Calorifier on gravity system

[RuK]

Hi , already had lots of useful advice from Bengo, smileypete & mike boilerman on my back boiler system thanks guys. Any advice on the following much appreciated:

 

On further investigation of my back boiler hot water/radiator gravity system design I've found that having my calorifier right behind the stove in a cupboard - under worktop height, I will only get a rise of 140mm on the flow from back boiler to the coil inlet. This will be about 300mm behind the stove, connected by 28mm copper and I can have a custom calorifier made with a 28mm coil. So this doesn't look like much of a height difference to try & get a thermosyphon flowing through the calorifier. Think I may be able to get the manufacturers to stretch the coil a bit and have the inlet a bit higher on the cylinder but it won't be by much. I'll have an air release valve at the calorifier inlet.

 

Alternatively I could put the cylinder back in the engine room at a higher level, with a rise of about 250mm from the back boiler but this could be increased to about 500mm by building a higher platform for the calorifier. This would be at about 5m away from the stove, with the pipe rising shallow & continuous to a radiator and then sharply up to the calorifier for the last bit, again in 28mm pipe.

 

I know it's tricky to get a gravity system to flow through a calorifier so my question is am I better off having the calorifier close by the stove with a low pipe rise (or will this likely not work at all?) , or far away but higher up in the engine room? I'd been planning on having it right behind the stove as I've seen this work quite nicely on gravity in another boat, but the pipe rise was higher there.

[BEngo]

Close is better. It's all about resistance to flow and density differences.

 

The 300mm of pipe will have a much lower resistance to flow than 5m of pipe so will need less drive to make it circulate. Close, you have a driving rise of about 460mm/m and the minimum practicable density at the to of the coil. Further away you have only 50mm/m or, with difficulty 100 mm/m. There also will be a much greater heat loss between the boiler and the engine room and this will reduce the effectiveness of the 100mm/m drive because the water at the coil inlet will be more dense because it's cooled some.

 

28mm coil is a V good thing- but if going bespoke, don't worry just about maximising the top location of the coil- get them to go as far down as possible, without going below the bottom of the boiler. The real drive in a calorifier loop is the increase in water density as the heating water gives up its heat to the tap water as it passes through the coil. The lower the bottom of the coil, the faster and longer it will circulate. You will also get the maximum of hot tap water.

 

N

[smileypete]

The problem you may run into is reverse thermosyphoning out of the calorifier.

 

Unless ALL the calorifier coil is above the top outlet of the backboiler, as the stove cools down some hot water may flow back into the top of the back boiler, cool down, and be replaced by more and so on.

 

This isn't a problem on domestic installs as the backboiler is usually on the ground floor and the calorifier well above it on the first.

 

So I'd expect a pumped calorifier circuit should work better, tee in as close to the backboiler outlets as poss, have the pipes go down to floor level then along and back up into the calorifier coil.

 

Put the pump in the return pipe and control with a pipe thermostat on the top outlet of the back boiler, make sure the pump outlet points up to help it to self bleed, add a couple of cheap 'ballofix' type isolation valves either side of the pump.

 

Also the flow/return pipes from the calorifer coil(s) should be very well lagged down at least as far as floor level, and the hot outlet lagged for about a metre too.

 

Pumps can be had on Ebay for as little as £18ish these days:

 

http://www.ebay.co.uk/itm/DC-12V-SOLAR-HOT-WATER-SUBMERSIBLE-CIRCULATION-PUMP-BRUSHLESS-MOTOR-6-5L-MIN-/301009053130

 

cheers, Pete.

~smpt~

[peterboat]

My old boat was set up with the calorifier right behind the backboiler, their was only a slight height difference between the top of the coil on the calorifier and the output on the backboiler it always seemed to work well with no reverse flow. However we set up a similar system on a friends boat the distance between backboiler and calorifier is about 3 meters the calorifier is a good 2 foot higher and yes it tried to reverse flow!! 28mm pipe everwhere the towel rail however was working perfik!! After a week of gentle fires it sorted itself out and works well now a mystery of life or at least it is to me

 

Peter

[smileypete]

Reverse thermosyphoning probably won't be a problem if the stove is always ticking over and staying in, but if it's left to die down at different times, the calorifier may lose some heat in the meantime....

 

cheers, Pete.

~smp~

Edited January 20, 2014 by smileypete

[RuK]

Thanks for your advice everyone, to be honest I have been coming round to the idea of a pump for the Calorifier.

 

At £18 for a 12v pump you can afford to have a spare on board in case of failure, but I have seen that jabsco do a very nice looking circulator with variable speed settings that goes down to 0.25A on minimum. Not cheap though, over £200!

 

But I think I will take a risk and try and make it work on gravity. I was thinking if I included a stop cock in the Calorifier pipework I could close it down over night or once it was up to temperature to stop any reverse flow happening. The radiator gravity circuit would take any further heat so no problem doing that.

[smileypete]

The little brushless motor pumps can be speed controlled quite easily.

 

Something that MAY (or may not!) work is a car thermostat in a tee into the pipe from the backboiler outlet, Renault did one that fits in a hose, can't remember the dia offhand:

 

http://www.ebay.co.uk/itm/OEM-Replacement-Thermostat-Renault-4-5-9-11-18-21-Trafic-Extra-Fuego/331094829710

 

As long as the thermostat is NOT in the pipe between backboiler and vent, but if the calorifer is on a completely separate circuit to the vent then it should be OK.

 

cheers, Pete.

~smpt~

Edited January 20, 2014 by smileypete

[RuK]

Been firming up the design- please excuse my plumbing ignorance but can anyone tell me how you plumb in the threaded fittings- like the malleable iron fittings recommended on this forum from bes?

 

They do 'nipples' which are pipes with threaded ends but the longest ones are just 18". I can handle compression, and you could couple raw pipe to these threaded fittings using it but that would add up to a big knot of fittings. Am I missing something?

[smileypete]

What type of connection is there on the backboiler, threaded, taper thread, male/female, BSP thread size?

 

Usual way from 28mm pipe onto a threaded backboiler outlet connection would be a straight compression to BSP adapter, adapters are on this page:

 

http://www.bes.co.uk/products/140.asp

 

A male taper thread into parallel female will seal OK with PTFE tape, a parallel male into parallel female will need a washer or jointing compound like Fernox LS-X or linseed putty like 'Boss White'

 

Malleable iron is a bit old school really, only found nowadays in old buildings. If the above doesn't make a lot of sense maybe just post up details of the connections on the backboiler, and where the pipes need to go from there, to get suggestions. A sketch or diagram of the pipe layout could be helpful too.

 

cheers, Pete.

~smpt~

Edited January 21, 2014 by smileypete

[RuK]

Thanks Pete yes they are male 28mm BSP out of the boiler, was just wondering about the malleable iron fittings as they have quite a few swept bends and a kind of swept T as well which would be good for assisting the thermosyphon.

 

I can do it all with compression fittings and a pipe bender but would have to use a regular T, but I'm probably over doing it worrying about that. The T could be arranged so the Calorifier takes the straight branch

 

Now to make this dream into reality, thanks for all your advice folks

[smileypete]

Worth checking the exact thread diameter with a table as say ~25mm across the thread is not 1" BSP...

 

http://en.wikipedia.org/wiki/British_Standard_Pipe

 

Swept bends sounds good, some central heating corrosion inhibitor would be also be good when up and running.

 

cheers, Pete.

~smpt~