Calorifier thermosyphon problem

[baffle]

Hello. I have just plumbed in a calorifier, a conventional panel radiator, and a length of finrad on a narrow boat. These are heated by a boiler I have made and fitted to a woodburning stove-cum-range thing (a La Nordica Rosetta). A description of my boiler and some pictures can be found

here

 

The pipe work is all 22mm copper. I have used a pipebender to put in curves where I can but I have used compression Tees and Elbows elsewhere. The cylinder came from Ernie Francis at DEP cylinders (no longer in business I believe), and is vertical with twin coils. I had it made to my spec a few years ago, and, here's the rub, I didn't specify the types of coil - only that there should be two.

 

I have now fired the stove up, and the boiler works fine, and heats the radiator and finrad effectivley and easily - the thermosyphon works fine and reacts quickly, but the calorifier is a different story and barely heats up. The calorifier, radiator and finrad are all plumbed in parallel, with the calorifier closest to the stove, and the finrad at the end of the system.

 

I have followed a drawing of a thermosyphon system installed by Uxbridge Boat Centre in the ?early 1990s with a permanent vent at the highest point of the system.

 

I had a similar system on my last boat which worked well all round for 15 years liveaboard- cylinder heated quickly and easily. Although the boiler was much bigger in the old system, things were otherwise the same as far as pipe configuration goes (i.e. rises and falls). However from memory the distance between the coil input (high) and output (low) on the cylinder was about 12". On the new cylinder it is 4.5", and it is not circulating well.

If I run the stove hard for a couple of hours, with the radiators shut down, I can force some flow through the cylinder, but the heat transfer into the cylinder is dire - the water might just be tepid after this, and also the stove boiler will be boiling. If I open the radiator valves, the boiler cools down, the radiators get toasty, but all flow through the cylinder stops.

 

Concerned there might be an airlock in the cylinder I did two things. Got me and the family out on the gunwhale and rocked the boat hard to try and shift any air causing a lock. This helped a little but did not help enough. So I then drained down the cylinder, removed the immersion heater and had a look at the coils. The rise in the pipe turn was not alot and I could see it would easily airlock. I tried to get a better rise on the lower coil - which is essentially a single turn of finned pipe with a narrow ID of maybe 16mm. This I did this with a broom handle to push on one side and a big shackle on a piece of rope to pull on the other. It moved a bit.

 

Got stove going again, and with cylinder still open could see that the calorifier coil was starting to steam.

 

Convinced I had sorted out an airlock which was the cause of the poor circulation, I got the stove roaring and planned for a bath. But in essence I got a bit of flow, but the heat was not transferring from the pipe into the tank anything like as well as it should do and I think it is the calorifier coil that is the problem. I like the thermosyphon system too much to want to put a pump in.

 

So it is dawning on me that I need to change the cylinder - either to buy a new one of the same size with as many fittings in the same places as possible, but most importantly with coils that are deep with lots of rise/fall, and big bore. Or to get someone to alter my existing cylinder to change the coils.

 

Anyone got any thoughts on the above and/or ideas on best performing coils designed for use in a thermosyphoning system? How should this be specified to a cylinder maker?

 

thanks in advance

Edited September 29, 2011 by baffle

[Smelly]

The loop in the backboiler tubing will airlock... Or it will fill with steam, either result is the same; no circulation. I wonder about radiation carrying heat past the calorifier.

 

Sorry but that loop; especially the stepped outbound feed looks like a good way to stifle a thermosyphon.That and 22mm pipe which is a bit small, especially over a 4m run to the first cooling point, looks like it's destined to fail.

 

Does it gurgle and fart when it gets hot? All that talk of extra heat transfer sounds scary to me... Hows it vented? Is the header tank stealing all the heat?

Edited September 29, 2011 by Smelly

[Bob18]

So when you run the stove hard you boil the water in the therm-syphon. That means the water isn't circulating beyond the boiler.

So, a few questions - in addition to Smelly's

Are the pipes hot - a few feet from the boiler

Do you have a real rise from the boiler to the furthest point in the system, with no horizontal pipes?

Do the radiators get hot, or do they stay tepid like the cylinder?

[baffle]

Thanks for replies folks. Some answers to questions:

Sorry but that loop; especially the stepped outbound feed looks like a good way to stifle a thermosyphon.That and 22mm pipe which is a bit small, especially over a 4m run to the first cooling point, looks like it's destined to fail.

The loop sits on an angle of perhaps 15degrees with inlet low and output high, so there is a gradual rise throughout the loop. Ive used 22mm on another system and that worked well for donkeys years.

Does it gurgle and fart when it gets hot? All that talk of extra heat transfer sounds scary to me

Yes when it can't circulate and boils. If the radiator valves are open it doesnt boil.

