Dual heating

[falasarana]

Hi
Is it possible to interconnect a Condensing open vented LPG Boiler and a solid fuel stove in order to heat radiators? the stove has an existing gravity circuit to the DHW cylinder.
will this damage the heat exchanger of boiler and void warranty?
has anyone installed a similar system?
thanks

 

[Shane007]

It is possible, however, solid fuel can cause pitching from overheating, thus re-oxygenating the system causing sludge. Gas boiler heat exchangers are much more restrictive than oil boiler exchangers so they can easily clog up with sludge. I would probably powerflush gas boiler systems with blocked heat exchangers approx 4 per month and sometimes more. I have 3 to do next week and none of them have solid fuel!

With regard to warranty, you would have to check with each manufacturer as some could vary.

 

[falasarana]

Thanks Shane007 for the reply,
Ive read that pitching can be minimised by the correct placement of the feed and expansion, ie. pre the pump. is this an effective solution?
do you have any views on something like a boiler buddy or similar magnetic filter, can they help to protect the heat exchanger or are they a gimmick?
also i wondered if open vented systems are generally more troublesome than pressurised systems?
thanks again
sorry for all the additional questions

 

[Shane007]

Yes pitching can be limited, but not avoided, by having a correctly installed overall system. Placement of both the feed & expansion pipe & the osvp are critical in relation to the position of the circulating pump, but you must also be aware of the placement of the circulating pump for the gas boiler, in that they are not either pulling against eachother nor pumping into to eachother. They should work in unison and circulate together. Pitching cannot be avoided if there is a pump failure or a power failure. Sludge will never be avoided completely with solid fuel. It is also to do with higher temperatures associated with solid fuel. Ensure you put in plenty of inhibitor in the system and use a good quality one such as Fernox F1.

Boiler buddies and the like are relatively new and in principle they seem good. I use one when carrying out a powerflush and they remove a lot of sludge. It cannot harm the boiler and they must be placed on the return pipe before the boiler. The stronger the magnet in the unit the better the performance will be.

Pressurized systems are less troublesome than open vented systems, in that it does not matter where the pump is located. The neutral point is exactly half way round the system (half in positive & half in negative), however, an open vented system is safer. You must try to design an open vented system with as much of the system as possible in positive pressure rather than in negative pressure. A well designed and installed open vented system will work very well. A pressurized system cannot have a solid fuel system installed so if you want one you do not have the choice of a sealed system.

A system that I am powerflushing next week incidently has a flush buddy installed but the sludge is caused by a chemical reaction between the heat exchanger and gun barrel installed in parts of the system. The particular boiler installed has an alloy heat exchanger that reacts with GB.

 

[falasarana]

Thanks Shane007 for the very concise answers,
I was originally sold on the idea of interconnecting the Stove and the LPG Boiler, however i have now decided to install an additional radiator in each room upstairs which will be fed from the Stove.
I found this schematic online
oilstoves.co.uk/webdocs/articles/Wood_-_Solid_Fuel_Boiler_Plumbing_Diagrams_03-06-08.pdf

I imagine that the same rules apply in order to prevent pitching in this system design. However this is where i lost you, when you explained that it is important to keep as much of the system in positive pressure, could you expand on this a little please?
In addition this system includes a injector tee, will this component not inhibit natural gravity flow prior to the pump kicking in?
Any other ideas on this system design would be very welcome.
thanks again for your help

 

[Shane007]

Re: The schematic link: The injector tee to my view adds restriction to the primary gravity circuit and not as they claim encourages. Injector tees are similar to TRV's in they open and close with a thermostatically controlled capillary tube. I would not install one on a primary circuit for solid fuel.

The schematic on page 3 is nonsense. They have an unvented oil boiler going into an open vented buffer tank. No harm in that but why unvented? They also have an unvented pellet boiler but that has no expansion vessel, etc. and it is only going through a coil in the buffer. It is also a very busy buffer tank with so many coils. Not much room left for water! All links from the document are broken or non-existent.

Basically, keep it simple. Heat will always try to find the easiest route to circulate so design the system with easy uncomplicated no fuss to encourage this and it will work.

With regard to positive pressure, if you think of everything in front of the pump is positive and everything behind is negative. The neutral point is the point where they meet. The neutral point should be kept as short as possible. It is hard to explain without a diagram, if the pump is on the flow the first tee from the boiler should be the osvp, then within maximum 150mm the f & e and then the pump. This means that all the system is ahead of the pump so it will be in + pressure and from the osvp to the f & e will be neutral and from the f & e to the pump will be - pressure.
If the pump is on the return then all the system will be negative. Nothing wrong with it and it will work fine, but all joints are on suck which microscopic air can enter. Generally, the pump on solid fuel will be on the return as they higher temperatures from solid fuel can damage the plastic impellor in the pump. Then the second pump would go on the flow. If both are on the return, they could pull against each other. This is where you find some people with dual systems find they cannot have the oil on at the same time as the solid fuel. They find pipes banging, etc. and this is usually caused by pumps pulling against each other and the heat can go nowhere.

If you pm me with your email address and an outline of what you have and what you are trying to achieve, I can do up a diagram for you.

BTW, I would not opt for LPG.