Hot water problem, please help!!

[fixitflav]

Thanks.

A bit more information might help diagnose the problem

How old is the heating system?
A sketch with rough elevations and pipe sizes would be useful.
It sounds like the problem is on the secondary side, but just to be sure, is the system fully-pumped with 3-port valve or two 2-ports, or pumped rads, gravity HW? Cylinder stat and room-stat? Might even be a Primatic HW cylinder.

 

[HomerJay99]

Sorry for the terse reply. I was rushing to work and didn't have the time to say any more other than acknowledging the reply!

I had a good talk with the plumber a few hours ago and as far as I understand this is the problem.

The main issue is that there are 5 x 20 litre tanks in the 'attic' which are connected together with a narrow diameter pipe and flow into the cylinder. The problem is that when a lot of water is used quickly - shower + bath for example, the water in these tanks don't re-fill quickly enough and just suck air in instead. This may not have been noticable before as the previous owners were an elderly couple but my daughter and partner have 2 young kids and their water usage is a fair bit more.

The tank arrangement in the attic and the lack of space for a bigger tank makes for a difficult, effective solution and given our daughter's limited budget, options are severely limited.

The plumber has already tried several times to flush cold water into the hot system but this has not removed the trapped air in the system. He's not charging us by the hour, just for the original job of getting the HW system working.

I have suggested adding a conventional size tank above the hot water tank instead and using a 'boost' pump to make up for lack of head height. This is being looked into as their water requirements will only increase as the girls get older in any case!

I'm sorry but I don't know if it's fully pumped / 3 port / 2 port etc - I don't know what these terms mean but I do appreciate you guys trying! Given my zero knowledge of plumbing, the best I can do is to try and regurgitate what the plumber tells me but I may (and probably am) losing something in the translation!

 

[Madrab]

Doubt they will be 20L, 20 Gal perhaps? How good is the mains pressure/flow? If anything I'd be looking to put a pump on the mains, if the pressure/flow is rubbish, certainly not the HW. Pumping the HW will only increase the draw on the cisterns and compound the issue. That being said 40Gal is a fair amount of water, they must be going some to run that dry unless the mains pressure and flow to it is really that bad?

If the cisterns are running dry then the mains feed to the cistern needs increased if the mains can handle it. If it's currently 15mm up to the cistern and they are tandem'd then the mains needs to be increased to at least 22mm along with the Ball valve and the comms pipe between the 2 cisterns needs to be 28mm. The solution here is to get more cold water into and through the cisterns quicker. Then if required adding a pump to the mains if it's possible.

 

[fixitflav]

The main issue is that there are 5 x 20 litre tanks in the 'attic' which are connected together with a narrow diameter pipe and flow into the cylinder. The problem is that when a lot of water is used quickly - shower + bath for example, the water in these tanks don't re-fill quickly enough and just suck air in instead.

It might be worth somebody getting the loft and seeing what happens when HW is used. The cold fill doesn't need to keep up with draw-off, but if the tanks empty before, say a bath is filled, it's too slow and maybe the cold fill pipe size could be increased.
If the pipe from the tanks to the HW cylinder is the normal 22mm it should be OK.

This may not have been noticable before as the previous owners were an elderly couple but my daughter and partner have 2 young kids and their water usage is a fair bit more.

I'm sure they'd have noticed waiting 10-15 minutes before anything came out of a hot tap, as in the original post! And that's the odd thing, which pipe and tank sizes discussed don't explain.

 

[HomerJay99]

Thanks guys, lots of valid comments here. I’ll call the plumber later and discuss these with him.

 

[fixitflav]

Doubt they will be 20L, 20 Gal perhaps?

Might need to be 20L because of the hatch size. 5x20L is a bit sparse but maybe in the circs....

If the cisterns are running dry then the mains feed to the cistern needs increased if the mains can handle it. If it's currently 15mm up to the cistern and they are tandem'd then the mains needs to be increased to at least 22mm along with the Ball valve and the comms pipe between the 2 cisterns needs to be 28mm. The solution here is to get more cold water into and through the cisterns quicker. Then if required adding a pump to the mains if it's possible.

None of that explains the 10 - 15 minute delay, that's the strange thing IMO.

 

[Harry Bloomfield]

None of that explains the 10 - 15 minute delay, that's the strange thing IMO.

My two penarth....

If the 5x tank are plumbed like this, then the flow might be slow...

CW In - [ T1] - [T2]-[ T3] - [T4]-[ T5] - to cylinder. The lack of much head, means the flow T4 to T5 means very slow flow, so T5 would run dry very quickly, thus you get a possible airlock.

A better scheme, would be for T5 to have both CW in and the outlet to the cylinder, with T1 to T4 each having their own direct connection to T5.

Star or parallel connected tanks, rather than series connections.

 

[fixitflav]

My two penarth....

If the 5x tank are plumbed like this, then the flow might be slow...

CW In - [ T1] - [T2]-[ T3] - [T4]-[ T5] - to cylinder. The lack of much head, means the flow T4 to T5 means very slow flow, so T5 would run dry very quickly, thus you get a possible airlock.

A better scheme, would be for T5 to have both CW in and the outlet to the cylinder, with T1 to T4 each having their own direct connection to T5.

Star or parallel connected tanks, rather than series connections.

Maybe they are, perhaps the OP will tell us. Aren't these tanks made to be ganged together, for just this restricted access situation?

 

[HomerJay99]

I think they’re joined together in series, like Harry suggested. Our plumber also said that the pipe between is narrow diameter so that’s not helping. a parallel set of connections to T5 would help but at this point, I’m not sure if the water supply could cope with 4 tanks rapidly emptying!

 

[Madrab]

Might need to be 20L because of the hatch size. 5x20L is a bit sparse but maybe in the circs....

Very true, missed the bit about 5 of them doh

 

[Harry Bloomfield]

I think they’re joined together in series, like Harry suggested. Our plumber also said that the pipe between is narrow diameter so that’s not helping. a parallel set of connections to T5 would help but at this point, I’m not sure if the water supply could cope with 4 tanks rapidly emptying!

The four (five) tanks emptying quickly, would make no difference at all to how quickly the water supply would need to replenish them. Nor would it make much difference to the cold flow at the kitchen sink, the sink tap would simply reduce the flow going up to replenish the tanks.

 

[Madrab]

I’m not sure if the water supply could cope with 4 tanks rapidly emptying!

It's about matching what comes in to what goes out, tandem cistern are never connected in parallel, always series, that ensures that the water in each is refreshed regularly, it's a standard. It should always run mains in to one, then run to the other and ongoing to the last one where the outlets are.

mains needs to be surveyed to understand max dynamic flow and then the mains supply to the first cistern and between each needs to be at least the same diameter as the outlet to the cylinder (usually 22mm).

It really isn't rocket science and your plumber should know this if he's got his quals, it's level 2 basic gravity HW design.

 

[Harry Bloomfield]

It's about matching what comes in to what goes out, tandem cistern are never connected in parallel, always series, that ensures that the water in each is refreshed regularly, it's a standard. It should always run mains in to one, then run to the other and ongoing to the last one where the outlets are.

Good point, I did not appreciate!

 

[HomerJay99]

Ok, scrub that then!

 

[Harry Bloomfield]

Our plumber also said that the pipe between is narrow diameter so that’s not helping.

Replacing the narrow pipes, with larger ones, would therefore seem to be the only answer. I would though before doing that, have someone observe what is happening to the water levels in the five tanks, when bath water is drawn, just to confirm that is the issue..