Boiler replacement. 15kw or 18kw

[Steve57]

Hi. Just joined up to ask some impartial advice. Had several quotes for a new heat only replacement boiler. One installer measured up and said 15kw would be more than enough, two others both said 18Kw. When I asked why not 15Kw, one of them said 'to be on the safe side, and anyway it modulates down’. The other one said it can be programmed to run at 15kw if I wanted to but you will have extra capacity in case you want to add to system at a later date. All Vaillant Ecotec boilers. Who is right? Will I be wasting fuel running an 18Kw boiler when a 15kw will do? I have no plans to expand system. Thanks.

 

[Riley]

Well they are right it can be range rated down and it would give you future flexibility. Depends on what you think the future brings

 

[Steve57]

Thanks. Just wondering if they can be adjusted down, why Vaillant make two boilers so close in output - why not just make an 18Kw boiler in the first place?

 

[Riley]

18kw is more expensive

 

[Steve57]

Thanks. Looking online there only appears to be about £90 difference between the two so I might as well go for the 18Kw just in case and have that set to 15kw. I suppose the extra cost would be for a bigger burner/heat exchanger or whatever it’s called inside - is that right? If so, I suppose having an 18Kw turned down to 15kw will put less stress on it or don’t they work like that?

 

[ShaunCorbs]

Will be the same components in both upto you

 

[king of pipes]

Heating systems need to be designed to operate at a comfortable temperature 21 - 22 0c when the outside temperature can dip to minus 10 or more with a wind chill better to have a little more in reserve in my opinion. Kop

 

[CHRISX]

Thanks. Looking online there only appears to be about £90 difference between the two so I might as well go for the 18Kw just in case and have that set to 15kw. I suppose the extra cost would be for a bigger burner/heat exchanger or whatever it’s called inside - is that right? If so, I suppose having an 18Kw turned down to 15kw will put less stress on it or don’t they work like that?

Hello Steve57,

Please excuse me for not looking at the Vaillant Boilers Installation Instructions before writing this but I defer to Member Riley for His knowledge / comment about these particular Boilers being able to be `Range Rated` in terms of being able to adjust their Input / Output.

A lot of Condensing Boilers have pre-set Gas Valves and cannot be `Adjusted` for a different Input / Output.

However - If You have a 18 Kw Boiler running on a system that `normally` requires no more than 15 Kw`s and which has a Room Thermostat it will just heat up the System / Property a bit quicker than a 15 Kw Boiler would and be turned off by the Room Thermostat when the set temperature is reached.

You would not use any [perceptible amount] more Gas than with a 15 Kw Boiler because when the Room Thermostat is satisfied the 18 Kw Boiler will be shut down until the Room Thermostat requires heat again.

The 18 Kw Boiler would be using slightly more Gas when the burner is running - BUT for shorter periods to Heat the property / to satisfy the Room Thermostat.

The 18Kw Boiler will heat up the System / Property a bit quicker and IF the Property Heat requirement has been calculated as close to 15 Kw`s you would have an `extra Heat Producing Capacity` for any period of extremely Cold temperatures as Member King of Pipes has mentioned.

If the Property Heat Loss Calculations have been done correctly and the figure is close to 15Kw`s I would definitely go for the 18 Kw Boiler because as Member King of Pipes has eluded to - Heat Loss Calculations are basically formed around `the coldest expected outside temperature of minus 2 degrees` [my wording] and various Room Temperatures of 18 - 20 - 21 degrees.

If your Property`s Heat Loss is close to the 15 Kw figure using that `coldest expected outside temperature of minus 2 degrees` / `Standard Room Temperatures` and there is an outside temperature drop down to perhaps `Minus 6 or 8 degrees` the 18 Kw Boiler is going to have the additional Heat producing capacity to combat the extra / quicker Heat Loss from the property that a much lower outside temperature causes.

This message is taking for granted that your Property`s Radiators are correctly sized and are currently capable of Heating the various Rooms ?

Otherwise the Heating System will not correctly heat the Property irrespective of the Boiler Input / Output.

I hope that this is helpful.

Chris

 

[Welder]

Heating systems need to be designed to operate at a comfortable temperature 21 - 22 0c when the outside temperature can dip to minus 10 or more with a wind chill better to have a little more in reserve in my opinion. Kop

The issue with that KOP is short cycling during the rest of its life, which translates to shortened overall life. We all know to get the best from a boiler it has to be run hard when it runs... unlike my ex wife...

 

[CHRISX]

The issue with that KOP is short cycling during the rest of its life, which translates to shortened overall life. We all know to get the best from a boiler it has to be run hard when it runs... unlike my ex wife...

Hello YorkshireDave,

When I wrote my reply to the OP I worded it to suit the idea that his property`s Heat Loss calculations found that the Heat Loss was close to 15 Kw.

Advising him to install a 18 Kw Boiler would NOT mean that the Boiler was so oversized that it would be `short cycling` any more than a 15 Kw Boiler would.

As long as there is a Room Thermostat controlling the Heating system either Input / Output Boiler would only be firing until the Room Thermostat was satisfied on each timed / temperature controlled Heating period with an occasional `Off / On` from the Boiler Thermostat while the system is heating up.

My idea of `short cycling` would be a Boiler that was operating without the control / `Boiler Interlock` of a Room Thermostat and which kept firing to satisfy the Boiler Thermostat.

