Dual heat only boilers on the same system! How, Why, When?

[Pongo]

I've just done a service on a pair of Halstead Best 50's plumbed into the same flow and return. One boiler worked fine on controls. Both boilers required an extensive strip clean. The customer said no.1 boiler rarely seen on, all on no.2 boiler. Could not get any response from no.1 so tested a few items, fan, electrical mains switching. No nearer sorting it. Control wires for each hidden by trunking. Inlet pressure no boilers is too low and drops to even lower when boiler no.2 is working.

I've not come across this set up or trained for it so any help or advice on how they work together, how they are likely wired and when these systems are used advantages, dis-ads?

 

[stevetheplumber]

i always set one stat higher than the other and reverse every service to even out usage advantage is you have always got some heating even if one boiler fails disadvantage is double the cost for servicing

 

[Pongo]

Are the boilers usually wired to fire at the same time on demand with one cutting out when temp is reached?

 

[gasmanrob]

Just a cascade set up set one up as a lead boiler and the other as a lag boiler and as already suggested swap them over every year. On a basic set up you'd common a single switch live to operate both units.

If boiler has correct voltage to it and all components operating as it should but rarely cutting in I would suggest it is getting false flow from the other unit satisfying the boiler stat before the boiler had the opportunity to fire up

 

[Ermintrude]

Check its getting flow, turn power off on number one put demand on and two should go to full tilt. If it's been poor hydraulic design boiler 2 may we'll be a sludge bucket

 

[Pongo]

Thanks gasmanrob and ermintrude for your comments. I hope to go back in a day or two and get a have another look.