Hi Guys, I'm having some problems with my central heating which I want to put out there to see if you guys can pitch in a few suggestions...

I plumbed my own house 5 years ago. Fitted an indirect central heating system with manifold heating to convection radiators in each room, which is common in europe. I Ran all the primary and secondary heating circuits (first and second fix plumbing), etc, etc... saved a fortune. (backround... My first real job was as a plumbers mate and worked at it for 2 years before moving onto a different trade, so I figured I had enough knowledge to plumb my own house....)

I over complicated the plumbing in order to precisely control the amount of energy that is used. From that stand point the house is super efficient but there are some side effects that I need to address. I fitted an 18inch "back boiler" in our fireplace as a secondary solid fueled source of heat.

On paper, it looked like it would work out fine but in practise I have some back pressure problems and/or surging which I am finding it hard to eliminate.

Some info on the electronic controls.. I have the primary circuit set out with 3 zones. Zone 1 - downstairs radiators, Zone 2 - Upstairs radiators, and Zone 3 - Hot water storage. All controlled by digital controller and independant thermostats for each zone. A motorised valve opens or shuts the zones under control. I can control each zone independantly or all together as would be found in a typical installation. This system works flawlessly, no problems there.

My problem is with the secondary solid fuel boiler. I have it set up in such a way with a thermostat switch, when the back boiler hits 65c, it throws an isolating relay which shuts the power signal to the oil boiler and only runs a circulating pump for the solid fuel boiler.

The problem is when the solid fuel boiler kicks in, a water surge forces water back up to the vent to the header tank, probably a gallon or more is forced out before the system settles down. I have tried lowering the set point of the thermostat but it still seems to force water back up the vent. I have also spent time balancing the radiators but still, the solid fueled boiler cannot be matched with the oil boiler for some reason which escapes me. When the solid fuel hits 65c, the circulating pump pushes water back up the vent. Even with all 3 zones open (wide open circulating system, no back pressure), the water still gets pushed up the vent..

Short of going and buying a PLC and getting deeper into the complexity of the system, I want to go the opposite direction. I am inclined to re plumb the back boiler directly into the circuit "in series" after the oil boiler which should simplify the control at least... All of this work can be done above ground and without too much mess. Not a big job by any means... I am leaning towards this ammendment but I want to see if you guys have any input first??

I know indirect circulating "hot water" radiant heat is not really a popular heating medium in the US, but there may be some of you out there that can pitch in?

Heres a simplified diagram of the primary circuit. I left out the secondary for clarity. I have also left out pressure relief valves, air vents etc...

All of the primary is plumbed using 1 inch pipe except for the vent and the hot water storage which uses 3/4 inch pipe.

Heres what I'm thinking of doing, pulling out the NRV's and simplfying the dual circuit. As both circulating pumps are controlled by independant thermostats, they can stay, I dont think they will need to be controlled in parallel? Once both hit 65c, they'll start spinning... I know it'll be less efficient and we'll probably start using more oil, but I cant allow the overflowing through the vent to continue. Besides, there should be a certain amount of radiant heat given off from the solid fuel boiler in that room, a second radiator if you will... so all should not be a total loss...