Hows it vented? Is the header tank stealing all the heat?

There is a run of 10mm up to the roof lining from the highest point in system - a permanently open vent.

Are the pipes hot - a few feet from the boiler

Pipes get hot right down to furthest radiator when radiator valves open - it is circulating through the rads, but not the cylinder

Do you have a real rise from the boiler to the furthest point in the system, with no horizontal pipes?

No, I have an initial rise out of boiler of 14" vertically, and then there is a fall down the length of the boat same as the boat. But this does not stop the thermosyphon it works as it did on my last boat. As long as there is the initial rise, that gives the potential.

Do the radiators get hot, or do they stay tepid like the cylinder?

Radiators get toasty hot.

[RobinJ]

...

The pipe work is all 22mm copper. I have used a pipebender to put in curves where I can but I have used compression Tees and Elbows elsewhere. The cylinder came from Ernie Francis at DEP cylinders (no longer in business I believe), and is vertical with twin coils. I had it made to my spec a few years ago, and, here's the rub, I didn't specify the types of coil - only that there should be two.

...

If the feed to the calorifier is quite high and the first from the boiler and it is vertical, will air and steam not collect around the inlet at the top? Is there a pressure relief or bleed valve at that point?

Depending on the size of the tee off, is the pipe at the calorifier getting to the same steamy temperature as the radiators?

[Evo]

It doesn't make sense...you cant have good circulation with 'toasty hot' radiators but a cold calorifier. The calorifier is just another radiator really. I suspect your plumbing.

[JohnB]

We have a calorifier from DEP, made after the business changed hands; after I installed it we could not get the water to heat from the Squirel stove though like you the radiators did work, as did the engine coil.

I will leave out the detail but the issue is the ciculating head on the thermo siphon system is not great enough to work through the greater resistance caused by the snall bore and convolutions in the exchanger coils.

I installed a pump, only on the calorifier part, which after a first run cleared the small air bubbles trapped on the coils and the system has worked fine ever since without the pump( it does need to be run if the back boiler cicuit is refilled).Please ask if you need more explanation....

 

John & Kathy

nb Sirius

[Evo]

So you are saying its not bled properly...?

[ditchcrawler]

We have a calorifier from DEP, made after the business changed hands; after I installed it we could not get the water to heat from the Squirel stove though like you the radiators did work, as did the engine coil.

I will leave out the detail but the issue is the ciculating head on the thermo siphon system is not great enough to work through the greater resistance caused by the snall bore and convolutions in the exchanger coils.

I installed a pump, only on the calorifier part, which after a first run cleared the small air bubbles trapped on the coils and the system has worked fine ever since without the pump( it does need to be run if the back boiler cicuit is refilled).Please ask if you need more explanation....

 

John & Kathy

nb Sirius

OK don't start shouting, I have a Dickinson diesel cooker and I was told to make sure the calorifier had large bore coils in it for thermosyphoning

[smileypete]

Anyone got any thoughts on the above and/or ideas on best performing coils designed for use in a thermosyphoning system? How should this be specified to a cylinder maker?

To get a thermosyphon you need a column of hot water acting against a column of cooler water. Now, the taller the columns and bigger the temp difference the better the thermosyphon will be.

 

I suspect in your case the back boiler and cal. coil are quite 'squat' and there's not a lot of height difference between. So it may well help if you can raise the calorifier even a foot or two.

 

Also there MUST be NO airlocks, to prevent these it might be worthwhile to fit a bottle vent at any high point between back boiler and calorifier.

 

cheers,

Pete.

[Bob18]

Two rules

Nowhere for airlocks to form

Hot cycle up hill

 

 

Oops

Three rules

Nowhere for airlocks to form

Hot cycle up hill

All pipework LARGE bore, most of the time, on small rise systems, 22mm is far too small...

 

 

Oops

Four rules

Nowhere for airlocks to form

Hot cycle up hill

All pipework LARGE bore, most of the time, on small rise systems, 22mm is far too small...

No sharp bends - compression type bend fittings count as very sharp bends...

 

 

(Any more rules anyone?)

[WotEver]

(Any more rules anyone?)

Buy a pump, it's simpler?

[bottle]

I ask this question.

 

Do the radiators on a thermo-syphon system need to be balanced?

[smileypete]

I ask this question.

 

Do the radiators on a thermo-syphon system need to be balanced?

Can be, as long as the rads don't get dangerously hot or the system boils when the fire is on full chat.

 

(Any more rules anyone?)

Proper feed pipe from header tank to low point in system.

Proper wide bore vent from high point above back boiler, pointing into header tank.

NO valves or restrictions between back boiler and vent pipe exit.

System must run unpressurised at all times and any circumstances

 

cheers,

Pete.

