Thermo syphon or pumped

[OllyO]

I'm hoping someone can help me here.  The question is in two parts.  I'd really appreciate it if we could limit the responses to people that have experience/ insight with that would be great as I'm already confused enough as it is!!

Question 1

Our stove has a flue boiler so the hot water outlet is around gunwhale height.  Therefore we do not have the normal height increase available to us that thermo syphon requires to work.  However, the boiler is a copper coil of about 18" height which equates to around 32ft in total length.  This is obviously quite a length of pipe that is in effect increasing in height over the course of the coil.

Our is a barge and we need the first rad to be situated in the back cabin which is right at the other end of the boat to the stove.

My question is, can a thermo syphon system cope with either a slight decent or horizontal for any distance and if so is there a formula/ calculation that can be made to work this out?

I have spoken to people who have successfully installed thermo syphon systems with decents and horizontals but I have not been able to get an details from them.  Presumably if there is enough inertia and volume in the initial head of heated water then it can over come the decent/ horizontal.

Question 2

If I have to admit defeat and install it as a pumped system them does the pump need to be running the entire time the stove is on?  It seems to me that it would not only be a constant drain on the batteries but also having the continuous hum of a pump working might be annoying?  

Or should every effort be made to install the system as a thermo syphon and then have a thermo switch that actuates a pump if the water gets sluggish at certain points ie if the water temp exceeds a pre set temperature?

My last option would be to remove the log store from the stove so that it is dropped by 6", this may then give me enough room to have an acsending pipe to the back cabin without horizontals etc but it will mean taking an angle grinder to a posh stove.

Any thoughts on this would be greatly appreciated.  I only want to do this job once!

 

 

[Ex Brummie]

Thermo syphoning will not be effective downhill, you will get some conduction, but nothing significant. You can install a small pump on the backward leg that you can control with a pipe stat. so that when the back cabin gets hot, the pump will switch off until your lower setting is reached. Your horizontal run may work, but will take a long time to warm up. I have just installed a pump controlled by the flow pipe temperature to assist a thermosyphon over 40 ft which is almost horizontal and has run satisfactorily(ish) for 28 years on 15mm pipework. There was only one radiator which omly ever got 1/3rd to 1/2 hot. I installed the pump on the return to the boiler, controlled by a thermostat on the flow pipe, set at 65oC. The radiator now gets hot all over within 5 mins, and the pump runs for a couple of mins at a time every 10-15 mins. Once the boat is warm, the pump can be switched off. Your difficulty will be that you probably need 2 pumps, unless you can put a pump on a commoned return into the boiler.

[David Mack]

A thermosyphon works by having less dense hot water on one vertical leg and cooler denser water in the other. So if you connect the top of the coil to the top of the radiator and bottom of the coil to the bottom of the radiator with straight (ish) pipes between the two, then you we'll get thermosyphoning, and it doesn't really matter whether the pipes have a slight rise or slight fall. Whether the amount of water flow is sufficient to provide as much heat as you want to the radiators will be dependent on the pipe length and diameter.

Apart from thermosyphoning considerations you should have a gentle rise from the coil to the expansion pipe, so that if the boiler boils the steam has an easy route to escape.

[BEngo]

My pump backed  thermo syphon two-radiators off a Morso Squirrel system works quite happily with a steady but gentle down hill run from the top of the vertical pipe at the stove to the radiators.

From the Squirrel boiler the pipe runs horizontal for about 100 mm then turns vertical and runs about 150 mm to just under the gunnel.  From there it is parallel to the gunwale all the way to the header tank at the back.   The return is parallel to the floor.  So, allowing for the bow up trim  of some 3-6 inches depending on water and diesel levels, over 46 ft , the flow pipe falls and the return pipe rises.

  There is a  thermostat  (from a domestic calorifryer)  on the horizontal bit from the boiler which switches on the centrifugal type circulation pump if the pipe gets hot enough.  I have adjusted it so the pump comes on just before the fire back boiler makes kettling noises.    It thermosyphons well enough that we lit the stove in early September and it ran continuously for over a month  without the pump ever coming on.

 

N

[OllyO]

Thank you both for getting back to me on this, it sounds promising.  When we moved onto this boat they had feed and return pipework all running at floor level so it required a pump running 247 when the stove was on which seemed to me mad.

It is good to hear that thermosyphon in less than ideal situations can be achieved at least in part with the assistance of a pump.  Certainly installing a pump on the return side triggered by a thermo switch set at 65 (or thereabouts) on the feed will not be a problem at all.

I have made the coil boiler and I am keen to see what kind of heat it will generate, its a project for a bit of fun really so if it all needs to be scrapped I won't be heart broken but I would certainly like to try and get it to work if possible.

Do you think if I used the length of coil in the pic ( I intended to cut it down to gunwhale height) and had the expansion pipe rising from the top of the coil and the feed dropping by say four inches to gunwhale height and then horizontal to the back cabin I might get away with it assisted by the pump or am I pushing my luck.  The entire length from boiler to cabin will be insulated.  However I also want to heat a towel rail in the bathroom on the return leg so we shall have to see how that goes.

Bengo- very promising.  Perhaps I'll just get on and install it and make adjustments if/ when required.  Any recommendations on pumps?  The quieter the better in my view

Edited November 7, 2017 by OllyO

[Ex Brummie]

Have you done any calculations to estimate the heat the coil is likely to produce in relation to the heating load? With so much exposed pipe in the coil it is likely to be very inefficient. A solution could be to insulate the coil and sheath it in another tube.

[bizzard]

3 hours ago, OllyO said:

Thank you both for getting back to me on this, it sounds promising.  When we moved onto this boat they had feed and return pipework all running at floor level so it required a pump running 247 when the stove was on which seemed to me mad.

It is good to hear that thermosyphon in less than ideal situations can be achieved at least in part with the assistance of a pump.  Certainly installing a pump on the return side triggered by a thermo switch set at 65 (or thereabouts) on the feed will not be a problem at all.

I have made the coil boiler and I am keen to see what kind of heat it will generate, its a project for a bit of fun really so if it all needs to be scrapped I won't be heart broken but I would certainly like to try and get it to work if possible.

Do you think if I used the length of coil in the pic ( I intended to cut it down to gunwhale height) and had the expansion pipe rising from the top of the coil and the feed dropping by say four inches to gunwhale height and then horizontal to the back cabin I might get away with it assisted by the pump or am I pushing my luck.  The entire length from boiler to cabin will be insulated.  However I also want to heat a towel rail in the bathroom on the return leg so we shall have to see how that goes.

Bengo- very promising.  Perhaps I'll just get on and install it and make adjustments if/ when required.  Any recommendations on pumps?  The quieter the better in my view

Looks like a Kayan woman.

[David Mack]

Having seen that picture I'm not so sure it will thermosyphon very well. Your radiators will have to be mounted quite high - if they are entirely below the bottom of th coil then there won't be any thermosyphon.

And as EB has said that coil may not collect much heat. It should be firmly in contact with the flue pipe over its whole length and ought to be wrapped in insulation. A back boiler would be more effective if one can be fitted to your stove.

[BEngo]

I use a mag drive Johnson. There are several advertisers of much cheaper look alikes on the solar heating sites.

THE BEST pump is a Bolin from Sweden but I think they are no longer producing them.

N