Cascading 2 boilers

[mike_s]

How was this done the old fashioned way?

I ask because I have a customer who has 2 heat only boilers, pressurised with external expansion vessell etc.

The system is between 5 and 10 years old, there are 4 non-return valves in total, one for each flow and return.

They have a problem with one boiler overheating when the other is on, boiler stat and over heat cut off seem to be working fine.

The pump is on the return, working and overunning fine however I'm going to move the pump to the flow, before the 3 zone valves (s plan plus system) as I'm thinking the non-return valves are preventing the heat escaping as the pump is incorrectly positioned.

Failling that, I will check out the non-return valves.

Does this sound about right? I've spoke to various manufacturers and these days they insist on low-level headers but the non-return valve method was the old school way so how should this be done?

 

[stevetheplumber]

never seen it done with non return valves ive done a fair few over the years and just linked them into suitably sized pipework with iso valves to allow repairs etc to either boiler the non return valves may indicate a previous problem i cant see how moving the pump will help is there a bypass to allow circulation on over run as s plan will shut flow completely?
is the pump wired from both boilers ?
if only one boiler is going to overheat have you tried running just that boiler see if it still happens if it does id be thinking boiler problem rather than system

 

[SimonJohns]

To cascade them the old fashioned way. You set the thermostat 5-10 deg c lower

And then on annual service you alternate the lead and lag

 

[mike_s]

Cheers guys, sorted it today, made up. We ended up taking off all 4 non-return valves and one was blocked solid, the others were falling apart, so we just joined the pipes back up without them and even just the one boiler managing to heat the 200 litre unvented cylinder and about 20 radiators, customer made up and says it's the best the system has ever worked.

 

[Dotty]

I was going to say take a butcher's at the nrv's but you beat me to it! Low loss headers are the way to go though if you ever need to cascade a pair of boilers in the future.

 

[mike_s]

cheers, he wants me to replace the boilers in a couple of years or when they break down and I'll probably go for Worcester RI's with a low less header, there's a technical bulletin on their site of how to do it and their technical services are great should I need any help designing the system.

 

[bill577]

you should have left the nrv on the flow pipes for each boiler, when one of the boilers (and pump) switchs off the other pump will push water back through the first boiler.

 

[stevetheplumber]

you should have left the nrv on the flow pipes for each boiler, when one of the boilers (and pump) switchs off the other pump will push water back through the first boiler.

there is only one pump so doesnt need nrv's there would be little heat loss through the boiler that wasnt firing

 

[mike_s]

yep it is only one larger grundfos pump on 28mm pipework. I replaced the overheat cut off incase that was faulty too.

 

[stevetheplumber]

you only need low loss headers to cascade boilers with internal pumps usually a pair of sytem boilers

 

[mike_s]

Technical Bulletins - Worcester, Bosch Group UK installer site
Not doubting you but the technical bulletin here on cascading boilers shows a low less header on regular boilers with external pumps, however it also has 2 unvented cylinders! I'll cross that bridge when I come to it. A good read here though.