Unvented vs vented cylinder

[ivixor]

So, planning some upgrades to our hot water.

We currently have an old vented cylinder, loft tank, and pump for both stored hot and cold.

Setting aside switching to a combi for now, as we already have the tanks and the pump, it seems like there's no advantage to unvented. In fact there are disadvantages, as the delivery rate from an unvented cylinder would be limited by the mains pressure and flow rate.

Can anyone come up with any reason to go for unvented as opposed to simply upgrading to a larger cylinder?

Cheers

 

[FiremanT]

Provided you have suitable supply, invented is head and shoulders above.
And, depending on circs, IMO better than a combi.

 

[ivixor]

You say that unvented is better than vented + pump, but why?

 

[muggles]

Unvented cylinders tend to have better insulation, and also almost always have bigger coils so they can heat up faster (potentially meaning you can get away with a smaller one). You don't get the noise and potential reliability issues associated with a pumped system either. Get your mains performance tested before making a decision either way

 

[ivixor]

It's been tested at the kitchen tap to about 15L/min. Might be Ok for a single shower but we have two. Don't want have to be yelling down the stairs when someone turns on the tap or flushes the loo!

 

[bernardgreen]

My preference is a vented system with a cold water storage cistern in the loft. Simple and safe, not reliant on over pressure valves and a yearly inspection and check.

There are minor disadvanages with vented systems but in my opinion and experience these are not sufficient reason to commit to an un-vented system if there is space for a cold water storage cistern a few feet higher than the highest tap.

 

[Dan Robinson]

at the kitchen tap to about 15L/min.

It depends on the tap.

 

[bernardgreen]

Are incredibly limited as to be almost meaningless.

That is your opinion, based on your limited knowledge of my experience.

 

[picasso]

I have never taken out an unvented cylinder to install a vented cylinder, unvented is always seen as an upgrade so to recommend a vented system shows more of a personal preference than any practical experience I reckon.

 

[fezster]

You say that unvented is better than vented + pump, but why?

No pumps to break down.
Less noise.
No running cost (though you need to factor in annual service using G3 certified plumber).
Faster recovery time.
Better insulation (IME, hot water stays hot more than a day later).
Mains driven (Potentially could be better than your pump - you need to check your pressure and flow rate. Just the kitchen tap isn't adequate).

 

[bernardgreen]

No pumps to break down.

Gravity never breaks down, a few feet of head from the cold water storage cistern provides good flow for basins and baths.

No running cost (though you need to factor in annual service using G3 certified plumber).

Vented have zero running costs ( other than the heat source )

Faster recovery time.

Recovery time depends on heat source, vented cylinders can have "fast recovery" coils.

Better insulation (IME, hot water stays hot more than a day later).

Vented cylinders can have insulation as good as that on an un-vented cylinder.

Mains driven (Potentially could be better than your pump - you need to check your pressure and flow rate. Just the kitchen tap isn't adequate).

Mains water pressure for high velocity showers is one advantage that the un-vented can offer. The same can be achieved with a vented cylinder with a secondary heat exchanger.

The secondary heat exchanger can be :-

(1) a plate exchanger with a pump to circulate water from cylinder through the plate heat exchanger
(2) a second coil in the top of the vented cylinder, no pump required for this option.

 

[picasso]

Its no wonder dans opinion of your advice is so low, you would actually fit a vented cylinder, plate heat exchanger, pump, flow switch, tank in loft rather than an unvented cylinder ?

 

[bernardgreen]

you would actually fit a vented cylinder, plate heat exchanger, pump, flow switch, tank in loft rather than an unvented cylinder

No..... In my cottage I fitted a vented cylinder with a second coil inside the cylinder for the mains pressure hot supply to the shower. Copious but gentle flow to the basins and bath from the un-pressurised water in the cylinder,

 

[hard-work]

You may have a better cold mains flow rate than you think. Removing the old mains stop tap and installing a quarter turn full-bore stop tap will improve the flow rate. So if you have `ballparked` at 15 litre/min a new full bore stop tap and 22mm pipe from the stop tap to reduce restrictions will improve flow, you may be over 20 litres/min. Then a combi is feasible, but with aerated shower heads. Handsgrohe call them `rainmaker` or `raindance` shower heads. They are used on cruise liners to reduce water usage. They use less water but still give a big shower sensation by adding air to the water flow. They have a `drencher` head that is 9 litres/min.
http://www.hansgrohe.co.uk/articled...cosmart-9-l-min-with-shower-arm-26470000.html

But if you really do have a low cold mains flowrate then keep the cold tank and vented cylinder. Fit a combi and have it supply the showers only, to give high pressures. The combi can also heat the rads and cylinder as per normal. All other outlets, except the cold water in the kitchen will be off the tank in the loft.

What is the probability of two showers being on at the same time? If it is very low then this a good solution. Also, when the cold main pressure is upped in the road in the future, you can remove the cylinder and have all the hot taps off the combi and cold mains. But I would advise keeping the cold tank to supply the toilet(s) and dishwasher and washing machine if possible, in case of a water outage to give a toilet, and divide & rule in water distribution.

 

[stonehouse]

Its no wonder dans opinion of your advice is so low, you would actually fit a vented cylinder, plate heat exchanger, pump, flow switch, tank in loft rather than an unvented cylinder ?

I'm with bernardgreen on this. I don't like the thought of having a pressurised system in my house. What if the relief valve gets stuck?

I also prefer gentle hot-water flow to the basins and bath.

However, I'm not sure about a plate heat exchanger with a traditional cylinder as I think it was intended to be used with thermal stores because according to Patrick Mitchell's book (published in 2008), there is very little change over of water in the store, which means that there is no continuing supply of calcium carbonate to produce lime scale. High temps can be maintained once all the lime from the water has precipitated, no more will arrive. The store cylinder can be readily kept at 70*C or above.

Maybe in areas of soft water (like where I live), it doesn't matter. If bernard says his shower works perfectly, I believe him.

Another thing, perhaps thermal stores are obsolete nowadays because modern boilers (condensing) cannot operate at 70*C, can it?