18kW or 26kW - higher or lower says Brucie

[ian.]

I currently have a 20 year old 24kW thermocompact system boiler which is costing a fortune to run.

I have 13 radiators, hive TRVs and a total heat loss of 7kW, my unvented cylinder takes 17.2kW to recharge.

The boiler I’m looking at is an Ideal Vouge Max, would the advice be to err on caution and go for the 26kW version accepting it will modulate down to 3.7kW but still oversized for DHW, or go for the 18kW version as this meets the load but with not a lot left in the tank.

Thanks

 

[PETE 123]

Work out your heating load to get your answer if you put smaller boiler in rads will not get as hot as they should on max in winter.

 

[gpbeck]

Both will do the hot water, both easily that heat loss if at -3. So I’d select the 18, particularly if it modulates lower.

 

[ShaunCorbs]

18kw or 12kw as your load is 7kw will take an extra 30 mins ish to heat the hot water etc

 

[ian.]

Both will do the hot water, both easily that heat loss if at -3. So I’d select the 18, particularly if it modulates lower.

Yes I worked it out a -3 outside and 20 inside

Thanks for your advice

 

[ian.]

18kw or 12kw as your load is 7kw will take an extra 30 mins ish to heat the hot water etc

Thanks for your advice Shaun, much appreciated

 

[ian.]

Work out your heating load to get your answer if you put smaller boiler in rads will not get as hot as they should on max in winter.

The heat loss calc was based on -3 outside, the consensus so far is 18kW boiler.

 

[Timmy D]

18kw or 12kw as your load is 7kw will take an extra 30 mins ish to heat the hot water etc

Agree. Loads of boilers are oversized, then you find out the gas pipe is too small.

I had a job in a flat where a new boiler was installed. 6 radiators and roughly a 200L unvented tank.

The new boiler was 30kw+, it was sucking the gas pipe dry. It was either in grade the gas pipe (scaffolding and knocking holes everywhere), or swap the boiler out.

Put a 12kw in, its now sized correctly for the gas pipe and the properties demand. The heating/hot water work perfectly.

Typically, if the system is balanced/return temp correct for condensing, the longer the boiler stays on, the better for efficiency.

 

[ian.]

Thanks all for your advice and guidance, much appreciated.

have a happy and prosperous new year.

 

[Desktop987456]

which size of boiler did you go for?

 

[ian.]

which size of boiler did you go for?

I went for the 18kW as this matches the DHW recharge loading time of my cylinder, also in CH mode, this size will give me plenty of ’heat from cold’ headroom and the ability to add radiators in the future should they be needed.

A 12kW boiler was a good suggestion and one I hadn’t previously considered, however, I felt the output didn’t give me much spare.

 

[gmartine]

The Ideal Vogue has HW priority so you don't need to factor in "spare" CH capacity as such, you already have it if set up correctly.