Gravity system unreliable after HW tank change

[Wumpu5]

This system was installed in the 70s - a basic gravity hot-water system fed directly from the boiler.
There's also a pumped heating system, using separate connections from the boiler. This works fine.
No diverter valves, as far as I know. Both systems use different outlets on the boiler.

Boiler is at ground level; hot tank is on first floor, open header tank in the loft.


About 5 years ago I had to have the boiler changed, the plumbers fitted a new Glowworm hideaway FF cast iron boiler.
The plumbers did say I should have 28mm pipes to/from the boiler, but my system only ever had 22mm ones and it would have been a nightmare to swap, involving ripping out wardrobes, floorboards, ceilings etc.
They also fitted an electric valve to the outlet from the HW tank, controlled by a thermostat fitted to the side of the tank.
Anyway, it all worked nicely for a couple of years.


Then the hot water tank coil sprung a leak, so I replaced the tank myself. The new tank is smaller capacity, but the biggest difference was that the top gravity pipe was not as high as the old tank.
Aside from that, the plumbing connections are exactly as per the old tank.
I made sure that all pipes run horizontally, just like the originals.
(no diverting old pipework upwards etc.) to avoid airlocks.

Ever since, we occasionally don't get hot water.
Particularly if somebody runs off all the hot water to have a bath.
It seems the gravity circuit isn't starting when it should.

Feeling the pipes near the boiler, when it's not working it seems that the gravity pipes (both flow and feed) are getting hot at the same time, so the gravity circuit can't start circulating.
The pipes near the tank don't get hot at all.
There are no vents near the tank, but I have tried cracking the top pipe open to vent it, and all I get is water.

I can get it started by putting a hosepipe onto the vent pipe in the loft and blasting water around with the rads turned off. The system then works until somebody runs off all the hot water again.
Blasting water around this way doesn't seem to cause any airbubbles in the open tank, so I don't think I'm shifting airlocks.

The electric motor on the tank valve seems to be operating correctly when I play with the tank thermostat.


Or is it simply a particularly stubborn airlock?
Do I now need a one-way valve in the gravity circuit, due to the height differences in piping to the new tank?
Should I think about fitting a low-powered pump into the gravity circuit?

Have I missed something obvious?

 

[Legion]

Then the hot water tank coil sprung a leak, so I replaced the tank myself. The new tank is smaller capacity.

Cylinders have a tendency to run out of stored hot water, especially smaller ones.

 

[Wumpu5]

******************************************************

Sorry, was that supposed to be a useful comment?

I didn't solve it, the system still stops working when somebody runs a bath.

 

[Legion]

Hot water runs out, only a liddle bit in da cylindeeeer.

 

[Wumpu5]

Yes, that bit's easy.

But why isn't the boiler putting more hot water back into the cylinder?

 

[Wumpu5]

Are you saying that a gravity circuit will only start if there is already hot water at the top of the system i.e. in the tank?

 

[Wumpu5]

No.

So how does using the hot water from the cylinder at the top of the circuit make a difference to the gravity system starting up again?

 

[corgiman]

the copil doesnt heat it up immeadiately and needs time to replace the heat

a grav system can take ages to do that, so when you denude the cylinder of heta expect about 40 mins to elapse afore you can have a hot bath again



legion you are a bugger funny but a bugger

 

[Wumpu5]

Thanks Corgiman.

Now I understand why Legion seems to be at cross-purposes too, which would explain why his posts don't make sense (even ignoring his shining wit).

I'm not getting hot water even the following morning.

The boiler runs from 5:00am to 7:30am every morning; there should be hot water after that.

I'm trying it again now; will run the boiler for a couple of hours.
But I don't think the gravity circuit will start without a visit to the loft.


Oh, and maybe Legion should take a look at a map - there's more to Merseyside than just Liverpool.

 

[Wumpu5]

Just my luck.

First post and I get hit by the resident troll.
http://www.flayme.com/troll/

 

[Legion]

I can get it started by putting a hosepipe onto the vent pipe in the loft and blasting water around with the rads turned off. The system then works until somebody runs off all the hot water again.
Blasting water around this way doesn't seem to cause any airbubbles in the open tank, so I don't think I'm shifting airlocks.

You are.

Or is it simply a particularly stubborn airlock?

Yep., down to Numpty who can`t plumb or does not understand heating system.

 

[Nige F]

This system was installed in the 70s -



, but the biggest difference was that the top gravity pipe was not as high as the old tank.
It seems the gravity circuit isn't starting when it should.



Have I missed something obvious?

wife like to visit Liverpool. we like the Museum there, we also like to visit our hubcaps.

How high was the old gravity pipe Was it near the Rim of the cylinder where the Dome starts...If it was you just swapped an Annulus heat exchanger with a coil ...Obvious to me , If that was the case and would explain why it don`t circulate so well now

 

[Wumpu5]

Nige, I think you have it bang-on. Many thanks.

Yes, the top pipe was up near the top of the tank, just below the dome.

The new one has the top pipe about 1/2 way up the tank.

Now I know what to google for, it definitely looks like it was an annulus system. I still have the old tank in the garden, and the heating component definitely looks like a large tube, rather than a coil.


I'm a bit annoyed because I did ask at the plumber's merchants whether there were different types of tank, and they told me the choice was just direct or indirect (with a coil)


OK, so what would you recommend I do?

I've got a plumber coming to do a boiler service on Friday anyway, but would like to know what my options are.

 

[Nige F]

Nige, I think you have it bang-on. Many thanks.Yes, the top pipe was up near the top of the tank, just below the dome.
I'm a bit annoyed because I did ask at the plumber's merchants whether there were different types of tank, and they told me the choice was just direct or indirect (with a coil).

They would have to be 55+ years old and ex- plumbers to know The merchant`s used to be staffed by people like that. Now it`s gone from a trade/craft to a technology... What I would do. Is if it could be fitted in the layout of the house - I would construct a extra "gravity loop" up into the loft and see if it worked better. Now there is not a 100% promise that it will, but it would be cheaper than converting to pumped Just the sort of job that interests me . But I`m miles away from scouseland Just for a laugh, you could ask the Boiler engineer what he knows about gravity primaries

 

[Wumpu5]

They would have to be 55+ years old and ex- plumbers to know The merchant`s used to be staffed by people like that. Now it`s gone from a trade/craft to a technology... What I would do. Is if it could be fitted in the layout of the house - I would construct a extra "gravity loop" up into the loft and see if it worked better. Now there is not a 100% promise that it will, but it would be cheaper than converting to pumped Just the sort of job that interests me . But I`m miles away from scouseland Just for a laugh, you could ask the Boiler engineer what he knows about gravity primaries

Getting any more pipes up into the loft from the tank wouldn't be that difficult. Getting more pipes right through from the boiler would be nigh-impossible without wrecking most of the house in the process.


What would be involved in going pumped?
I assume it's a bit more complicated than simply sticking a pump inline with the cold return from the tank, controlled by the same thermostat/feed that operates the tank valve.
Or maybe it isn't. I've read of people doing just that.


I don't really want to go closed system unless I have to; I don't think the pipework would stand any extra pressure.