Even a Boiler that was massively oversized [Input / Output] for the Property / System would only be firing for long enough to Heat the property / satisfy the Room Thermostat.

Incorrect Bypass installations have been a major cause of `short cycling` for decades where a gate valve has been installed and opened to some extent allowing a permanent flow immediately back to the Boiler`s Heat Exchanger - often `satisfying` the Boiler Thermostat many times per hour and therefore turning the Burners Off & On far more times than was desirable.

Chris

 

[king of pipes]

Sorry Dave have to disagree mate providing the auto bypass is fitted correctly being 22mm and at least 1.5m away from the boiler and has a flow rate of 12lmp through the boiler when system thermostat is satisfied I cannot see it short cycling anymore than a 15kw unit,the burner module will modulate down to low as 5kw as the return temperature increases, the KW output can be adjusted as can the differential across the boiler it's all about controls fit the correct controls and you will have better performance. Kop

 

[Knappers]

I'd chose a 15kw over an 18kw if the gas supply was borderline or if I needed a boiler that may be ok on a 15mm supply (manufacturer permitting).
Come on guys are you really gona argue over 3kw?

 

[Steve57]

Thanks all. I thought I read somewhere that you’re better having a boiler working flat out to get the best out of it? Anyway, after reading your comments I’m going for the 18Kw Vaillant Ecotec now as there’s not much in it and as has been said, I’ll get a quicker warm up from cold and then it can modulate down to whatever it’s comfortable with.

 

[CHRISX]

I'd chose a 15kw over an 18kw if the gas supply was borderline or if I needed a boiler that may be ok on a 15mm supply (manufacturer permitting).
Come on guys are you really gona argue over 3kw?

Hello Knappers,

We were advising the OP / Steve57 on what Member King of Pipes and I maintain to be the best choice of two Boilers regarding the Input / Output produced by either a Vaillant Eco Tec 15 Kw or 18 Kw model.

We were not arguing with Member YorkshireDave who is a well respected Member on the Forum - we do have slightly different opinions on this subject.

However IF the property`s Heat Loss Calculations are correct / approximately 15 Kw either of the two Boilers installed with the correct Time and Temperature Controls and an Automatic Bypass Valve with correct pipework [as mentioned by Member King of Pipes] would heat the property without causing any problems in terms of `Energy Efficiency` / excess Gas usage.

There has been no mention of the Gas supply being an issue regarding supplying either Boiler.

Steve57, there are opinions in the Heating Industry that a Boiler is `more efficient` when working to its maximum capacity - that theory is also related to the `Whole House Heat Loss Calculation` theory for sizing a Boiler - where the Heating Engineer measures the exterior of the whole Property rather than the individual Rooms.

I can see the theory behind that because the Boiler must be capable of heating the Internal space in total - but I personally don`t use that method because I feel the `old way` is more accurate.

Chris

 

[doitmyself]

I can seinfluence on e the theory behind [Whole House Heat Loss Calculation] because the Boiler must be capable of heating the Internal space in total - but I personally don`t use that method because I feel the `old way` is more accurate.

Presumably because there are more calculations involved.

 

[CHRISX]

Presumably because there are more calculations involved.

Presumably because there are more calculations involved.

Hello doitmyself,

The `Whole House Heat Loss Calculation` / `Whole House Boiler Sizing Method` idea was introduced by the `Energy Saving Trust` to try and stop Heating / Boiler Installers just installing a `like for like` Boiler when they were replacing an existing appliance.

The `EST` had discovered that Heating Engineers / Boiler Installers were often installing Boilers that were many Kilowatts / BTU`s oversized - sometimes almost double the Input / Output that was required for the property because correct Heat Loss calculations had never been done.

There were also many instances of Boilers which were NOT capable of correctly Heating the property and heating the Hot Water Cylinder just being replaced with a similar output appliance.

In past decades [1970`s - 1980`s] there were probably nearly as many untrained `Heating Engineers` installing Central Heating as there were `Professionals` in the Industry.

Mandatory CORGI Gas Installer registration [1991] hopefully gradually got rid of a lot of those.

Boiler Inputs / Outputs and Radiators were frequently determined by `guestimates` and because of misunderstandings about the Boiler capacity that should be allowed for Hot Water Cylinders some of those `Boiler Installers` would routinely add 5 - 10 Killowatts to the Radiator requirement guestimate.

Others would add nothing and install the smallest Input / Output / cheapest Boiler that they felt they could get away with.

The `Whole House Heat Loss Calculation` was supposedly an `easy method` of calculating the Input / Output of the correct sized Boiler so that a `like for like` / possibly incorrect sized Boiler would not be installed.

How many times have We seen `Heating systems` where the Radiators are visibly far too small for the rooms and a Boiler which is either just about capable of serving those Radiators and the Hot Water Cylinder or is about the correct size for the property but will NOT Heat it correctly because the Radiators are far too small ?

When I survey for a New / Replacement Boiler I carry out Heat Loss Calculations for all of the Spaces / Rooms to be heated within the property and I also check all of the Radiators to see if they are correctly sized for the Rooms using my Heat Loss calculations - that is what I meant when I wrote that I do it the `old way`.

Chris

 

[Steve57]

Well, thanks all. We went for the 18Kw boiler just before Christmas and it’s been running fine so I’ll leave well alone!