Replacing two hot water cylinders with one

[HRP123]

HI there. I've posted another thread on this forum regarding my high gas consumption. This thread is specifically regarding the awkward hot water cylinder setup I've inherited with the house but am now looking to change.

The house came with two Geminox EBS-1 200 litre hot water tanks which seem to be plumbed in parallel with a secondary hot water circulator. Since living in the house, one tank has always drained first and the second tank later, although the pipework shows them both feeding the same hot water pipe going throughout the house.

The plumber some time ago installed balancing hot watert flow restrictors on the tanks, suggesting that this would help to balance them out. It did not work as if I close both restrictors fully, the hot water flow reduces to a trickle. If I open only the slow tank, it stays at a trickle. If I close the slow tank and open the fast tank, it flows normally. However as mentioned occasionally after a lot of people showering, if I check the tanks, even the slow one has fallen maybe 10-20 degrees (the fast one will have fallen 40 degrees).

Perhaps there is an issue inside the tank, but I think its time to change the system rather than investing further in a bad one.

Being a large house with multiple bathrooms, it does happen that different kids / adults may have showers at similar times and in theory be using 300 litres of hot water for 3 x showers within a short space of time. So far we have never run short of hot water - although I cannot tell whether my capacity is the one 200 litre working tank, or the second slow tank is kicking in after the fact.

I am looking to replace these two tanks (now around 15 years old) and put one tank instead. This will eliminate the confusing unbalanced drawdown problem and ensure the boiler is only heating what is required, rather than filling the slow drawing tank because of heat loss and occasional (and unexplained) drawdowns.

I've seen the the heat loss on the old tanks is 2.331kwh/24h according to the tech specs so x 2 tanks is 1,679kwh/year.

The Megaflo Eco Solar PV Ready tank I was looking at says 617kwh/year. So there is a saving already.

The reason I am looking at the Megaflo Eco Solar PV Ready one, as I already have a solar array which has surplus energy to my battery storage some days, so this could boost the tank.

Questions

1. Anyone have experience of this specific tank? I know Megaflo has brand premium attached, but I'm looking for a tank that will cause the least problems for the next 10 years for heavy usage and this also comes with the iboost kit integrated.

2. The current tanks are on the third floor of the house on a timber intermediate floor. Assuming we strengthen the base under the new tank (more concentrated load from one tank than two), plumbing into the same pipework would be easier than plumbing the new tank on the ground floor near the garage and rerouting pipework. Any other considerations to think about having the tank on the third floor as opposed to putting it in the garage?

3. I'll be replacing the hot water brass circulator with a new one - the shop recommended the Grundfos Alpha version of the same product because of the Auto Adapt function. Any opinions on this? I know a timer can be connected to the secondary pump (I have this already but it is on all day as our pattern of usage is unpredictable) or PIR sensors (difficult to wire into all bathrooms / wireless ones won't work because of the second floor/block wall construction) ... so does the Auto Adapt actually help if the pump is on all daytime? It costs more than the standard brass pump (which we currently have set at speed setting one).

Thanks a lot for taking the time to read and for your replies in advance.

 

[oldbutnotdead]

2 cylinders feeding the same circulation pipe is a very odd way to go.
Especially in a larger house, having an additional cylinder local to the bathroom being served (so losing the running costs of secondary circulation) sounds sensible. Your setup sounds Odd- one large cylinder will be thermally more efficient than 2 small cylinders (surface area/volume ratio is better).
With your setup, it sounds as if 1 cylinder is taking most of the load and not really sharing the load properly with the other cylinder (how you'd implement a system like that unless the tanks are adjacent I have no idea). The second cylinder is thus being an energy sink (you are warming it and not using the hot water).
Have you tried running the house on only 1 cylinder?
And replacement- keep the cylinder near the heavy use points (bathrooms/showers) unless floor reinforcement would be prohibitive.

 

[Bod ...]

I've fitted the auto adapt grundfos with learning mode. Had no complaints.
Very important for long runs and bathrooms not used often so essential set up.

Do try to insulate the hot and the return in its entirety.

 

[HRP123]

2 cylinders feeding the same circulation pipe is a very odd way to go.
Especially in a larger house, having an additional cylinder local to the bathroom being served (so losing the running costs of secondary circulation) sounds sensible. Your setup sounds Odd- one large cylinder will be thermally more efficient than 2 small cylinders (surface area/volume ratio is better).
With your setup, it sounds as if 1 cylinder is taking most of the load and not really sharing the load properly with the other cylinder (how you'd implement a system like that unless the tanks are adjacent I have no idea). The second cylinder is thus being an energy sink (you are warming it and not using the hot water).
Have you tried running the house on only 1 cylinder?
And replacement- keep the cylinder near the heavy use points (bathrooms/showers) unless floor reinforcement would be prohibitive.

