Mixing heat sources within a CH system

[reb78]

Back in the early 90's when gas came to our village, we got central heating in my parents house. They had a gas boiler, a solid fuel Rayburn and an open fire with a back boiler and combined them all via a Dunsley Neutraliser. The system has been faultless for years.... The unit sits under the hot water cylinder and has been problem free.

We want to do something similar in our place. Solid Fuel Rayburn, oil fired boiler and if we can we want to put a back boiler on the huge log burner in the lounge. The latter is less likely to happen.

I am sure I have read people on here saying that the Dunsley neutralisers are no good. Thats not our experience, but i wondered what else was out there to do this? We are 30 years on since mum and dad had their system installed so I dont know if other methods have developed? I would be keen to hear what the experts on here think...

 

[julianf]

Our central heating (recently installed) comes off a thermal store, as does the domestic hot water.

Today, the rayburn was lit, so the oil boiler was not firing. There is also solar thermal (almost) connected to the store. We have PV to go on the roof, and that will probably feed to an immersun style dump to the store also, but there may be no point, as, if the sun is shining that much, the solar thermal tubes will keep it hot anyhow.

Today the rayburn was burning wood. In an emergency i could chuck coal onto it.

 

[waddycall]

Thermal stores give a lot of flexibility. In your shoes I’d be looking at one of those.

 

[Kayos]

As @waddycall says a thermal store is a "modern" solution. Like cylinders they can be custom built to your requirements, for not much more money than an off the shelf item.

As for Dunsleys, like most plumbing it comes down to how it is fitted, they are less popular now as G3 says you cannot have an uncontrolled heat source feeding an unvented cylinder, with or without a neutraliser

 

[addjunkie]

We have as you require done with a thermal store. Where things get complex is modern heating is a pressurized system, but your log burner with back boiler is an open system. Not sure if ours is the best way but it works. Thermal stores is heated by oil boiler. Also has dump imerssion heater from solar pv. The back boiler adds heat to the system via a heat exchanger, to allow the open vented system in the event of it cooking. But this requires pumps. Biggest down side is getting it to control. As the boiler heats the stores to 90 ish the log burner has a damper that closes at 85 to stop it over heating. So in reality unless the system is taking g all the heat input from the fire, the boiler is off. Gave up trying to get it to control, so now we have a relay that stops the boiler when the water in the back boiler loop reaches 45degrees.
If I did it again, Id just have a normal log burner. Then you have geat in a power cut, with out a generator. I do like the heststore though, it holds enough to heat the house, for about 24 hours if its just the underfloor heating on.

 

[Mikes]

I've linked a bale burner and an oil burner serving a very large old farmhouse. I built a simple low-loss header and each heat source had its own primary pump. On the secondary side of the header there were two pumped circuits. One served a large indirect hot water cylinder and the other the radiators. The whole system was open vented. Control was simple. Just a time clock to serve the oil boiler to allow it to fire early in the morning to give hot water before everyone had got up or via the override if they were too lazy to feed the bale burner. It all worked very well and never gave any trouble. In reality, the bale burner rarely saw a bale but was mainly fed mixed demolition waste. The oil burner ran on 35 second oil. A simple thermocouple and light circuit showed the temperature from the bale burner and gave an indication in the house of when stoking was required. The oil burner was a small sectional cast iron unit with pressure jet burner that used to be the summer hot water boiler on a job of mine that was being stripped out. All in all, the best way to keep these systems linked is simply. Recently worked on a job where there were some hot water cylinders served by thermal solar panels and a top-up gas boiler but also with a preheat vessel which was heated by a heat pump. All far too complicated and in my mind not worth the effort as the hot water demand was very low.
At the other end of the scale, a friend who has oil fired heating and a couple of wood burners wanted to link the two. After showing him what was required and the disruption to the two systems, He made a small back boiler for one of the wood burners and linked this to a couple of additional radiators . All open vented via its own F&E tank (the oil boiler has a pressurised system), again it works fine and doesn't have any of the interlink problems. The oil boiler is a combi so there wasn't any stored hot water to dump heat into. The back boiler is a welded-up fabrication made from various odds and ends and gives a good lot of heat to the radiators. The whole lot cost very little as even the radiators were nearly new freebies.