New Boiler sizing

[Guzzles]

Hi, I am getting a Vaillant 600 series System Boiler and auroSTOR unvented cylinder with a plan for the addition of solar next year. I have had several corgi plumbers around to give quotes, but they all seem to leave me to decide what size boiler I want. I tell you what I know, and I would be grateful for any opinions from any installers out there

I currently have 13KW of radiator load (across 14 rads) in a large 4 bed detached house with 1.5 bathrooms. The SEDBUK website calculates my boiler requirement to 11KW (including 2KW for DHW). I have no plans to extend the house. The SEDBUK calculation is I think conservative, and my house does take a good couple of hours to warm up at the moment if cold, but I put that down to a very old Netaheat 10/16 which probably kicked out 16KW on day one and not now, 20 years later.
The Cylinder has a 22KW coil. I don't know whether the SEDBUK calculation which added 2KW for DHW is correct if you are using a fast recovery cylinder like myself. As the auroSTOR will only hold 130 litre when using the top coil i.e winter/no sun, I want the tank to recover quickly. But I have heard a strong arguement that you should rate the boiler so that it is running flat out as much as possible. The boiler sizes I can choose from are 12, 15, 18, 24 and 30 KW.

Which size should I go for?

 

[plummerman]

As i',ve recently installed the same. apart from mines the 310litre aurostor system in my house, i opted for the 24 kw option because as you say running the boiler flat out conserves energy due to return temp of the heated water on the secondary heat exchanger. baring in mind tho as you so rightly put the cylinder require's 22kw to heat the water. I to haven't installed my solar as yet an until that time i've temperarly piped up the bottom coil to the heating. Having discussed this with VAILLANT who say that is fine as long as the system is throuhly flushed and inhibited while in use on the heating and once disconnected flushed again as not to damage solar panels once installed. So for a short time i only have 2kw of heat energy for rads etc, but the heating will be properly set up to create my optimum return temps and fingers crossed i shouldn't notice to much and hopefully the solar should be on giving me more kw to play with

 

[gas4you]

I would fit a 618 for you and time the HW to come on about 30 mins before any heating load in the mornings, as this is the only time your system may need full output from the boiler.

As you only have 13Kw of rad output, what is the point of having a 24Kw boiler?

A condensing boiler will be at its most efficient when ticking over, rather than running flat out, you need the return temp as low as possible.

With the 618 you could have it range rated down to 15 Kw and see how you get on, rather than have a 615 installed.

Why not add a VRC430 as the control, this will add to the efficiency of your system. The 560 control unit with the solar will control all aspects of your system when installed later, letting everyting talk to each other via ebus for the greatest energy efficiency for you.

 

[Guzzles]

So a bit of a debate going on between the 18 KW and the 24KW. Focussing on the statement 'its best to run your boiler flat out', what do they mean? Presumably, with a fully modulated system, and so long as you have your rads well balanced so that the return temp is the lowest, having a bit of over capacity shouldn't be a problem? Is the boiler efficiency the same at 50% as it is at 100% ?

 

[gas4you]

Not wishing to provoke any form of disagreement or argument, all condensing boilers are less efficient when run flat out.

Hence why combis are less efficient when on for HW.

Any decent manufacturer will always tell you to range rate their boiler to your system, so the boiler can never fire on more output than can ever be needed.

Unless of course the manufacturer is Ideal, who's boilers are the most efficient for gas use on the market, because they are broken down more than they work, thereby saving the householder gas

 

[Guzzles]

Any decent manufacturer will always tell you to range rate their boiler to your system, so the boiler can never fire on more output than can ever be needed.

Is this something that is done on commissioning, or does the boiler do it by itself by modulating

 

[JoeMar]

on a vaillant boiler (dont know about other makes) it is possible to range rate it but as the boiler is fully modulating it is not necessary...

 

[mickyg]

on a vaillant boiler (dont know about other makes) it is possible to range rate it but as the boiler is fully modulating it is not necessary...

it is absolutely necessary. It will still ramp up to its full rate on initial fire before it modulates. The boiler can't guess the demand before it fires up

 

[gas4you]

Yes your installer will range rate it to suit your system after commissioning has been completed.

The Vaillants DO need to be range rated, as apart from making the boiler more efficient to run, as it then only modulate between the max heat output your property needs and its minimum output.

