Replacement Hot Water Cylinders

[SimonDen]

Hi All,

Please forgive my basic knowledge, I will try and explain as much as possible.

I have a flat that currently has a hot water cylinder. Now there is no gas in the flat (electric only) and the cylinder appears to be a combi type (with the header tank in the top half of the cylinder) with an electric immersion. I understand that this is a direct type (as there is no boiler, just an immersion) and is vented.

The flat is currently being renovated and I have a few problems with the current tank as follows:

a) It is in a cupboard in one of the bedrooms, as it is a top floor flat with a humungous loft space above, i hope to free up some space (and get rid of the ugly cupboard) by moving the hot water cylinder to the loft

b) The current pressure is not enough for the new waterfall taps that i have bought for the bath which appear to need higher pressure.

c) Very few things require a hot water feed as the heating is electric underfloor, the dishwasher/washing machine are cold feed only, electric shower. The only hot water feeds are the kitchen sink, the bathroom sink and the bath. Due to the weird bathroom size/shape it has a custom bath that is 300litres!

My idea is to move remove the current cylinder and put in the loft a header tank, feeding a 90/100 litre cylinder (with immersion), feeding a 50/60 litre cylinder (with immersion) in series which then feeds the taps. the idea behind this is that the 50/60 litre tank will be the normally on cylinder for the bathroom/kitchen sink taps and then when a bath is required the 90/100 litre cylinder immersion can be turned on to give 150 litres for the bath.

My questions are:

a) Will moving the cylinder up in the loft (about 1 metre higher than current) give it enough pressure?

b) will the idea of putting 2 cylinders in series work?

c) If b) above will work, is it OK for the cylinders to be the same height?

Thanks in advance.

 

[Dan Robinson]

Scrap all that and buy a thermal store such as a Gledhill.

Then you get mains pressure hot water.

 

[Agile]

I think that before buying totally over the top taps and bath that you should have contacted a competent plumber!

How will 150 litres of hot water fill your 300 litre bath?

Have you considered the cost of buying and heating all this water?

Will your ceiling carry the weight of a 300 litre + cold water tank?

How are you going to raise this 300 litre tank at least one metre above the loft based cylinder?

 

[SimonDen]

Scrap all that and buy a thermal store such as a Gledhill.

Then you get mains pressure hot water.

Thanks - does that work off electricity? I can't use other sources without the freeholders permission. It also needs to be able to be cheap to run as the flat is in a very high unemployment/low income area.

I think that before buying totally over the top taps and bath that you should have contacted a competent plumber!

How will 150 litres of hot water fill your 300 litre bath?

Have you considered the cost of buying and heating all this water?

Will your ceiling carry the weight of a 300 litre + cold water tank?

How are you going to raise this 300 litre tank at least one metre above the loft based cylinder?

The building is an old Victorian hotel. In the loft there are still the old water tanks from before the hotel was converted in the early 70's, 6 of them each are around 8 feet long and 2 feet wide, so while I assume the roof is strong enough I will of course get a structural engineer to check it before they are installed.

I am a bit confused about your comments re the bath. My understanding is that a) a bath will not be filled to the overflow, b) displacement of the body will reduce the water requirements further and c) baths are not totally hot water, but mixed with cold. My main residence has an extremely oversized bath (approx 350 litres) and my 170l cylinder allows for 1.5 baths.

I am also confused about the need for a 300l cold water tank! there is currently no (used) cold water tank as it comes off of the mains. Why would I need a 300l cold water tank?

 

[Dan Robinson]

Yes they do an electric one.

Running costs are irrelevant.

Electricity costs what it water, and the specific heat capacity of water is the same regardless of what you use to heat it.

A well insulated tank will not cost much more to run than a small one.

 

[Agile]

Hi All,

Now there is no gas in the flat (electric only) and the cylinder appears to be a combi type (with the header tank in the top half of the cylinder) with an electric immersion. I understand that this is a direct type (as there is no boiler, just an immersion) and is vented.

My idea is to move remove the current cylinder and put in the loft a header tank, feeding a 90/100 litre cylinder (with immersion), feeding a 50/60 litre cylinder (with immersion) in series which then feeds the taps.

Thanks in advance.

It is you who said your existing cylinder is a combi type with built in header tank.

You also said you will have a new header tank in the loft!

So I don't understand how you expect to provide the 300 litres of water now without a loft tank which will need to be at least 1 metre above the top of the cylinder!

Tony

 

[Armo74]

I hope you have also notified the correct people that you plan on installing a bath over 230litres.

 

[swbjackson]

Why not just have a dual element unvented cylinder fitted?

 

[SimonDen]

It is you who said your existing cylinder is a combi type with built in header tank.

You also said you will have a new header tank in the loft!

So I don't understand how you expect to provide the 300 litres of water now without a loft tank which will need to be at least 1 metre above the top of the cylinder!

Tony

Thanks for youre response, however I am still confused. I am not a plumber (or even someone that attempts plumbing) so please forgive my ignorance

I understood that the header tank is only a small tank above the cylinder to reduce mains pressure down to gravity pressure and that the tank size would only need to be the difference between outlet flow rate and inlet flow rate, so for example if inlet flow rate was say 13litres per minute and outlet was say 15 litres per minute then the difference is 2 litres per minute so over 10 minutes it would need minimum 20 litres plus enough to allow the tank time to refill so say 30l.

If i really do need a 300l header tank then please can you explain as I will need to stop the guy going up tomorrow to remove the wooden wardrobe (as I will not be going ahead).

I hope you have also notified the correct people that you plan on installing a bath over 230litres.

