation.

Tony

 

[Deleted member 105431]

I run my 937 on 'analogue' control using the VRT 360F rather than 'digital' on-off control. This means the circulating water temperature varies according to the heat demand and it should be condensing all the time. It isn't fixed at 80degC or whatever it used to be.

I find that to get the correct temperature downstairs, where the thermostat is, I have to set it to 21degC which is higher than on the old system. It's a 3-storey modern townhouse.

What thermostat are you running?

I also notice that the thermostat actually measures wall temperature not air temperature due to it being mounted on a concrete wall. (Checked with IR temp gauge from Maplin) I intend to insulate it from the wall with some polystyrene before next winter so it better reflects air temperature.

good advice but he uses a h/well cm 927.

 

[snb]

wih the size of lounge you have it would need in the region of 2400 watts with 71 dgree return temp. but 5000w with 50 degree return temp.

My guess would be that the biggest problem is that your previous thermostat was in accurate ( reading 19 when it was achieving 20!).

With the new stat you are asking it to maintain a lower temp and that it is exactly what it is doing. Have you tried increasing the temp to 21.5 and seen what the effects are?

if you have the stat set at 19.5 it will not turn back on till temp has dropped below 19 but will then try to reach 20 before it turns off.

 

[D_Hailsham]

the average lounge 5.5m x 3.5m with cavity wall insulation would require around 1800watts but if you wanted to have 50 degree return temp then this means you need rads with around 3600 watts output.

You don't need to double the rad size just because the differential is 20°C (70/50 instead of 75/65). You would need to install a nominal 2500 watt radiator.

The lounge is 4m x 4m with an extension that is 3m x 3m, the extension has a 2135w radiator and the older part has a 1600w radiator.

The extension is smaller than the lounge and has insulation; yet it has a larger radiator than the lounge which is uninsulated. Doesn't make sense.

Still waiting for the results of the EST boiler sizing calculation.

 

[D_Hailsham]

I also notice that the thermostat actually measures wall temperature not air temperature due to it being mounted on a concrete wall. (Checked with IR temp gauge from Maplin) I intend to insulate it from the wall with some polystyrene before next winter so it better reflects air temperature.

Didn't know that IR themometers could read air temperature.

Modern digital themometers are designed to read a combination of the air and wall temperature; so insulating the thermostat from the wall is not a good idea.

 

[glasgowgas]

Include some reflective foil in the insulation.

Tony

do NOT use any metallic materials near the stat.

 

[David937]

My guess would be that the biggest problem is that your previous thermostat was in accurate ( reading 19 when it was achieving 20!).

With the new stat you are asking it to maintain a lower temp and that it is exactly what it is doing. Have you tried increasing the temp to 21.5 and seen what the effects are?

if you have the stat set at 19.5 it will not turn back on till temp has dropped below 19 but will then try to reach 20 before it turns off.

The previous thermostat did not work and had to be set at 30C to achieve a near decent temperature, I used some thermometers in various areas to check the temperatures and the temperatures downstairs never got near 20C.

Increasing the stat to 21.5C results in the boiler staying on constantly and the room temperature staying the same, radiators are boiling hot but they make no difference to the room temp.

From what I have seen the Honeywell CM927 begins to cycle the boiler as the room approaches the set temperature and once the room is at the set temperature the stat will call for heat occasionally to maintain the set temperature.

Still waiting for the results of the EST boiler sizing calculation.

Sorry for the delay, I have not got round to doing it yet, when I have the results I'll post them.

We're not trying to get a 20C differential, as this would involve changing the upstairs radiators which are just about adequate, all we want now is a nice warm house all throughout winter. Guests always feel cold at our house.

 

[D_Hailsham]

Increasing the stat to 21.5C results in the boiler staying on constantly and the room temperature staying the same, radiators are boiling hot but they make no difference to the room temp.

Back in January you told us:

I measured the radiators today and checked the outputs in the Stelrad Elite catalogue, I added all the outputs and it added up to approx 13kw.

I have a feeling that when you get round to using the EST Calculator, you will find that it gives a result much higher than 13kW. If so, the only thing you can do to make the house warmer is put in larger radiators.

 

[David937]

Back in January you told us:

Yes I remember that, when I measured up that time I forgot some radiators and rounded some of the outputs down....I don't know what was going through my head.

 

[D_Hailsham]

Yes I remember that, when I measured up that time I forgot some radiators and rounded some of the outputs down.

It would be worth doing again and comparing the total rad kW with the EST boiler calculator result.

 

[David937]

I have done the radiator calculations and they add up to 16.8kw, I believe this is quite high for the size of the house.

This is including the Myson Kickspace 1535w.

 

[D_Hailsham]

I have done the radiator calculations and they add up to 16.8kw, I believe this is quite high for the size of the house. This is including the Myson Kickspace 1535w.

May look high, but what you need depends so much on the construction of the house. With a 100 year old house with solid walls, large rooms and high ceilings you will inevitably need more heat compared to a modern fully insulated house. For example, a 7m x 7m 2-floor detached house with 2.5m high rooms will require 16.5kW if it has no insulation or double glazing, but only 6.5kW if it is fully insulated - EST figures.

It's now just a case of getting the EST figures.

 

[David937]

Boiler sizing results:

Main house: 12.7kw

Extension 1: 2.0 kw....this is the the room with the *2135w heater.

Extension 2: 2.0kw....this is an extension to the kitchen and is exactly the same size as extension one but only has the *Myson Kickspace mentioned earlier!

So the whole house including extensions require 16.7kw.

And BTW remember that that only 3 radiators are fed off 22mm pipework, and the rest are all fed by 15mm pipework.

*This 2135w heater was not fitted when the boiler was installed, it was installed by the builders when the extension was built 5-6 years ago.

*The builders had fitted an electric plinth heater with an output of 1kw, this was replaced by a fan convector when the boiler was installed.

The whole kitchen has a 1535w Myson Kickspace and a 564w radiator, the older part has the 564w heater, however a bigger heater couldn't be fitted due to a lack of space.

 

[D_Hailsham]

Boiler sizing results:

Main house: 12.7kw

Extension 1: 2.0 kw....this is the the room with the *2135w heater.

Extension 2: 2.0kw....this is an extension to the kitchen and is exactly the same size as extension one but only has the *Myson Kickspace mentioned earlier!

So the whole house including extensions require 16.7kw.

And you have 16.8kW of radiators. Can't get much closer than that.

So the question now is, are the rads in each room the correct size? Without going to the lengths of using a rad size calculator (e.g. Stelrad Stars) the quick and dirty method is to relate the rad size to the volume of the room. Example Total house volume = 480 cubic metres, living room volume = 40 cubic metres. Total rad size= 16kw. So living room required 16 X 40 ÷480 = 1.33kW You only need to do this for your main house. Obviously this makes a lot of assumptions about each room - temperature, heat loss etc, but it's a starting point.

remember that that only 3 radiators are fed off 22mm pipework, and the rest are all fed by 15mm pipework.

As the rad output is only just enough, we will have to forget about running with a 20°C - you will loose 15% of the output if you do - 10°C it is. But this means the 15mm pipes are carrying far too much heat. You won't need to replace all of it. This diagram show how you work out the loading of the pipe work. You should be able to do this for your situation. The maximum going down a 15mm pipe should be 6kW.

Untitled

• D_Hailsham
• 18 Dec 2008
• 3