New Viessmann boiler

[D_Hailsham]

height - 7

I don't think so.
It's the floor to ceiling height which is wanted, not the total height of the house. Just above the boxes it says:

Enter the dwelling length, width and room height (all in metres) and number of floors

If we assume a 2.4m room height, the boiler size works out at 13443W. Deducting 2kW for hot water, we get a heating requirement of approx 11.5kW.

You say you only have 8kw of radiators. Does your house get warm enough, particularly in the recent cold weather? Or did you have to resort to open fires, electric heaters etc to keep warm?

I used 1 external wall (width) previously to get 30kW as on one side there is an empty garage.

But it, presumably, is an unheated garage and therefore an external wall.

 

[sloons]

It's the floor to ceiling height which is wanted, not the total height of the house.

haha

If we assume a 2.4m room height, the boiler size works out at 13443W. Deducting 2kW for hot water, we get a heating requirement of approx 11.5kW.

My installer says the boiler will modulate down to 6kW so I guess that will be ok.

You say you only have 8kw of radiators. Does your house get warm enough, particularly in the recent cold weather? Or did you have to resort to open fires, electric heaters etc to keep warm?

We do have a log burner but its used irregularly. We keep the thermostat down to 17C during the day and 15C at night which is probably a bit cold for most people. I think the house shouldn't be too warm in winter, you should wear a jumper plus lpg is so expensive

 

[D_Hailsham]

My installer says the boiler will modulate down to 6kW so I guess that will be ok.

Your installer is wrong. The 26kW combi modulates down to 9kW and the 30kW combi down to 11kW.

If you keep your house at 17C the required boiler output, assuming -1C outside, will be 9.5kW.

That means the boiler will be running at its lowest output most of the time, As it cannot modulate any lower, it will revert to on/off mode for the majority of the year, which is less efficient.

It is very difficult to find a boiler which modulates below 8kW, which is ridiculous as many UK houses need them. One make which does is the Remeha Avanta. All models go down to 6kW. They are available in gas and LPG versions.

Does the installer know that you keep your house at a relatively low temperature? He will have assumed 21C.

Have you had your water flow rate and dynamic pressure checked? There is no point installing a large combi if the flow rate is not high enough. You need 2.5kW for each litre per minute. This will give a 35C temperature rise, i.e a 10 litre/min rate needs 25kW. If you install a 25kW boiler and the rate is faster, the temperature rise will be less, and vice versa. The pressure determines if you can have more than one outlet running at the same time without one influencing the other too much. Obviously the flow rate will reduce on each.

We keep the thermostat down to 17C during the day and 15C at night which is probably a bit cold for most people. I think the house shouldn't be too warm in winter, you should wear a jumper plus lpg is so expensive

Let me guess: you go skinny dipping in the sea on Christmas day.

 

[Agile]

Its a little trick which works in a lot of cases if you dont have access to a north wall.

Fit in on the underside of your eaves !

Tony

 

[sloons]

Your installer is wrong. The 26kW combi modulates down to 9kW and the 30kW combi down to 11kW.

I put this to him today and he said he was thinking of the Remeha Avanta model which he also fits (he said this himself I did not mention your post) He says he has had some problems with the Remeha Avanta and the Viessmann has a five yr warrantee. He also says Viessmann has better support and are able to sort out repairs much quicker if something does go wrong due to better infrastructure.

If you keep your house at 17C the required boiler output, assuming -1C outside, will be 9.5kW.

That means the boiler will be running at its lowest output most of the time, As it cannot modulate any lower, it will revert to on/off mode for the majority of the year, which is less efficient.

Is it worth giving up the extra warrantee for the increased efficiency? Is the difference in performance significant?

Does the installer know that you keep your house at a relatively low temperature? He will have assumed 21C.

No however I suppose I could increase the temperature if the gas wasn't to expensive

Have you had your water flow rate and dynamic pressure checked?

15L per minute out of the kitchen cold tap but I would get the smaller model anyway as it modulates down to 9kW

Thanks for all your advice it has been really helpful

 

[sloons]

Its a little trick which works in a lot of cases if you dont have access to a north wall.

Fit in on the underside of your eaves !

Brilliant idea! That will also help with wiring

 

[D_Hailsham]

Is it worth giving up the extra warranty for the increased efficiency? Is the difference in performance significant?

Be careful about warranties. The Viessmann five year warranty only applies if the boiler is installed by someone who has attended their training course. In all other cases it is 3 years. Remeha also advertise a five year warranty, but I'm unsure of the exact details.

It's impossible to say what the difference in efficiency would be. According to Viessmann's literature weather compensation can save up to 20% on fuel bills, but they also say that it increases the Sedbuk efficiency rating by only 2%. One thing that concerns me is how weather compensation works if the boiler can only operate in on/off mode, because it cannot modulate low enough. The whole aim of weather comp is to control the flow temperature of the boiler, which it does by adjusting the modulation. How does it do this when the boiler is either on or off?

On the hot water side, you say you have a flow rate of 15 litres per min. If you go for the smaller 26kW boiler, at full flow the water temperature will only rise by 25C, which might be OK in the summer, when the mains water could be as high as 20C, but in the winter, when mains may only be at 5-10C, the water would not be hot enough for a shower or bath. Slowing the flow to 10 litres per min, by not having the bath tap/shower full on, would give a temperature rise of about 35C, i.e a water temp of about 40C in the winter.

If you have more than one cold tap fed direct from the mains, e.g the kitchen sink and a garden tap. You need to check the flow rate when both taps are full on. This will give you an indication of the flow rate when two hot outlets are running at the same time.

 

[Agile]

One thing that concerns me is how weather compensation works if the boiler can only operate in on/off mode, because it cannot modulate low enough. The whole aim of weather comp is to control the flow temperature of the boiler, which it does by adjusting the modulation. How does it do this when the boiler is either on or off?

Unlike most RGIs who dont understand the importance of fitting the the correctly sized boiler you do appreciate this.

The simple answer is exactly what you say, the efficiency falls if a high power combi which is recommended by most installers is fitted in a small property.

Like all, other energy saving devices they only save if they are used !

Probably 80% of TRVs that i see are set at max !

Tony