Backboiler problems

[vintagescubaman]

A friend has recently purchased a 70ft narrow boat and has what appears to be a peculiar back boiler setup. It has a few problems and i'm hoping the collective wisdom can shed some light on it. I have searched the forum but haven't found a specific answer.

 

 

 

 

The log burner with back boiler is located at the bow, the top pipe runs to the rads via a pressure relief valve and a header tank (which is mounted in a locker in the cratch), the bottom pipe runs to the rads via a 12v pump. The peculiar thing about this setup is that the pump runs backwards forcing water from the bottom of the boiler around the loop and back into the top of the boiler. I would have thought that this runs counter the the natural law of heat rising. The syetem has never really got hot and is at best lukewarm.

 

The pump has been turned round, but the system just will not bleed or heat up, it only wants to run in this direction, so it has been put back as it was originally.

 

Can anybody explain why the system is set up this way? I would have thought it would be better to have the header tank at the stern. I was thinking that blanking off the front header tank and installing one at the back of the system would help, does anyone have any views on this system?

[canals are us?]

It's badly designed!

 

It should have the header tank at the highest point ie where you say is a pressure release valve, sure it's not an auto air vent? If the valve isn't releasing air/water when the system is refilled it's faulty., but I would install the header tank there. A photo would help.

 

2 the pump on the return should pump towards the stove.

3 in the event of pump/power failiure there is no provision for gravity heating to the one rad to dissipate heat from the stoves boiler.

 

I would drain it down flush it and the rads and install the header tank at the stove end at the highest point and turn the pump round.

 

Probably needs a plumber to look at it and repair and clean it out.

 

If it has a domestic hot water boiler in the stove it may only heat 2 rads max depending on the size.

 

James.

Edited September 6, 2013 by canals are us?

[vintagescubaman]

Yes the pressure relief valve is indeed an auto air vent, and it is at the same height as the header tank. The rads have been removed and flushed, the pump was turned round but the system just wouldn't work

[canals are us?]

Yes the pressure relief valve is indeed an auto air vent, and it is at the same height as the header tank. The rads have been removed and flushed, the pump was turned round but the system just wouldn't work

When you refilled the system did air or water come out of the air vent? Some are automatic and some are manually operated by loosening the little cap. What colour was the water when you drained it down?

 

James

[vintagescubaman]

Just air, the water was pretty clean

[canals are us?]

Just air, the water was pretty clean

 

Sounds like it's very airlocked or pump not working great or a blockage/restriction. I would drain it again and disconnect the auto air vent and refill the system with all rad vents open then see if it will circulate ok. Is the pump working properly?

 

James

Edited September 6, 2013 by canals are us?

[BEngo]

This is very badly designed. Not only is the pump fighting the natural circulation of the boiler but, in the event that it boils, hot water is likely to be violently ejected from the header tank. Potentially dangerous in itself but also increasing the likelihood that the whole thing will boil dry.

 

 

The bottom of the header tank should be connected to the bottom pipe before the pump, not to the top one and there should be a vent/expansion line from the top pipe to the top of the header tank. This can be quite small- 3/8 is fine- and it could replace the air vent.

 

Are there any non-return valves in the system? Some radiator valves will only work one way and prevent flow the other. Have a careful look at them to ensure that any flow arrows point toward a pipe going the base of the stove (into the rads at the top, out of the rads at the bottom.) Have a good look at the back of the stove and at any isolation valves either side of the pump.

 

You have shown both pipes going into the base of the rads. It should work like that all other things being OK but will be more effective if the flow pipes are connected to the tops of the rads and the return to the opposite end at the bottom.

 

Is this a typical boat with the bows in the air and the pipes parallel to the floor? If so what sort of pump is it? With the bows up, the pipes to the back actually slope down hill and make the pump's task of assisting natural convection more difficult.

 

Is the pump connected correctly so that it is going round the right way? Most DC motors will happily run either way, depending on the polarity of the connections. This one may be running the wrong way.

 

N

 

N

[vintagescubaman]

Yes I agree, it is extremely badly designed.

 

There are no non return valves in the system, the pipes are parallel to the floor at skirting level, and the pump is wired correctly.

 

The pump has been tried running the other way but it makes the system even worse. I was thinking about getting rid of the forward header tank and replacing it with one at the stern.

 

Its a bit of a sticky one, I think it needs some major re working.

[Bee]

Bengo is right, it could be dangerous, The hot water should come from the top pipe. The whole thing is trying to go backwards and forwards at the same time. Ideally the hot water should rise all the way to the expansion tank on the end of the uphill pipe run and then fall all the way to the bottom pipe of the boiler, this will work without a pump. Quite how much you can depart from this ideal by using a pump is difficult to say but I wouldn`t use it like this, the circulation could reverse suddenly if it overheated or the pump failed and crack the boiler or worse. Sometimes on boats it is difficult to achieve really neat pipe runs and you have to live with pipes running at a slant.

Richard Cooper

[smileypete]

Could be a right can 'o worms, how long do you really want to spend on it?

 

A set of photos uploaded to a photo hosting site might help people better spot the problems with it.

 

cheers, Pete.

~smpt~