Gravity HW not circulating

[PotatoHead]

I recently replaced a indirect cylinder that had split. The old cylinder had a 28mm coil with 35mm pipe running to it via gravity.

When I replaced the cylinder I reduced the 35mm pipe back approx 1/2 a metre from the cylinder into 28mm (including 2 elbows in the flow and 2 in the return).

Over a period of a month the customer has had no HW on 2 occasions. When I've got there the HW circuit had already begun circulating heating up the water.

I'm hoping someone can help sort out the problem. I've made sure there's no air in the circuit. I'm wondering if its because I've reduced down the pipes earlier than the previous cylinder (never had this problem before the change).

Cheers for any help.

 

[DP]

Modern cylinder is for used with pumped primaries- poor performance with gravity circulation.

 

[picasso]

did you replace it with a cylinder suitable for gravity ? very few cylinders sold nowadays are, only on special order I think.

 

[Boilerman2]

Can you post pics of airing cupboard end?

a few years ago (when it was still leagal) I moved a boiler about 6ft in a utillity room to allow for an extension to be built, we reconnected the existing Gravity circuit, had huge problem getting it to circulate, we deduced that air was trapped somewhere in the gravity loop, we tried back filling it worked for a short while and stopped again, in the end we created a boiler overheat (under controlled conditions) to force any air out of the system, after this it all worked OK

Good Luck!

 

[PotatoHead]

I didn't realise there were cylinders specifically for gravity, I just assumed 28mm would be up to the job, Its a 450 x 1200 telford cylinder. I tried checking but couldn't find any details, I noticed a few other cylinders say suitable for gravity or pumped, is there a difference in the coil for gravity ones?

They don't have mains water at the house which makes back filling a bit more awkward.

 

[Goldspoon]

I have fitted lots of bog standard PLUMBFIX cylinders on gravity circuits and never a problem. As long as 28 mm pipe rises it will work. Ignore this as a potential problem. Could be trapped air. Follow circuit is there anywhere air can rise to but not escape? Trapped air stops circulation.

 

[Servotech]

I recently replaced a indirect cylinder that had split. The old cylinder had a 28mm coil with 35mm pipe running to it via gravity.

When I replaced the cylinder I reduced the 35mm pipe back approx 1/2 a metre from the cylinder into 28mm (including 2 elbows in the flow and 2 in the return).

Over a period of a month the customer has had no HW on 2 occasions. When I've got there the HW circuit had already begun circulating heating up the water.

I'm hoping someone can help sort out the problem. I've made sure there's no air in the circuit. I'm wondering if its because I've reduced down the pipes earlier than the previous cylinder (never had this problem before the change).

Cheers for any help.

As Goldspoon says the cylinder you have should work on gravity as long as the primary flow and return pipes rise (level at least) towards the cylinder.
Could the new elbows you've fitted have backfall on them?
Is this SF heating (sounds like it is)?
How much circulating hieght have you got?

 

[DP]

A bogstandard cylinder is made for pumped primaries. These cylinders are fast recovery (often heat in some 30 minutes). Indirect coil is much longer, it may even be multiple strands of microbore pipe sitting like a pancake at the bottom of the cylinder.

Changed a cylinder connected to a backboiler recently. All connections were same for new cylinder (no adjustments heightwise or for plumbing). Circulation was poor. Had to convert to fully pumped.

Cannot see how you can get airlocked 28mm that rises all the way to a cylinder. Can be blocked, but airlocked? Have difficulty visualising that.

 

[penfold46]

we created a boiler overheat (under controlled conditions) to force any air out of the system, after this it all worked OK

Good Luck!

Always best to have your apprentice in the loft when you do that.

 

[Servotech]

How much circulating height have you got?[/quote]


Circulating height is the distance in height between the centre of the flow and return connections on the cylinder and the flow and return on the boiler.
The greater the distance in height between the two the better it will work, if you have very little circulating height it will struggle to work and in some cases not at all.
Gravity circulation as been with us since Roman times and therefore is a proven method of heating water but there are set rules which must be adhered too (a lot have been forgotten by the modern plumber of today).

 

[DP]

Gravity circulation may have given the mother hot water when Adam was a wee boy, but modern times dictate efficient use of fuel. Boiler cycling is to be avoided as is unchecked heating of hot water.

Sensible answer therefore is pumped and controls to peg hot water temperature that does not scald. Boiler thermostats are to control boilers

OP is it not possible to convert the system to pumped HW?

 

[gehngus]

Hi.
The circulation in mine was fine, but i have now added a floor to the bungalow and raised the cylinder to the 1st floor (was in a cupboard on ground floor).

the dump tank is now in the new attic.

so all in all the circulation height has increased from about 5 foot to about 10 foot. i would have thought this to be better.

all the pipes are the same (extended) except the flow and return now include an elbow and slow bend to follow the offset wall.

when 1st tested i noticed it wasn't circulating so allowed water to be discharged from the lowest part of the system. this forced water round the system and all seemed to be well.

the next time was fine with hot water a plenty.

then on the next test it wasn't working again. let it overheated into the dump (which is frightening) but the water is still not circulating.

I have now fitted upper & lower thermostat set at 90 & 50 and am ready to add a pump. but have two questions.

where is best to fit the pump (higher or lower on the system) and is it to circulate water system or into the rads (still to be added).

2. should i sort this out regardless of pump as a precaution if the pump goes faulty the system will still circulate?

i cant see where air would be trapped, but would it be worth draining and refilling?

thanks for any input.