Twin Boiler System

[Harry Ziman]

We have an old farmhouse with the hot water/heating system confidgured as follows.

Boiler 1: 110,000 BTU 16 radiators in the "front" of the house

Boiler 2: 110,000 BTU 6 radiators, hot water in the "back" of the house

Aga: Hot water in the front of the house

In case you are wondering the house was once divided into two each with its own heating and hot water system - they are independent with their own header tank, timer, pump, hot water tank.

Needless to say this is not very convenient - to heat one of the bedrooms we need to put the second boiler on. The two boilers sit next to each other in the boiler room so we would like to combine the systems. The question I would like to ask is how to organise the boilers. Do they go in series (i.e. water through one then the other) or parallel (i.e. split the water flow so it can run through either boiler and then recombine). CanI safely run one boiler and when I need the extra heat switch the second one on? One person has todl us that the boilers must go in series and both be controlled identically - same temperature and same time-controls.

Before anyone asks, I know that the header tanks need sorting out so there is only one. Also, I do not want to go to the expense of a new boiler with the capacity for the whole house.

We would also like to rationalise the hot water system so the Aga heats water for the whole house. This is a little more involved. Can I direct the flow through two separate hot water tanks? If not, how about a valve and a timer to regularly switch alternate from one to another. Failing that I guess I will need to remove one of the tanks and accept that it will take a long time for hot water to flow to some parts of the house.

Do let me know if any of this fails to make sense and I will try to explain further.

Thanks

Harry

[IMM]

No.

Yes.

Yes.

Not so.

Have a vertical 32mm header pipe. At the top have an automatic air vent. Just below each boiler tees in its 28mm flow pipe. The same at the bottom of the header pipe for the boiler return pipes. Each boiler has a pump on its own 28mm flow pipe. In the centre have the flow and return to the DHW and CH, with both tees as close as possible.

Is your mains pressure good? If so install a direct heat bank and eliminate the existing cylinder(s) and cold water storage tanks and have full mains pressure. A heat bank will also work with DHW fed from a cold water storage tank too. The heat bank creates a neutral point for all heat apliances to connect and also the CH circuit. Have 3 flow tappings and three return tappings on the heat bank. Each boiler, each with a pump, and the aga connect to the heat bank cylinder. Take the CH flow and return off the bottom of the heat bank using a pump in either the flow or return. The DHW will be taken off the heat bank too. If the heat bank is sized up right you will not need a radiator to dissipate heat in summer from the aga.

See:

Look at the examples of customer systems.

I doubt the Aga will cope heating two hot water cylinders.

[fred]

Keston have quite a lot of info on boiler sharing on their site that I came across when researching my own (single boiler) installation. They do a lot of it on their commercial boilers using a plenum chamber arrangement which looks as if it would actually short circuit the boilers' outputs, but by clever pumping a balance is achieved. There is also a simpler arrangement for use with the lower output Keston Celsius 25 boiler. Not your product of course, but you may find useful info.

Can't find all the links, but there is a useful diag in this download (450k pdf):

useful, but less intuitive diagrams at: more to search at: of drawings under the CAD section.

HTH

[Pete C]

Hi,

I'd put both boilers in parallel, with a decent size pump feeding both.

Then put a 'pipe stat' on the return to switch off one of the boilers when the system is up to temperature and the heating demand is low.

If both boilers were in series you might get 'kettling' in the second boiler when both are on, or the second might cut out when the first cuts in.

cheers, Pete.