CH output on vaillant

[marsaday]

We have a Vaillant 837 and you can adjust the heating output. As i understand it, houses require a low heating output compared to the water.

So does anyone know what output i should put the boiler on ?

We have a 4 bed semi over 3 floor. We have 10 rads and one under floor heating loop in the kitchen.

I reckon 12 KW is fine, which is the minimum setting. Am i way out here? does anyone know any different?

 

[doitmyself]

Use the Boiler Size Calculator to find out what the heating requirement is for your house. The result includes 2kw for a HW cylinder, which you don't have as the 837 is a combi, so you should deduct 2kW from the result to give the recommended output.

You may have to increase it a bit if it gets too cold outside; it's a case of experimenting.

 

[marsaday]

Thanks for that. it is saying i need 21 Kwatts. What will happen if i run it on 15 ? It was set to 20 my the engineer and that was fine. i was thinking of turning it down to 15 you see.

 

[oldplumber]

if you have the turbo max plus 837 or thermo compact why are you messing with it in the first place, read the instructions as you have a fully modulating boiler that doesnt require it to be range rated, leave alone what you obviously dont understand.

 

[marsaday]

yes my boiler is fully modulating. so why do they have a range rating facility on them in the first place?

i am not sure what modulating actually means - care to enlighten me ?

 

[doitmyself]

Thanks for that. it is saying i need 21 Kwatts.

That's a lot of heat for a three bed house. Is it very large or very old with no insulation, eg solid walls, single glazed sash windows etc.

As you have a semi, the number of front to back walls is only 1. Did you enter 2 by mistake

What will happen if i run it on 15 ? It was set to 20 my the engineer and that was fine. i was thinking of turning it down to 15 you see.

The worst that can happen is that the house will not get warm enough.

I am not sure what modulating actually means.

Older boilers had a fixed output, which could often be adjusted by the installer to meet the specific requirement. These were know as range rated boilers. Control over the water temperature was by a thermostat, which is just a switch which turns the boiler on and off to maintain the required water temperature. When the boiler was on, it ran at the set output; when it was off, the output was zero.

With a modulating boiler the output is continuously variable between the min and max. Instead of having a thermostat, a temperature sensor is used. When the boiler reaches the required temperature the output of the boiler is automatically reduced so the required water temperature is maintained. The boiler only turns off if the boiler cannot modulate low enough.

Range rated boilers are like a dodgem car - it either goes or it stops. A modulating boiler is like a car with an accelerator - you use it to adjust the engine output to maintain the required speed.

yes my boiler is fully modulating. so why do they have a range rating facility on them in the first place?

To prevent the maximum output exceeding the maximum requirement of the house. When a modulating boiler ignites, the output is set to maximum and then reduces. If the maximum is much higher than required, the temperature of the water can rise so quickly that the boiler shuts down, waits for the temperature to drop and then tries again. The result can be that the house takes ages to reach temperature, if it even gets there. This is particularly true of combi boilers where the requirement for hot water could be as much as 40kW, while the house may only need 10kW. The max heating output of a 40kW combi boiler may be 30kW.

Another problem, particularly with large combi boilers, is that the minimum CH output may be above the CH requirement of the house. In this case the boiler cannot modulate low enough, so the boiler reverts to on off mode.

Lastly, the required output of a boiler varies with the weather: colder= more output, warmer = less output. The boiler is always sized for the worst conditions, normally an outside temperature of -1C. This means that the boiler needs to modulate lower if the outside temperature is higher than -1C.

If the lowest output of the boiler is only slightly below that required for a -1C temperature, then, in warmer weather, the boiler will revert to on-off mode.

 

[marsaday]

ok thats for all that. Yes i did enter 2 walls, i will change that. The house is solid walls and d/g at back and sides. The front is single sashes.

We also have 125mm kingspan in the roof as i am converting my attic this very moment. So the house will be a 4 bed semi soon.

I will see what the calculator says when i make the wall change.

ps. the calc now says 17KW with full d/g selected. We have a 1/3 not D/g so i could add on an extra 1kw.

When the calculator says 75mm insulation in pitched roof is this referring to the rolls of insulation or the kingspan stuff ?

 

[doitmyself]

We also have 125mm kingspan in the roof as i am converting my attic this very moment. So the house will be a 4 bed semi soon.

So it's really a two storey house with a room in the roof.

The top (roof) floor will not need so much heating as it is a smaller volume, even though the floor area is the same as the floors below. I would work out for a two storey house and a three storey and take the average.

the calc now says 17KW with full d/g selected. We have a 1/3 not D/g so i could add on an extra 1kw.

It's easy to calculate for no d/g, then full d/g and see what difference d/g makes then you can work out what to add for the 1/3 not d/g.

When the calculator says 75mm insulation in pitched roof is this referring to the rolls of insulation or the kingspan stuff ?

It's assuming that you have the insulation laid on the ceiling, not in the rafters. But 125mm Kingspan will be better than 75mm roll. This means you will probably be slightly over estimating the heat requirement, which isn't a bad thing as it means the house will stay warm at lower temperatures.

What controls do you have at the moment - thermostat and programmer?

 

[marsaday]

we have a honeywell room stat/prog in the hall, which is very good and clicks on and off when it reaches the right temp.

Right i have done the calcs and it comes out that we need 17KW. Off the top of my head i thought we needed 15, so i wasnt far off. The boiler is set to 20, so i will reduce this.

Thanks for all the info.