Header tank for backboiler

[blackrose]

How big do these need to be? Is there some correlation between size of backboiler & header tank?

 

I've got a tank from an old Merc in a scrapyard. I guess it would take about 2 litres. The volume of the backboiler must be about 3 litres and its supplying a 25' (return length) 1" dia pipe run supplying two small 3000btu radiators.

 

I'd rather use a plastic tank where I can instantly see the water level, than an expensive stainless steel tank.

[DHutch]

I dont know the answer to your question. But obvously you are right, the size of the header tank would have to be suitably large for the size of the system. (you could easly work outthe volume of this, adding the backboilers 3litres to the stated size of the radiators, and the volume of the pipe runs)

 

We have a closed loop system, which incorparates a 2liter (air filled rubber menbrine) expantion tank.

 

 

Daniel

[NB Esk]

Hi blackrose, can't really see a problem with your idea. If I could just add a couple o' points. Don't (obviously) fill right up with water, original MAX mark should be fine.

If you are using original pressure cap, you may want to disable this by running an 1/8 drill thro' it. (may save a nasty accident)

 

As to wether it's suitable for the job.........I think it's a case of trial and error. Cheers............

[blackrose]

I dont know the answer to your question. But obvously you are right, the size of the header tank would have to be suitably large for the size of the system. (you could easly work outthe volume of this, adding the backboilers 3litres to the stated size of the radiators, and the volume of the pipe runs)

 

We have a closed loop system, which incorparates a 2liter (air filled rubber menbrine) expantion tank.

Daniel

 

Surely the volume of the header tank doesn't have to equal the volume of the rest of the system? The tank would be enormous!

 

Hi blackrose, can't really see a problem with your idea. If I could just add a couple o' points. Don't (obviously) fill right up with water, original MAX mark should be fine.

If you are using original pressure cap, you may want to disable this by running an 1/8 drill thro' it. (may save a nasty accident)

 

As to wether it's suitable for the job.........I think it's a case of trial and error. Cheers............

 

I'd fill it up to the minimum level in the tank to account for expansion. My system is open to allow gasses to escape but if this leads to humidity in the cabin I might put a hose from the top of the tank to a skin fitting on the roof with some kind of lip fitted about an inch higher to stop water running in and the whole lot covered a mushroom vent.

Edited August 6, 2006 by blackrose

[timboharticus]

Hi,

 

If you were to allow 15% of the system volume for expansion, that should be more than adequate.

(10% would probably be ok)

 

 

I don't see why you can't maintain the system under pressure using the pressure cap. This would raise the temp in the pipes and stop steam forming which would prevent heat transfer. Need to make sure there is enough air in the pressure tank to allow for expansion!

[John Orentas]

If a 'closed loop' system means it is un-vented I would say that is a very dodgy thing to do, far better to allow the system to vent to atmosphere with the air space in the tank only to take up expansion of the coolant.

 

Fit the tank as high as reasonably possible with a largish pipe connecting to the highest point of the system, a couple of pints capacity should be enough for the largest system leaving it half empty when cold.

Edited August 6, 2006 by John Orentas

[blackrose]

I don't see why you can't maintain the system under pressure using the pressure cap. This would raise the temp in the pipes and stop steam forming which would prevent heat transfer. Need to make sure there is enough air in the pressure tank to allow for expansion!

 

I think the reason most people don't do this is because that as a one-off self-built system which hasn't been properly tested it's quite difficult to know what pressure to keep the system at, and if you get it wrong it could be disasterous - the stove itself produces some extreme variations in temperature after all.

 

All the systems I've seen have been open. Steam leaves via the header tank which is at the highest point so it's self-bleeding. There's really no advantage to a pressurised system. The pipes get very hot without being pressurised & I also think pressurisation may negatively affect thermocycling (there is no pump on my system).

Edited August 6, 2006 by blackrose

[DHutch]

Surely the volume of the header tank doesn't have to equal the volume of the rest of the system? The tank would be enormous!

No, not at all. But a larger system obvously needs larger expantion tank.

- I dont know how much, but i would go along with timboharticus's 10-15% figure in the lack of any further information.

 

If a 'closed loop' system means it is un-vented I would say that is a very dodgy thing to do, far better to allow the system to vent to atmosphere...

Yes, closed loop, unvented system.

- Just a short lenght of pipe extending about 10inchs above the highest point of the system with a bleedpoint, the expantion tank to accomodate the expantion of the water, and a PRV near the calorfier.

 

Whats "dodgy" about that? Thats exactly what every single car has! (bar the odd 2cv and other aircooled vehicles)

 

 

 

Daniel

[ChrisPy]

I bought the smallest float operated cistern not much bigger than a toolbox (easily fits in a small wall cupboard) from screwfix for about a tenner, complete. saves designing something from scratch. and always replenishes automatically if the water level drops for any reason.

 

the tank is remote from the boiler, so I also installed a local vent to allow the system to blow locally if there are bubbles of steam in the system. it is a 'boiler' after all. the vent is higher than the header tank cistern.

Edited August 7, 2006 by chris polley

[Sir Nibble]

I bought the smallest float operated cistern not much bigger than a toolbox (easily fits in a small wall cupboard) from screwfix for about a tenner, complete. saves designing something from scratch.

