Boiler Replacement Size?

[hengus]

hi

Is there a guide on sizing a new condensing boiler to replace a 24kW conventional system boiler with a 250L unvented cylinder? I ask because I have been offered a range of outputs ranging from 18 through to 32kWs.

 

[joni os]

A more efficient boiler of 22Kw will replace your existing but will be too small to fully exploit the advantage of your 250L unvented. Matching the boiler to cylinder, 32 Kw will give a recharge time of less than 30 minutes (dependant on coil). If your hot water demand is sufficient with old boiler there is no advantage of sizing new boiler greater than 24Kw.which would still recharge cylinder in approx. 40 minutes.
Another consideration is 24 Kw will heat 8L of water/min so a shower running at 12L/min will only be taking 4L of stored hot water, allowing you to shower at 12L/minute for 60 minutes.
With water stored at temperature higher than 43C these figures are conservative.

 

[ShaunCorbs]

What is the heat load of your rads, your gas safe engy should of calculated this first

No point in have a 32kw boiler if your rads ar only 12kw

 

[hengus]

A more efficient boiler of 22Kw will replace your existing but will be too small to fully exploit the advantage of your 250L unvented. Matching the boiler to cylinder, 32 Kw will give a recharge time of less than 30 minutes (dependant on coil). If your hot water demand is sufficient with old boiler there is no advantage of sizing new boiler greater than 24Kw.which would still recharge cylinder in approx. 40 minutes.
Another consideration is 24 Kw will heat 8L of water/min so a shower running at 12L/min will only be taking 4L of stored hot water, allowing you to shower at 12L/minute for 60 minutes.
With water stored at temperature higher than 43C these figures are conservative.

Thank you. That is helpful. To be fair. the installer that I am considering to do the work has been very up front. His calculations suggest that a 18kW boiler would be sufficient to heat my home (19 rads in a 2000 built 5 bed house) but HW heat up time might well be an issue. The proposed 24kW boiler is a compromise between HW heat up time and efficiency.

 

[ShaunCorbs]

Thank you. That is helpful. To be fair. the installer that I am considering to do the work has been very up front. His calculations suggest that a 18kW boiler would be sufficient to heat my home (19 rads in a 2000 built 5 bed house) but HW heat up time might well be an issue. The proposed 24kW boiler is a compromise between HW heat up time and efficiency.

Normally 5-10kw for a cylinder so that 23-28kw would be ideal

 

[joni os]

The fury of a female emerging from a shower run cold is off the scale of normal instrumentation, but with 18Kw as the base adding, (2Kw X Number of Females in house), should be seen as minimum for cordial domestic existence.

 

[cdodds2004]

always should oversize the boiler slightly, It will modulate down anyways 🙂