Setting Automatic Bypass Valve for an old Baxi 381 boiler

16 Aug 2008
Reaction score
United Kingdom
I'm upgrading my central heating system with Thermostatic radiator Valves and an ABV.

Do you know the minimum flow rate for an old (but good) BAXI WM 381 CHB boiler set at 51700 BTU/15.15KW, the system use a grundfoss selectric 4 pump, 10 radiators and a mix of 10mm and 8mm microbore.

Or is there a standard setting that should be used or some trial and error method when the minimum flow rate is not known? The ABV is a Honeywell DU145.

Thanks, Kevin
Sponsored Links
It would probably be about 25 li/min but it all depends on how the system is set up.

Microbore often does not provide sufficient flow and the temperature differential across the boiler often exceeds the 11°C original design figure in many installations.

However, thats pretty irrelevant as its normal to set them so that they "just" start to allow a flow when all but three or four of the rads are off.

Different people have different ideas and luckily its not at all critical. Some people set them to about 3m head and leave it at that.

You should not have any TRV on the rad in the room where the room stat is situated. On an non-microbore system this is normally adequate to provide sufficient flow for an older boiler anyway.

Its possible the FAQ section on this site has some suggestions too.

Honeywell MI's tell you to close all rads off, then open ABV until you feel the water circulate through it, then turn it back 1/2 a turn.
Sponsored Links
Thanks for your fast responses. I performed the method suggested by Dave and the setting is now 2.5, not too far off the 3 mentioned by Tony. The 25litre/min suggestion indicated a setting of about 2 (based on pump data and the graphs included with the valve).

As well as fitting the bypass valve I removed the 9 radiators, flushed them, altered the plumbing, fitted new TRVs & lockshields.
Having drained, refilled and bled the radiators. I was concerned when the radiators would not get hot, I thought the pump might need replacing. However I ran the hot water circuit for about 3 hrs to clear more of the air out. Then turned on the heating and the radiators came on as they should. I just need to balance the radiators to finish off.

In case someone reading this is thinking of doing a similar job this took me 2.5 days, and the total cost (in 2008) was £170 materials. An expert would, I suspect, be quicker.

Thanks again for your advice.


DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local