Edited September 30, 2011 by smileypete

[baffle]

Thanks for all these very useful thoughts and observations....

 

So perhaps before resorting to pump/redesigned coils, I need to find a method of filling my system and bleeding it, when the convoluted coil in the cylinder is very prone to getting an airlock.

 

OK, so now I know what bottle vent is, but how would this help get air out of the coil itself? I do believe that in my system because there is a permanent vent at the highest point that air in the system that is immediately outside the coil will bleed out. It is the coil itself that is the problem, and the problem is how to get the air out of it without a pump.

 

thanks again

Edited September 30, 2011 by baffle

[baffle]

We have a calorifier from DEP, made after the business changed hands; after I installed it we could not get the water to heat from the Squirel stove though like you the radiators did work, as did the engine coil.

I will leave out the detail but the issue is the ciculating head on the thermo siphon system is not great enough to work through the greater resistance caused by the snall bore and convolutions in the exchanger coils.

I installed a pump, only on the calorifier part, which after a first run cleared the small air bubbles trapped on the coils and the system has worked fine ever since without the pump( it does need to be run if the back boiler cicuit is refilled).Please ask if you need more explanation....

 

John & Kathy

nb Sirius

 

This is of course very useful for me to know John, and gives me the incentive to persevere for a little longer yet, and maybe having to go for a pump ultimately. What sort of pump did you go for, and out of interest how quick does your cylinder heat up in the winter? I was used to the tank on our old boat heating up fully in perhaps 2 hours in the cold of winter i.e. starting off at a few degrees C.

Thanks for your help

[bottle]

After re-reading the original post, this

 

 

If I run the stove hard for a couple of hours, with the radiators shut down, I can force some flow through the cylinder, but the heat transfer into the cylinder is dire - the water might just be tepid after this, and also the stove boiler will be boiling. If I open the radiator valves, the boiler cools down, the radiators get toasty, but all flow through the cylinder stops.

 

 

seems to me to indicate that the system has a design fault, the water is taking the least line of resistance, hence no flow to the calorifier.

 

Hence my question about balancing the rads but I now do not think that balancing them will solve the problem.

[smileypete]

So perhaps before resorting to pump/redesigned coils, I need to find a method of filling my system and bleeding it, when the convoluted coil in the cylinder is very prone to getting an airlock.

Fit a couple of valves next to the lower end of the calorifier coil such that the system can be filled through the lower end of the calorifier coil to the upper end, pushing air out.

 

Also would be useful to get hold of a 'water level', to find out what the vertical distance between backboiler lower inlet and calorifier upper inlet actually is.

 

cheers,

Pete.

Edited September 30, 2011 by smileypete

[JohnB]

I didn't have time yesterday to go into more detail as we were setting off with another boat to continue along tht K&A. So here goes....

 

The essence off the matter, as I confimed with the chap who used to run DAP, is to get a thermosyphon to work with a low forcing pressure needs very low flow resistance. When I specified the cylinder I asked for large bore coils but as the outfit now trading as DAP din't seem to have coped with the importance of all this and had only fitted large bore connections. I only discovered this after several, pointless, attempts at reorganising the pipe runs and cylinder height The detail of this is all a bit tedious so I'll leave that out and hopefully answer the points you ask about.

 

One must as you and others have noted have a route for air to escape the system the principle of ALL RUNS RISING to an air escape point be it bleed valve or opening is essential, BUT, small air bubbles trapped on the surface of a pipe are very sticky and do not flow to the outlet but restrict the bore even more.Hence these circulation problems. It seems that these best practice princples are nowadays not so vital because of the brute force design of modern systems an so have been forgotten (no I'm not a grumpy old school plumber but I have had to track down some very old textbooks and you have to seriously respect those bloke that could make things work and systems balance)

 

I used a small Johnson pump,its a bit noisy but I had to slow it down and it was quiet enough then. It would pull heat from the stove fast enough to put out the fire lighting and had to be turned off untill the stove was properly alight.

We discovered that the air bubbles had cleared when we had hot water from the taps after a couple of hours even though we had not turned on the pump!

 

The pump now serves to clear the air when the system is filled and after that the set up works well enough but I may try to see if things can be improved this winter.

 

Right I think thats it - ask if I have missed anything

Have to go boating now :-)

 

John & Kathy

[baffle]

Moved header/expansion today from filling hot circuit near top, to filling cold circuit at very bottom. Refilled very slowly and fired up again, but the story is the same. The radiators heat up quickly and easily, but the cylinder does not. So despite all the rules that I am breaking (pipework runs down hill after initial rise to partway up cabin side, 22mm pipe, compression fittings etc) the thermosyphon *is* working. What is not working is getting the fluid through the thin bore flat spiral coil at the bottom of the tank. It is the coil which is at fault, not my plumbing.