I like that,

but I have a little metal jug painted with roses sitting on a shelf with a plastic sight level tube up one side and the pipe to the system coming out under the shelf.

[dor]

How big do these need to be? Is there some correlation between size of backboiler & header tank?

 

THe Aldes have their own header tank built in. It holds about a litre, and in my system - 4 rads & calorifier coil, the levle doesn't change more than a few centimetres between cold and very hot.

 

I don't think you need a big header tank.

[ChrisPy]

I like that,

but I have a little metal jug painted with roses sitting on a shelf with a plastic sight level tube up one side and the pipe to the system coming out under the shelf.

poser !!! pity the sight glass is plastic though

Edited August 7, 2006 by chris polley

[Sir Nibble]

[

glass is plastic

[ChrisPy]

[

hmmmmm...... yes, well........ on reflection - not worthy of comment

[NB Esk]

I don't see why you can't maintain the system under pressure using the pressure cap. This would raise the temp in the pipes and stop steam forming which would prevent heat transfer. Need to make sure there is enough air in the pressure tank to allow for expansion!

 

No technical reason why you can't. The reason is that someone who doesn't know what it's all about, may open it........a future owner........a relative or friend of the present owner. Having seen many people scalded and driving one apprentice to the burns unit, I can assure that it does happen..........

[blackrose]

I bought the smallest float operated cistern not much bigger than a toolbox (easily fits in a small wall cupboard) from screwfix for about a tenner, complete. saves designing something from scratch. and always replenishes automatically if the water level drops for any reason.

 

Thanks, I'll have a look for one.

[blackrose]

I bought the smallest float operated cistern not much bigger than a toolbox (easily fits in a small wall cupboard) from screwfix for about a tenner, complete. saves designing something from scratch. and always replenishes automatically if the water level drops for any reason.

 

Trouble is they're designed for cold water. I expect the water in my header tank to get pretty hot.

[DHutch]

Trouble is they're designed for cold water. I expect the water in my header tank to get pretty hot.

Yeah, some may fail. (ones with plastic parts)

- You can get all metal ones, they are used by steam boaters all the time, the one on our boat i subjected to temperature close onto 100c (80 atleast), and presures of upto 200psi, never mised a beat yet, after 15 years. However, its a well known fact that any plastic-containing units usally fail very quickly when used like this.

 

However, its starting to get a bit complicted, isnt it....?

 

 

 

Daniel

[ChrisPy]

Trouble is they're designed for cold water. I expect the water in my header tank to get pretty hot.

why?

 

check the header tank connected to your hot water storage tank in your loft, if you have one. mine is plastic, it's 30years old and the water is stone cold even if we don't draw off any hot water for hours.

 

There is no reason why hot water will circulate into the header tank, the only heat connection is conduction up a small diameter water-filled copper pipe 1metre long that branches off the 28mm circulation pipe. If it does get too hot for plastic then there's a serious loss of heat in the system which needs to be addressed. Probably best to minimise the connecting pipe size though.

[stuart]

why?

 

check the header tank connected to your hot water storage tank in your loft, if you have one. mine is plastic, it's 30years old and the water is stone cold even if we don't draw off any hot water for hours.

 

There is no reason why hot water will circulate into the header tank, the only heat connection is conduction up a small diameter water-filled copper pipe 1metre long that branches off the 28mm circulation pipe. If it does get too hot for plastic then there's a serious loss of heat in the system which needs to be addressed. Probably best to minimise the connecting pipe size though.

 

Exactly the same as mine, the water in the header tank is luke warm - its about 1.5m higher than the rads, connected with 15mm pipe to 22mm rad pipe.

[blackrose]

Yeah, some may fail. (ones with plastic parts)

- You can get all metal ones, they are used by steam boaters all the time, the one on our boat i subjected to temperature close onto 100c (80 atleast), and presures of upto 200psi, never mised a beat yet, after 15 years. However, its a well known fact that any plastic-containing units usally fail very quickly when used like this.

 

However, its starting to get a bit complicted, isnt it....?

Daniel

 

Not really complicated - mine's about the simplest thermocycling system you could build. No pump, not pressurised...

I was going to use a plastic header tank from a car - I'm pretty sure these are made from heat resistant plastics - but the one I've got is only about 1litre vol & might be a bit small. Difficult to find a bigger one.

 

why?

 

check the header tank connected to your hot water storage tank in your loft, if you have one. mine is plastic, it's 30years old and the water is stone cold even if we don't draw off any hot water for hours.

 

There is no reason why hot water will circulate into the header tank, the only heat connection is conduction up a small diameter water-filled copper pipe 1metre long that branches off the 28mm circulation pipe. If it does get too hot for plastic then there's a serious loss of heat in the system which needs to be addressed. Probably best to minimise the connecting pipe size though.

 

I'd have thought any hot water tanks were made of heat resistant plastics. Why do the chandlerys sell stainless steel header tanks if heat isn't an issue? I'm not sure a house system is a good analogy - I don't have a house, but the tank can be a lot higher.

 

Exactly the same as mine, the water in the header tank is luke warm - its about 1.5m higher than the rads, connected with 15mm pipe to 22mm rad pipe.

 

1.5m higher than the rads? Either you've got some very low radiators or a very high roof!

My highers 28mm pipe run comes up under the gunwhale so the bottom of the header tank would only be 2 ft above this.

Edited August 12, 2006 by blackrose