Replacement Combi bolier

[Fixitrite]

Hi
It's become time to change my combi boiler , currently an Ariston MFFI27.

Going to put a Baxi in , looking at a mini,mum of the 33KW model but I'd like to go for the 40KW because of the better HW flow.

Is there any reason not to go for the higher output model although I don't need the extra CH capacity of it ?

I usually do all my own DIY ... re plumbed the house , rewired / fitted bathrooms / wet room / double glazing etc etc .

Local plumbers have quoted in the region of £700 cash job to swap my boiler.
This seems a bit high to me given that he has quoted the job will take 1 day, possibly 1 1/2 days.
Is this really what I can expect to pay ?
Looking at the installation literature it looks like 5 pipe connections , the flue and power. Does it really cost £700 for a boiler swap ?

 

[lightning]

If you dont like the price get a couple more quotes???

Does it cost £250 to service my van!!! yet you dont hear me moaning

 

[gasman26]

Looking at the installation literature it looks like 5 pipe connections , the flue and power. Does it really cost £700 for a boiler swap ?

Yes, if customer provides the boiler we charge 600+ fitting charge, if it is a straightforward swap. If the rest of the system requires upgrading, i.e TRV`s, room thermostat, scale reducer, system filter then expect to pay more....

But like you say only 5 pipes, flue and electrics...... Give it a go what can go wrong?
If you are competant you can fit your own boiler in your own property, I did....

 

[D_Hailsham]

Going to put a Baxi in , looking at a minimum of the 33KW model but I'd like to go for the 40KW because of the better HW flow.

Is there any reason not to go for the higher output model although I don't need the extra CH capacity of it ?

Putting in a larger boiler does not give you a better flow rate. All it will give you is the same flow rate, but at a higher temperature.

The best place to measure the flow rate is at the garden tap (if it is straight off the mains). Do it with the kitchen tap full open and then closed and use the smaller figure in your calculations. If you have no garden tap just use the kitchen tap.

You can do this with a watch and litre-marked bucket. You need to work out the flow rate in litres/minute. Multiply the result by 2.5 and you will get the correct kW hot water output boiler.

 

[gasman26]

Putting in a larger boiler does not give you a better flow rate. All it will give you is the same flow rate, but at a higher temperature.

Thats incorrect......

 

[gasman26]

You can do this with a watch and litre-marked bucket. You need to work out the flow rate in litres/minute. Multiply the result by 2.5 and you will get the correct kW hot water output boiler.

I have a dynamic flow rate of 21 l/min in my place, does that mean I need a 52.5kw combi boiler ?

 

[D_Hailsham]

Putting in a larger boiler does not give you a better flow rate. All it will give you is the same flow rate, but at a higher temperature.

Thats incorrect......

If you have a flow rate of 10 litres/minute, a 25kW boiler can raise the temperature by a maximum of 35C. A 50kW boiler could raise the temperature by 70C. However the boiler will have a temperature control to prevent the actual temperature rising above about 60C.

I have a dynamic flow rate of 21 l/min in my place, does that mean I need a 52.5kw combi boiler ?

If you wanted to raise the temperature of water flowing at that rate by 35C then, yes, you would need a 52.5kW boiler.

 

[Efrygasservices]

HAHAHAHA, great calculation with a bucket!!! Must try that one with a customer!

 

[Agile]

Putting in a larger boiler does not give you a better flow rate. All it will give you is the same flow rate, but at a higher temperature.

Most combi boilers have a flow rate limiter appropriate to the flow rate relevant to the power of the boiler.

The maximum temperature rise is limited to about 60 C to prevent the danger of scalding.

Tony

 

[gasman26]

Putting in a larger boiler does not give you a better flow rate. All it will give you is the same flow rate, but at a higher temperature.

Thats incorrect......

If you have a flow rate of 10 litres/minute, a 25kW boiler can raise the temperature by a maximum of 35C. A 50kW boiler could raise the temperature by 70C. However the boiler will have a temperature control to prevent the actual temperature rising above about 60C.

Your nearly there, let's use the remeha avanta range as example - combi range 24,28,35 & 39c flow rates provided - 10,11.5,14 & 16 l/min all at a 35 degrees temperature rise, the varying output boilers have different sized burners, DHW & main heat exchangers to accommodate these figures.

I have a dynamic flow rate of 21 l/min in my place,does that mean I need a 52.5kw combi boiler ?

If you wanted to raise the temperature of water flowing at that rate by 35C then, yes, you would need a 52.5kW boiler.

 

[DP]

Higher boiler output ARE for greater flow of hot water. Combination boilers are normally spec'ed for 35 degrees ̊C. Obviously 37kw boiler will give greater flow at the designated reference temperature than 32kwatt boiler, which will beat 24kw model no problem

 

[D_Hailsham]

Higher boiler output ARE for greater flow of hot water. Combination boilers are normally spec'ed for 35 degrees ̊C. Obviously 37kw boiler will give greater flow at the designated reference temperature than 32kwatt boiler, which will beat 24kw model no problem

Provided you have the nessary flow in the first place. You can't have water leaving a tap at a greater rate than it enters the boiler.

 

[D_Hailsham]

If you have a flow rate of 10 litres/minute, a 25kW boiler can raise the temperature by a maximum of 35C. A 50kW boiler could raise the temperature by 70C. However the boiler will have a temperature control to prevent the actual temperature rising above about 60C.

Your nearly there, let's use the remeha avanta range as example - combi range 24,28,35 & 39c flow rates provided - 10,11.5,14 & 16 l/min all at a 35 degrees temperature rise, the varying output boilers have different sized burners, DHW & main heat exchangers to accommodate these figures.

But that's a different matter.

You disagreed with my assertion that putting in a larger boiler does not give a higher flow rate, only a higher temperature.

I stand by that; it's the laws of physics.

If you have a 20 litre/min incoming flow rate and a 24kW boiler, the temperature rise will be about 17C. The flow rate will have to be reduced to about 10 litres/min to produce the required 35c rise. If you then replace the boiler with a 36kW, you will be able to increase the flow rate to 15litre/sec. But you will still be using less than the full incoming flow.

 

[DP]

Provided you have the nessary flow in the first place. You can't have water leaving a tap at a greater rate than it enters the boiler.

DH, regret that would not happen as (I do replace boiler, but it is like eclipse of the sun) taking stock of gas line and water flow are the first things I check, else a mismatched combi is not fitted. Furthermore, in Scotland, poor cold water flow is a rare beastie indeed.

 

[Agile]

If you have a 20 litre/min incoming flow rate and a 24kW boiler, the temperature rise will be about 17C.

Thats not usually the case.

As most have a flow limiter, the flow rate is limited to about 9-10 li/min for a 24 kW boiler so that its able to heat the water towards the 35 C increase or any lower output temperature set on the boiler hot water temperature setting.