 

Also would be useful to get hold of a 'water level', to find out what the vertical distance between backboiler lower inlet and calorifier upper inlet actually is

The boiler inlet is 28" from the boat floor, and the calorifier upper inlet 10". The ports are 4m apart, and I reckon there is up to 1"/metre fall down the boat. So in real terms the boiler inlet is 22" (18" + 4") above the calorifier inlet. The radiator inlets will be lower than the boiler inlet, but only by a few inches.

 

One must as you and others have noted have a route for air to escape the system the principle of ALL RUNS RISING to an air escape point be it bleed valve or opening is essential, BUT, small air bubbles trapped on the surface of a pipe are very sticky and do not flow to the outlet but restrict the bore even more.Hence these circulation problems

 

Following from this, I will try to refill the system tomorrow with a pressured hose and have an outlet draining out of the boat to really bleed it through, and I will then relight and see what happens. If this fails, I will take the cylinder out, and get bigger bore coils with a big rise/fall put in to replace them. Andy at Newark Cylinders has offered to do this. Although I can see that a pump might sort it, I am going to stick with thermosyphon as it is so elegant, and I know that a longer big bore coil in the cylinder will be better at heating the water than the skimpy thing that is in there now.

 

These forums are brill for sharing problems like these and getting really useful pointers and advice (and the rest of it). Thanks again for everyone's help, and I'll keep you posted.

[baffle]

Have now flushed system through with pumped water to bleed out any residual air, but the story is the same. So cylinder to come out, and taller big bore coil to be fitted.

[smileypete]

The boiler inlet is 28" from the boat floor, and the calorifier upper inlet 10". The ports are 4m apart, and I reckon there is up to 1"/metre fall down the boat. So in real terms the boiler inlet is 22" (18" + 4") above the calorifier inlet. The radiator inlets will be lower than the boiler inlet, but only by a few inches.

If I'm reading this right, the back boiler is above the calorifier ?!?!?

 

Might help to have a rough diagram showing the relative heights of back boiler and calorifier inlets and outlets. It does sound as though the back boiler is towards the front of the boat and the calorifier 4m towards the stern.

 

cheers,

Pete.

Edited October 2, 2011 by smileypete

[baffle]

Hi Folks, here is an update. I perservered with the thermosiphon as I am still unsure that what you are trying to heat, has to be above the boiler. This is quote from here GreenBuildingForum

 

Thremosyphon is quite a strong force if you follow a few simple rules. Pipe work should be larger than for pumped systems and elbows and tees should preferably be swept. Pipes should also go up until they go down and then down until they go up i.e. no horizontal runs, at the most gentle rises or falls. If you consider the circuit as a circle the heat source can be anywhere except at the top but the closer it is to the bottom the better. And given that heat rises it depends on which side of the circle you put the heat source as to the direction of flow. There are lots of ‘rules of thumb’ about gravity circuits but they are just that, general guidance, without seeing the exact location it is difficult to say how it might or might not work.

 

and also from Dutch Barge Forum

 

Depending on layout, the flow pipe can also raise vertically as high as possible and then fall to the radiators and cylinder, this is essential if the boiler is not below the level of the cylinder/rads. The vent and f/e tank should be at this highest point and all pipework, boiler, cylinder coil and radiators should readily vent to this point.

 

So I have increased the vertical pipe out of the boiler to 28mm, and replaced all the tees with swept ones (at a price), but the results are still not good enough for the clyinder, the heating results can be seen here

 

 

I used a £10 digital thermometer from Tool Station and stuck it through a plastic blanking cap on the tapping of the spare coil. I then logged the temperatures using Excel. The sharp rise on the blue line, is when I gave up and turned the immesion heater on!

 

So, although slightly improved, this is not good enough - too slow and not hot enough. I now intend to install an ecocirc pump ideally to run the cylinder only, governed by a thermostats on the boiler out pipe, and on the cylinder, with a manual over ride.

 

As the rads are heat leaks, I do not want to put any valves in between them and the boiler. I do need though, to prevent the rad circuit being pumped with cold water when the pump is running: will my swept tees installed above and below the cylinder be sufficient to govern the flows that occur when the pump runs? Are the fluid dynamics of these things very clever - they look quite evolved? I have put together a little piccy to help. This shows the original system with slight modification below the cylinder for the pump. The absolute rises and falls are pretty much accurate.

cheers

 

 

Edited October 27, 2011 by baffle

[baffle]

Multiple reply deleted

Edited October 27, 2011 by baffle

[bag 'o' bones]

We have the same problem with our thermosyphon system fitted by my dad who is a retired plumber. We narrowed the problem down to the calorfier heating coil being too narrow. Solved the problem by fitting a small 12 volt pump to give it a kick.