Change from open vented to system boiler?

[D_Hailsham]

If you work for a proper firm they pay you the proper rate. The current JIB rate for a technical plumber is £14.55/hr

Is that all British Gas etc pay their top installers?

OP
You said earlier that you had calculated 16.53kW, including 4kW for hot water. That means just 12.53kW for heating. Remeha boilers can be used with weather compensation, in which case the system is configured as HW priority. So a 15kW boiler would be sufficient and the 18kW more than adequate.

 

[phil1235]

Thanks for that confirmation, DH.

My self-employed installer told me that he had been lecturing on British Gas training courses, and bar talk indicated that their repair technicians were paid bonuses for persuading customers to replace rather than repair boilers (doubly good business if customer was on a maintenance contract). This tallied with my experience 10 yrs ago when weeping heat exchanger on the old boiler would have been "risky" to replace (under maintenance contract) and would need £500 (approx) powerflush (not included in contract), but new boiler would only need £300 (approx) powerflush. Why the difference? "Different company". Going independent got me new boiler, and replacement cylinder (which BG hadn't suggested despite hard water area) for £several 00 less than BG's quote. So there's perhaps a boost to the £14.55/hr. Tell me it's not true, someone!

 

[DP]

So a 15kW boiler would be sufficient and the 18kW more than adequate.

If the cylinder is looking for 24kwatts, 18kw would be 6kwatts short or, as indicated in the table posted earlier in thread, one might need to have their breakfast as well before water is available for 'rain' shower as opposed to just having a cuppa.

It would seem each poster has their own idea what power the boiler needs to be to heat water in a cylinder. Some have indicated a miniscule 2kwatts (heaven knows how long the boiler would need to run to provide a full cylinder of hot water) to some others who suggest equal matching for what the cylinder coil is rated at. The choice rests with the installer who fits the boiler that he thinks will do the job. It takes a brave man to undersize when satisfactory job completion equals final payment made.

 

[RichieHoneybucket]

I 100% agree with Danny(DP).

 

[phil1235]

As a non-expert, I calculated the house heat loss using both SEDBUK and IDHEE models, which gave 12-12.5kw. SEDBUK said add 2kw for HW (which assumes I guess that HW and CH would not both be flat out at the same time), but I added 4 to be safe, so concluded that with some HW prioritisation (by timer, sensor or whatever) 18kw would suffice. (Who are these SEDBUK and IDHEE people anyway?) If my wife insists on running say 120l of hot water into a 200l bath from our 140l fast recovery cylinder she can wait a while for the CH to kick in! With a high thermal mass (pre-war house, all solid cavity-filled walls), ambient temp holds up pretty well.

 

[D_Hailsham]

Who are these SEDBUK and IDHEE people anyway?

SEDBUK stands for Seasonal Efficiency of Domestic Boilers in the UK. They are responsible for maintaining the boiler efficiency database used for SAP ratings of properties.

IDHEE is the Institute of Domestic Heating & Environmental Engineers.

The boiler size calculator they each provide is identical, it's just that the IDHEE version is easier to use. It was developed by the Building Research Establishment.

An HW cylinder might be rated as 24kW, but it will only be supplied with 24kW from a 24kW boiler if the HW is timed to only run when heating is not required. In a typical Y or S plan the CH and HW times overlap, so the HW cylinder will frequently get only part of the boiler output.

 

[phil1235]

Is a 10-yr old fast recovery, vented yellow-covered cylinder going to take 24kw?? (It's about 1200 x 450). How do I find its rating?

 

[D_Hailsham]

Is a 10-yr old fast recovery, vented yellow-covered cylinder going to take 24kw?? (It's about 1200 x 450). How do I find its rating?

There should be a label on the cylinder, though it's probably come off or is hidden round the back!