heating system on a canal boat, automatic vent valves

[stevet]

Hi

I've been helping a friend get her canal boat 'ship shape' and the heating sytem is most odd, there's a log burner with some sort of back boiler and pipes that run from one end of the boat to the other, via the radiatoirs. The pump (a small 12v affair) is at the rear in the engine bay and there is a loop of pipe that goes up and over before going to the inlet of the pump. This loop seems to alway get air in it, it does have a bleed valve but it's a pain to keep having to bleed it. Also you have to bleed it with the pump off otherwise the pump just sucks more air in. My question is this, can I put an autmatic vent valve onto this loop, bearing in mind it's on the inlet side of the pump and only 6in away from it, or will it just result in more air being sucked into the system. If it's not a problem then it will make life easier in keeping air out of the system.

many thanks

STeve

[BigWallop]

The little rise in the pipe is actually there to catch the air bubbles, before they get to the pump, and bleed them off. It might actually help to extend the pipe, that the bleed nipple is on, further up. The extension will catch more bubbles at a time, and should make it easier to bleed the system of all the bubbles at one time.

Why does it actually need to be bled frequently anyway?

[Terry]

radiatoirs.

A stupid suggestion? A canal boat is probably rarely in rough water? So why does the system have to be a closed loop? Could it not be an open system with a small 'header' tank at the high point in the system? Any air in the system would circulate to the high point and 'bubble off' to be replaced by water through gravity?

[Dave Liquorice]

For the same reason that domestic systems are closed loops, to keep oxygen out of the water. Even open vented systems the the water does not circulate through the header tank (or shouldn't).

Got to admit I don't like the idea of a woodburner with boiler that doesn't have gravity as the means of circulating the water. If the pump fails or auxillary battery goes flat the water in the boiler will boil PDQ and the resultant steam has no where to go...

[Nick Atty]

[brian]

I saw a rather "well used" Hudson boat last year in Braunston with a can on the roof and a bit of plastic pipe going into the beam across the roof at the front of the cabin, so I asked the owner about it and the explanation was as follows.

He was going to make the roof beam into a header tank for his heating system as it was the highest point of the boat and fully welded on all seams. From this he had run a large diameter copper pipe the length of the boat just below the ceiling inside the accommodation and another along the floor. These were the flow and return for his heating system. He had spaced radiators down the length of the boat just below gunwale level connected between the high and low level pipes flow and return. His back boiler on his stove which was in the front corner of the cabin was also connected to these pipes. He now has a gravity feed central heating system that requires no electricity/pumps etc no danger of an air lock getting in the boiler, in the absence of a leak no chance of the boiler running dry when hot. It all just works

-- Brian From Sunny Suffolk

[Geoff]

An automatic vent has to be on outlet side, ie positive pressure. You need to find where the air is getting into the system and cure that, and, or move the pump down-stream to a position where it wont suck air. Air, or rather oxygen in the system on a continuous basis is a 'very-bad-thing' it'll rot your radiators in no time. There is some mechanism in the system to allow expansion of the heated water I hope, is air entering thru that, as when the system cools. g

[Geoff]

I lied, move the pump UPstream, sorry

[stevet]

Geoff

A bit more info about the system, it's an open system with a header tank that's inside the boat, it can only hold about 5 litres. It's not at the highest point in the system though as the pipe from the burner goes up and right over the top of the cabin and is probably a couple of inches higher that the tank. There's also a vent outlet that goes out through the roof that is permamently vented, it has a fitting with some fibre washers in it that seem to allow air through. The other heath robinson bit is that the rads are connected in series so you can't turn any one of them off or down as it would stop the flow completely. As to getting air in the sytem, If you let the burner get too hot without the pump running then it vents steam out through the vent valve and the system looses a lot of water and air 'gets in', it then takes ages to bleed everything again, hence the desire for an automatic valve. My plan is to put a pipe thermostat on the outlet pipe from the burner and get it to turn the pump on if the water gets too hot. The other suggestion I like from BigWallop is to extend the bleed valve near the pump up higher so that it can accomodate more air before it affects the pump operation. Or I could reposition the pump so that it's before the loop and use an automatic valve.

thanks for all your help.

STeve

[Geoff]

Hi Steve, I'll need to spend time pondering your post, but, I do feel your going about this all wrong. In my opinion you 'must' prevent air from entering the system. If your getting boiling up into the header then you must take steps to prevent this, bloody dangerous if nothing else, can you not have the pump running whenever the stove is alight?

[Drifter]

burner and

affects the

system. If

'very-bad-thing'

Re-plumb it correctly. It'll also be quite inefficient with rads connected in series.... and the open header tank SHOULD be the highest point. I doubt you'll ever get it sorted the way it is. Tony ex-plumber on the Basy

[stevet]

Geoff

Your right, I plan to put a pipe thermostat on the outlet pipe of the log burner and have this switch the pump on, it'll probably need a relay near the pump as there's about a 50 foot run to the back of the boat and at 12v there would probably be a lot of loss unless I used thick cable. I though of using a timer delay relay that keeps the pump running for a time after the stat has gone open to stop the pump from cycling on and off. That should stop any more air getting in and then once it's all bled it will hopefully stay that way.

many thanks

Steve

[Geoff]

Remember the pipe stat will probable switch the wrong way, ie. switch off at high temp so you'll need the relay to correct this. I agree with Drifter, I dont think the system as descibed is ever going to be much good, in fact it sounds a bit unsafe, rip it out and start over. B'ain't big dollar :) g

[brian]

If you are going to do that I would suggest that you have a short vertical pipe run at the back of the boiler so the hot water in the boiler can rise and heat the switch. If the pipe is horizontal out of the boiler, the boiler will probably boil before the heat gets to the switch. RS use to do switches that switched the way you want that you strapped on pipes. Also the timer is a very good idea, otherwise the boiler heats up, the pump starts, pulls cold water from the rads and pump cuts out, water stops flowing and heats up, pump starts, more cold water , pump stops. You can get a plug in timed relay that they use for heated car back windows that would do the job nicely.

What makes you think I've been there.

-- Brian From Sunny Suffolk

>

[stevet]

Brian

The pipe up from the boiler is vertical up to the ceiling of the boat, I'll put the pipe stat on that run. I did phone a local auto electrical shop for the relay and I think I've found one that will do the trick. It's a job lined up for this weekend so I'll keep you all posted. The 'rip it out and start again' option is definitely the best long term but with a boat that needs to be lived in pretty soon and lots of other work needing to be done it may have to wait until the summer.

cheers

STeve