Thanks for your reply, yes it is odd to have two cylinders - I think it could have been because of where they fitted them in the loft eaves, even thought they could have moved them slightly and had a taller one.

I really cannot understand why the cylinders do not drain evenly as they both feed the same hot water pipe going out through the house. The pipework for one cylinder is perhaps about 0.5 metres further from that pipe that the cylinder which drains quicker, so not sure if this is the issue.

I am doing a series of tests to work out if I can either live with the one cylinder, or if I can force the slow draining cylinder to come back on. Thanks for your help!

 

[fixitflav]

The plumber some time ago installed balancing hot watert flow restrictors on the tanks, suggesting that this would help to balance them out. It did not work as if I close both restrictors fully, the hot water flow reduces to a trickle. If I open only the slow tank, it stays at a trickle. If I close the slow tank and open the fast tank, it flows normally. However as mentioned occasionally after a lot of people showering, if I check the tanks, even the slow one has fallen maybe 10-20 degrees (the fast one will have fallen 40 degrees).

You say

If I open only the slow tank, it stays at a trickle

Sounds to me like there is a restriction somewhere in the slow tank circuit (apart from the added restrictor).
The principle of putting a restrictor in each path is sound. The idea is to estimate the difference in headloss of each stream, at a given flow, and make the restrictor loss a few times greater, depending how accurate you want the flow sharing. Starting from scratch on an industrial job you might be able to increase the total pressure drop to allow for the restriction, but clearly here you're stuck with the mains pressure. But I'd be surprised if you can't achieve something good enough to avoid taking the tanks out.

 

[Johntheo5]

Has each cylinder got its own expansion vessel?. and have they got separate PRV sets?

 

[HRP123]

Has each cylinder got its own expansion vessel?. and have they got separate PRV sets?

Yes to both - each has a vessel and PRV

 

[Johntheo5]

The plumber some time ago installed balancing hot watert flow restrictors on the tanks, suggesting that this would help to balance them out. It did not work as if I close both restrictors fully, the hot water flow reduces to a trickle. If I open only the slow tank, it stays at a trickle. If I close the slow tank and open the fast tank, it flows normally. However as mentioned occasionally after a lot of people showering, if I check the tanks, even the slow one has fallen maybe 10-20 degrees (the fast one will have fallen 40 degrees).

What are these restrictors?, are they lever or gate valves?
If you shut the cold water supply to the fast cylinder and open up the restrictor to the slow cylinder does the flow increase from the slow one?.

 

[HRP123]

What are these restrictors?, are they lever or gate valves?
If you shut the cold water supply to the fast cylinder and open up the restrictor to the slow cylinder does the flow increase from the slow one?.

Yes so I tried that - but when I shut both restrictors (gate valves) the hot flow slows to a trickle - when I open the slow cylinder, it stays at a trickle. When I open the fast cylinder, the hot water flows out like normal. I did not shut the cold inlets though as I figured the tank was already full so it would not make a difference right?

 

[fixitflav]

when I open the slow cylinder, it stays at a trickle.

Doesn't that suggest there's an additional restriction elsewhere in the slow cylinder circuit?

 

[HRP123]

I've also noticed that the plumber fitted both diverter valves which are connected to the thermostats on each tank on the return to the boiler from the cylinder. Each cylinder has a diverter valve on its return, so one cylinder can be open. However this means that if its on the return to the boiler, the hot water is still circulating into the coil of both cylinders, even if just one cylinder is calling for heat?

Also any advice on what sort of pump should be installed to get from the boiler which is in the garage up to the loft which is 2.5 floors up so lets say 6-7 metres higher? The current pump is a Grundfos Alpha 15-50

 

[Johntheo5]

Where are these restrictors installed, on the hot outlets or on the cold feeds and if on the cold feeds are they before the PRVs?, a little sketch wold help.

 

[HRP123]

Doesn't that suggest there's an additional restriction elsewhere in the slow cylinder circuit?

Yes, I'm just confused how I would figure this out. Does it basically mean you should open the lid of the cylinder and look inside?

I guess I'm wondering if it is worth sinking in time and money to get this cylinder operational, or if it is better to just invest in one cylinder to replace them both, with better heat loss characteristics and probably a better / faster reheat time.

Cheers

 

[Johntheo5]

Didn't realise these are vented cylinders if hyou can look inside? Must be unvented if you have exp vessels

 

[HRP123]

So I managed to find the manual for this range of cylinders. The gate valve has been put in number 2 which is the domestic hot water outlet.