Lead and lag boiler wiring design

[kiaora]

Hi
Customer has very big house, using whole house sizing method, it requires 44 kW to heat house.
in my view I would like to install 2 open vent 24 kW boilers, the cost is about the same, but the efficiency will be better with only one boiler working on most days and only the 2 when temp drops, ps we are in South England .
also we will have a back up boiler.
i want to design a lead lag wiring system, any ideas?
I was thinking of timing the return pipe to see how long to get to a fixed tem, befor bringing in no.two boiler?
any thoughts?
Regards
peter

 

[Killy Bing]

Cant you just go for a sequencer ?

 

[stevetheplumber]

run one boiler stat slightly lower than the other and reverse every service

 

[Wet dream]

run one boiler stat slightly lower than the other and reverse every service

Simples like it !

 

[Ermintrude]

Pid controller or just smart start stats and a weather comp. two logic heat only on LLH . Can use the reverse return cascade but LLH better as boiler can be drained and dropped out easier. If pressurised use logic systems. Set s/l on a pipe stat for each boiler sets stats slightly different 10c and alternate the slave boiler every 12 months. ( turn one up and one down)

 

[oz-plumber]

There is one thing to consider when cycling package boilers - to have one switch off at a lower temperature, is that you lose half the pump pressure when one boiler shuts down.
This may cause the heating system not to operate correctly when only one boiler is operating.
At 44 kW you may be OK, but any larger you may have to have the boilers piped to a manifold that has a larger pump. ( primary pumps - boilers - secondary pump - heating system ) You will also require non return valves on the outlet of each boiler so when one shuts down you don't short circuit the system through the slave boiler.

By the time you set up the system - non return valves, secondary pump, electrical controls and so on, it may be more cost effective to put in a single boiler that can cope with the heatload and have a suitable pump for the heating system.

Oz-Plumber

 

[Ermintrude]

There is one thing to consider when cycling package boilers - to have one switch off at a lower temperature, is that you lose half the pump pressure when one boiler shuts down.
This may cause the heating system not to operate correctly when only one boiler is operating.
At 44 kW you may be OK, but any larger you may have to have the boilers piped to a manifold that has a larger pump. ( primary pumps - boilers - secondary pump - heating system ) You will also require non return valves on the outlet of each boiler so when one shuts down you don't short circuit the system through the slave boiler.

By the time you set up the system - non return valves, secondary pump, electrical controls and so on, it may be more cost effective to put in a single boiler that can cope with the heatload and have a suitable pump for the heating system.

Oz-Plumber

Use a system boiler with a zv on flow if that's an issue nrv going to stick.

 

[Roger Welton]

Straight sequencer

Boiler Sequencers / Step Controllers

Or with weather comp, and DHW control

Satchwell DC1400 Optimiser Compensator

 

[kiaora]

Hi all
thanks very much for all your comments, I was planning to install the pipework to form a low loss header, the system has 28 mm flow and return, heating only, the two boilers have 22 mm connections at the top, each boiler will have it,s own pump fed by boiler/ over run, and the system will have a separate pump.

the house is not used a great deal in the winter, so often the heating load is ok with a smaller boiler, I need to install in a way the house can be heated if needed.
Hence only one will be needed most of the time, but the potential will be there if the temp drops dramatically. Or if it's fully occupied .

i like the idea of running one at a lower seting, simple, but I am trying to come up with a simple way to satisfy the performance, without lots of system controls.
i may be approaching this the wrong way, but hey! Isn,t this what a forum is for?
thanks again for the input
regards
peter

 

[lame plumber]

if there is little use in the winter why are you going for open vented, surely an unvented system would make more sense, less chance of a freeze in the roof space.

 

[Killy Bing]

if there is little use in the winter why are you going for open vented, surely an unvented system would make more sense, less chance of a freeze in the roof space.

And if its not used all that much it removes the possibility of a burst pipe emptying into the house for a few days or week till found.

 

[kiaora]

hi
it is occupied all year, but only a couple, also the reason we are upgrading the boiler, it,s an old over-sized 80kw cast iron natural draft with a massive stainless steel external flue,
and the customer has trouble sleeping with the noise of the meter dials going round!
(ok that was a feeble attempt at a joke)
regards
peter

 

[Ermintrude]

You zoning it off aswell then?

 

[stevetheplumber]

use open vent boilers but seal the system with just the one pump

 

[Ermintrude]

Bio mass and a thermal store any one?