DHW supply - thermal store vs traditional hot water tank

[themilnes]

Hi guys

I hope someone can help me out here as I'm struggling to find an answer. I've very recently had my heating changed from a gas boiler to an electric heat pump and, as part of this, the plumbers have taken out the existing 210 litre thermal store (Nu-Heat EnergyMaster) that supplied the hot water for our heating system and heated our DHW as it passed through, (apologies for the explanation if it's not necessary - just not sure that I'm using the correct terminology). They've replaced it with a traditional hot water tank.

The tank is in our attic and there's restricted space so the plumbers were meant to supply a custom built tank that was created to the max dimensions that we could easily get up there, (ie through the loft hatch), whilst retaining as much capacity as possible. We only realised after they'd installed the tank that it was actually just a slim-line 150 litre tank and we're concerned that this just won't hold enough hot water for us - we're a 4-person household.

We're not hugely impressed with this as it isn't what was ordered and are worried that it'll be less efficient and that we'll run out of hot water. However, they're now claiming that this 150 litre tank can supply more 45 degree water than our old 210 litre thermal store, (which was at 70 degrees with a max flow rate of 26 litres/min to raise cold mains water to 40 degrees). Is this correct? I don't even know where to start to work this out so would hugely appreciate any knowledgeable people helping me out here. If that's right then perhaps we don't need to worry?

Any thoughts?? Thanks in advance...

 

[ericmark]

I don't know how you would work out the thermal store capacity, as no idea at what point the water passing through it would run cold, the cistern slightly easier, it has a set amount of hot water in it, from the hot coil up, but with both systems the water heater is heating water as it is being used, so if filling a bath it will run out of hot water at some point as one is running water faster than it is being heated, but using a shower it may not actually run out of water, I moved from storage tank to instant gas, and for a shower only change was water pressure, but bath took longer to fill.

 

[bernardgreen]

The size of my vented cylinder heat store was "calculated" to be able to supply more than enough hot water for at least 3 five minute showers ( using data from a cylinder manufacturer ).

The original idea was to use a plate exchanger and circulating pump to heat mains pressure water to the shower. The cylinder would also supply low pressure hot water to basins and bath.

The plate exchanger was not used, the complexity of flow switches and having to have power to the circulating pump when showering meant that idea was dropped but only after I had bought the plate exchanger.

The system was re-designed to use a second coil in the top of the cylinder to heat the shower hot water supply, the surface area of this coil is roughly the same as the area of the plate exchanger.

My minor error was overlooking the reduced volume of water in the cylinder due to the second copper coil. Showering time is reduced but still more than adequate for when I have guests.

The system works very well, and the shower works during a power cut.

The cost of the bespoke cylinder with the second coil was less than the cost of a "normal" cylinder and a circulating pump.