Thank you - Can you point me in the direction of the regulations for this so I can check with the person who installed the bath. I am a stickler for ensuring correct notifications/approvals. The manual for the bath says 'capacity 300 litres' but i assume this is to the top as the reseller had to drill the overflow hole, maybe it is less than 230 litres to the overflow. FYI - the bath is a Reva industres BT602 corner bath

Why not just have a dual element unvented cylinder fitted?

Thanks for your reply, I understand that unvented cylinders need holes in the wall (which I am not allowed to do) and also require regular maintenence. I suspect my idea (if feasible) would be cheaper to run than a dual element tank as i suspect day to day hot water usage would be very little. The area the flat is in is a very deprived area and I would rather pay a few thousand now to save the tennants a few pound per month in bills. I was originlly looking at the Heatra Sadia Megalife HE120 however it was the dual elements that put me off (and the heat up time if ony a singel element is used).

nb: As previously mentioned I am no expert, so where I am wrong, please tell me I am wrong, i would rather get it right, than go down a path that needs undoing.

Thanks all

 

[Agile]

You are quite correct that the loft tank can be sized to take into account the fill rate.

However, most loft tanks are specified because the mains flow rate is poor. Furthermore, the flow rate can reduce considerably as other cold water in the property is used and by district pressure reductions.

It cannot be assumed that the fill rate is all that is required because other water consumption like flushing WCs and washing machines or garden hoses may well use up or exceed the fill rate while your oversize bath is being filled.

Practically I would usually expect to size the loft tank so that a bath can be filled without assuming any refilling. If it were to run dry then that can ruin shower pumps and create airlock problems quite apart from the inconvenience to the bath user.

Putting cylinders in series can also risk implosion if the refill is not maintained.

Tony

 

[Charnwood]

Fit your existing fortic (combination) cylinder in the loft along with another for the bath, piped to the bath only. Feasible?

 

[SimonDen]

You are quite correct that the loft tank can be sized to take into account the fill rate.

However, most loft tanks are specified because the mains flow rate is poor. Furthermore, the flow rate can reduce considerably as other cold water in the property is used and by district pressure reductions.

It cannot be assumed that the fill rate is all that is required because other water consumption like flushing WCs and washing machines or garden hoses may well use up or exceed the fill rate while your oversize bath is being filled.

Practically I would usually expect to size the loft tank so that a bath can be filled without assuming any refilling. If it were to run dry then that can ruin shower pumps and create airlock problems quite apart from the inconvenience to the bath user.

Putting cylinders in series can also risk implosion if the refill is not maintained.

Tony

Thanks - there is no way that I am risking even the possibility of implosion and I don't want a large header tank - so I will need to look for another solution

Fit your existing fortic (combination) cylinder in the loft along with another for the bath, piped to the bath only. Feasible?

Now that might work (although i will replace the current tank as i don't know how old it is and it looks like it has overflowed in the past) - Is it possible for the smaller cylinder to just feed the 2 sink taps and for both cylinders to feed the bath?

 

[Charnwood]

Is it possible for the smaller cylinder to just feed the 2 sink taps and for both cylinders to feed the bath?

Yes.
The smaller one would probably need seperate outlets, one for the bath, one for whatever else it's feeding, although you could possibly tee off a single outlet.

 

[SimonH2]

OK, a few ideas, and a few comments.

Do you have a dual rate tariff (eg something like what used to be called Economy 7) ? If not, then that will probably make sense and is what dual immersion cylinders are designed for. The idea is that you heat the whole cylinder at night on the low rate, and only use the upper heater during the day if you run out of hot water. As such, it's important that you size the cylinder correctly so that you can heat all the water you need in a day during the previous cheap period. This can make a difference of 75% of the cost of heating the water - but you need to look at your usage and the tariff carefully as the day rate tends to be higher than a single rate tariff. Mind you, we're all suppose to be on variable rate smart meters in the next few years

Now, cylinder options.
"Simplest" is another open vent cylinder. But your header tank needs to be well above it. If there isn't enough head, then when drawing off water, the pressure drop in the pipework from header tank, through cylinder, and as far as where the vent and service pipes separate can be such that you draw air down the vent pipe. You can't block the vent or fit a non-return valve as the cylinder will then "suck in" instead. So normally the header tank goes in the attic, while the cylinder goes in the living space (usually in an airing cupboard).

As mentioned, an unvented cylinder may fit your bill. If your dynamic pressure and flow rates are adequate, then it'll give you mains pressure hot water. It might not need a hole in the outside of the building as I believe there are permissible ways to use a drain (but I'll leave qualified people to correct me on that, or say what the rules/restrictions are).

A thermal store would allow you to stay open vented (and hence avoid the safety drain issue), and would probably be a similar cost to the unvented. You can get models similar to your current setup with a F&E tank integrated on top - and some of these are manual fill so you'd avoid the need for an overflow (I think).
What's your water like ? If the water is hard, then thermal stores are subject to scaling up - this reduces the heat transfer and so your hot water flow rate (for a given temperature) reduces (or conversely, at a given flow rate, the temperature reduces).
I suspect that that immersion heaters in vented and unvented cylinders also scale up - but as they'll still put the same heat in until the element gets so hot that it fails.

Note that with any of these options, since you are electric only, there is little penalty in running them at a higher temperature and tempering down with a TMV. Eg, to give an example - say your mains was at 10˚ and you want 60˚ flow. You can heat the cylinder to 60˚, and what's in there is what you can use. If you heated it to (say) 85˚, but mixed it with cold to get 60˚, you'd use 2/3 hot and 1/3 cold - so effectively you'd get 50% more hot water out of the tap than you have in the cylinder. There will be higher standing losses - but a well lagged tank will keep this down, and if it's in the living space then it contributes to the heating.