Apart from the above, Vaillant boilers need to range rated to prolong the useful life of the HE fitted to them. They are very sensitive to too much un-needed heat being produced.

In fact this aspect was one of the contributing factors over the burner seals failing in several of the cases that this happened. Also forget the MI's, never let the CO2 setting run higher than 9% NG or 10% LPG (case off) otherwise the increase in KW output this produces can again cause premature failure of the HE, especially if anything they tend to drift upwards as they are used over time.

 

[JoeMar]

on a vaillant boiler (dont know about other makes) it is possible to range rate it but as the boiler is fully modulating it is not necessary...

it is absolutely necessary. It will still ramp up to its full rate on initial fire before it modulates. The boiler can't guess the demand before it fires up

Hi

I was just quoting from the installation manual of my own boiler.

A few questions if i may. I understand the fact that the boiler will 'ramp up' to it's full capacity on initial fire but why is that a bad thing? if anything the water in the CH circuit will heat up faster hence the house warms up faster...that to me is a good thing.

 

[gas4you]

If for example you have a 630 installed on a system that only requires (lets go extreme here) 18Kw and you don't range rate it then you will have a boiler that is producing 30Kw but only a heting system that can 'emit' 18Kw. Therefore you have 12 Kw of unnesseary enegry being used. The boiler running at 18Kw will be more able to condense more therefore acheiving greater efficiency over its set heating curve range.

If you range rate the boiler to 18Kw, then on initial fire up it will only ramp up to 18 Kw, not the 30Kw.

This is also advantageous when using weather compensation.

Also as previously mentioned, the excess power in the boiler cannot be used to the detriment of the HE in the boiler, so hopefully a longer lifespan.

 

[mickyg]

If for example you have a 630 installed on a system that only requires (lets go extreme here) 18Kw and you don't range rate it then you will have a boiler that is producing 30Kw but only a heting system that can 'emit' 18Kw. Therefore you have 12 Kw of unnesseary enegry being used. The boiler running at 18Kw will be more able to condense more therefore acheiving greater efficiency over its set heating curve range.

not to mention the fault codes flashing at you indicating incorrect flow and return temps, and the kettling noise from the boiler.

 

[Agile]

It seems that I am the odd one out!

I would rate the boiler at 15 kW amd that would be exactly what the energy efficiency course would teach plus 2 kW as thats justified by your larger installed rads so would give faster heatup.

You say it takes a long time to heat up. In spite of what you imagine your Netaheat is probably giving the right heat output but you can check that by gas rating measurements at your meter.

The Vaillant boilers should be rated according to the heat loss in spite of what so many people above are saying. That is clearly taught on the energy efficiency course which all installers should have attended.

However, I think that I am correct is saying that the Vaillants do start on reduced power and only ramp up after a few minutes if the return has not shown a significant temperature rise.

The full calculated heat output is only needed when its -1° outside. Thats only a few days a year. For the rest of the year only 3-5 kW" are likely to be needed.

Seriously over rating a boiler will mean that its cycling on/off which greatly reduces the efficiency. Most boilers only modulate to about 33% of their maximum. If the heat output is less then they have to cycle!
Hot water is catered for by starting the HW 30 minutes before the heating so the full boiler output is available for just the HW.

Tony

 

[simond]

Given your SEDBUK calcs I really don't think it is going to make much difference between a 15kw and 18Kw 6 series.

But with solar the size of the cylinder is very important.

So if you must have solar (actually I don't think much of it) you should have the biggest cylinder you can, so you can make heat while the sun shines. This will make far more impact than +/- 3Kw on the boiler.

 

[Agile]

i opted for the 24 kw option because as you say running the boiler flat out conserves energy due to return temp of the heated water on the secondary heat exchanger. baring in mind tho as you so rightly put the cylinder require's 22kw to heat the water.

Thats another falacy! The cylinder does not require 22 kW ! Thats the maximum power the cylinder can absorb when the water is completely cold. Many times its still shown as the power transferred with a flow of 80C rather then the 70C which a condensing boiler should be set for as a maximum.

If the controls are set correctly then it makes very little practical difference if there is 22 kW or 11 kW available for heating the water. In most situations the water is only reheated once or twice in a day for less than an hour.

The recommendations on boiler rating were very carefully worked out and over rating a boiler just reduces efficiency as a result of cycling on heating!

Tony