This is a text-book/ideal-world type answer.
Bear in mind that I do not know whether your boiler can be controlled by an external controller or whether there is a domestic, reasonably-priced, controller capable of doing what I describe. Maybe one of the domestic heating engineers can advise.
The Raypak diagram is a good illustration of your system.
• The boilers are controlled to maintain the system/secondary flow temperature, Tf, as measured by the ‘main temperature sensor’ .
• If you have a weather compensation system (highly recommended) then Tf is varied from say 40 to 75 degC as the outside temperature, To, varies between 18 and 0 degC. The temperatures used here are just for illustration, they aren't recommended for your system.
• When To = 18, Tf = 40.
If To = 0, Tf = 75.
If To = 9 degC , Tf =57.5, etc..
• A weather compensation system would need an outdoor temperature sensor (not shown) to measure To.
• Note that, because you are reducing the system flow temperature in mild weather, any TRVs will only be acting as ‘trimmers’ to provide control of the individual room temperatures. They will not be trying to shut down completely, as they might be in a conventional fixed flow temperature system where Tf would be 80 degC at, say To = 16degC.
This will ensure there is always a high flow rate in the secondary system.
This is important; see below.
• If the secondary flow were less than the primary/boiler flow, then the
flow direction will reverse in the common pipework and you will get a mixture of primary flow and secondary return water entering the boilers, raising the boiler return temperature and reducing the efficiency.
• Also, the boiler pumps are intended to provide adequate flow when the boiler is connected directly to a conventional heating system. In your system, the primary/boiler circuit has very little pipework and few fittings compared to such a conventional system. The primary system has a very low hydraulic resistance and the boiler pumps will produce a large flow rate; this is not good, see above.
The flow rate through the boilers needs to be regulated. This would usually be done by reducing the pumps' speeds and fitting a double regulating valve (DRV) to each boiler to impose an additional, adjustable resistance. The DRVs would make the reverse return pipework arrangement unnecessary. The DRVs would be adjusted to provide the recommended temperature differential across the boilers.
• The controller should provide 0 to 100% PI (Proportional & Integral) control of the boilers’ modulating burners to maintain Tf at the required valve. If the demand for heat is 0%, all boilers are off. If the demand for heat is 100% both boilers are firing.
• The lead boiler fires at 0 to 100% as the demand varies from 0 to 50%. Above 50%, both boilers are fired at the same rate. At 60% demand both boilers are fired at 60% output. The idea is to keep the flow temperature of each boiler (and so the standing losses) to the minimum required to maintain Tf at the required value.
• There should ideally also be an indoor temperature (Ti) sensor. This would adjust Tf up or down if the indoor temperature is less than the required value.
If, as above To were 9 degC, then a simple weather compensation system would fix Tf at 57.5 degC. This would be inadequate if Ti were 12 degC; you’d want Tf to be 80 degC until the house had warmed up to 18 or 19 degC or so.
Sorry that’s a bit long-winded; I can’t condense a couple of decades of experience into a smaller package.
I would control this with a BMS system, knowing that the £3kish starting price would not be a significant part of the (commercial) equipment cost.
The control requirements for a two boiler 2,000 kW installation are the same as for a 2 boiler 50 kW job.
I’d also employ a controls contractor, who I’d expect to know much more about this than I do; I have sometimes been disappointed.
