Baxi 228 combi boiler hot water flow rate poor?

[Ric2013]

Hi,

Just checking I'm not talking rot.

Today I visited a house with a Baxi 228 combi boiler. The technical spec for the DHW output is shown in the image. Boiler was installed 27 months ago.

Baxi is claiming 13.3 litres @ DT 30.

I set the boiler DHW temperature to a high setting. I was getting about 11lpm from a nearby tap, but measured temperatures at the supply and DHW output pipes at the boiler were 9/36°. So DT 27 even with a lesser flow rate than the technical spec suggests. I make it about 25% less output than claimed.

The flow rate is presumably dictated by pipework and fitting resistance and pressure and is not of concern as such, but I'm wondering if the low DT could be indicative of a scaled plate heat exchanger (hard water area) or whether the technical data tends to be a bit like the Volkswagon car that performed in a certain way under very specific emission test conditions and should be taken with a pinch of salt.

Obviously I will suggest that a Gas-Safe Installer could be asked to comment on whether the plate heat exchanger needs cleaning or replacing, but not if the experience of others on this forum is that claimed DHW flow rates are rarely achievable in real-world conditions.

Any thoughts?

 

[ShaunCorbs]

Did the tap have a flow restrictor in ?

 

[Ric2013]

Did the tap have a flow restrictor in ?

I don't think it did. Actually I initially got a higher reading than 11 litres (weir gauge), but it seemed to have dropped when I re-checked (washer springing back?)

In any case, wouldn't the boiler find it easier rather than harder to achieve a greater DT if the flow rate is slightly less than design?

 

[ShaunCorbs]

Depends if it modulated as the water in is fixed due to flow restrictor in the boiler

 

[Ric2013]

Depends if it modulated as the water in is fixed due to flow restrictor in the boiler

In other words, to do the test properly I'd need to bring the flow up to to 13.4 litres?

 

[John.g]

Don't know if its sorted out or not but you are correct in saying that its output is down by 25% with a dT of 27C at 11LPM, the dT at that flowrate should be, 28*860/60/11, 36.5C, to give a flow temperature of, 9C+36.5C, 45.5C. Your output was 11*60*27/860, 20.72kw, (74%).

 

[Ric2013]

Hi John, This was actually a house I'm probably buying so should have plenty of time to investigate. Most likely cause is that my hitherto extremely accurate infrared thermometer is now giving inconsistent readings and I need better test equipment.