A question about hot water tanks

kah22

I have recently switched from warming my hot water by oil to electtricity because of the hugh hike in the price of oil.

Anyway I was talking to a friend the other day and he was telling me that he has hot water from his tank within five to ten minutes. I have to leave mine on for a good 40 minuites to get any amount of useable hot water.

On asking a few more questions it seems he has two hot water tanks in his loft: one the large traditional one like I have and the second a much smaller one which gives him that almost instant hot water.

Can anyone give me inforrmation about this (indeed is he correct)? And can you also give me some information on whether or not it is cost effective.

I have the facilites to heat water either by electricity or oil. Gas is not an option in my part of the coutry. My oil is partioned so I can use either hot water or heating or both at once. I can only heat water by electricity.

Cardew

Quite often a HW tank will have two immersion heaters. The main one is 3kW and a smaller one in the top of the tank that heats a small amount of water for quick use.

I have never heard of two HW tanks.

Unless you have an Economy 7 tariff and heat water overnight, I very much doubt if electricity will be cheaper than oil.

Electricity costs around 12p/kWh. Oil costs around 6p/kWh. However you have to factor in the efficiency of the oil boiler which can range from 60% to 90%. So oil would cost between 7p/kWh and 10p/kWh.

If you have a well lagged HW tank the losses are not high. They are tested to a British Standard and with a full tank of water at 65C an average loss is 2kWh in 24 hours.

In practice, your losses would be lower than that, and the heat isn't really 'lost' as it warms the fabric of the house - which is why many tanks are in the airing cupboard.

EricMears

kah22 wrote: »

On asking a few more questions it seems he has two hot water tanks in his loft: one the large traditional one like I have and the second a much smaller one which gives him that almost instant hot water.

Can anyone give me inforrmation about this (indeed is he correct)?

I'd agree with Cardew : it would indeed be unusual to have any hot water tanks (let alone two) in the loft.

Might the friend have misunderstood the purpose of the two loft tanks ? A large one acting as a cold feed header tank for the hot water tank (which would usually be in an airing cupboard) and a small one as header for the central heating system is quite common. In that case, the large one should be clean water and the smaller one should have central heating inhibitor added. Both would usually be black plastic tanks or (in very old systems) galvanised iron. The actual hot water tank is usually made of copper.

If indeed the actual hot water tank is in the airing cupboard, the two stage heating described by Cardew would be fed by two separate electric cables and (if you have off peak metering) the lower one would be fed from offpeak meter and the higher one from the day rate meter.

jimjames

How old is your hot water tank?

We had ours replaced last year and the difference is amazing. We are in a very hard water area and I think the tank had significantly furred up over time. The new one both heats faster and retains heat better than the one that was 20+ years old.

onamissionEU

What your friend may have is an electirc waterheater which turns on when a tap is opened and only heats the water being used (or sometimes with a very small inbuilt tank to store a gallon for washing hands etc). They are usually fitted to individual taps (say, under the kitchen sink) and are not generally used where a room/tap has another source of hot water. They are very common in Europe but not so in the UK & are usually deployed in flats.

rogerblack

EricMears wrote: »

I'd agree with Cardew : it would indeed be unusual to have any hot water tanks (let alone two) in the loft.

Mine is.
This as I understand it will have been especially common where there was gravity fed circulation systems.
Also - a tank in the loft can provide lots better flow unpumped.
In times past, the losses were less important as fuel was rather cheaper.

EricMears

rogerblack wrote: »

Mine is.
This as I understand it will have been especially common where there was gravity fed circulation systems.
Also - a tank in the loft can provide lots better flow unpumped.
In times past, the losses were less important as fuel was rather cheaper.

Usual arrangement would have been to have a hot water cylinder in (say) the bathroom where the waste heat would have been put to some use and a cold water header tank immediately above it in the loft.

Since the cylinder was a sealed system, the pressure in the hot water system is a function only of the height of the (cold)header tank.

It's certainly possible to have a HW cylinder in loft with a cold header tank also in loft (but slightly higher of course) but it's unusual. Usually only done where space in house was considered more important than wasting fuel.

kah22

My own hot water tank was originally in a hotpress situated in the bathroom but was moved when the bath was taken out and a shower installed. House built mid 60's

My friends bunglow, built in the early 70's, originally had a large tank in the loft. He replaced it with the smaller one earlier in the year and it now supplies his instant hot water.

Think I understand now what it is all about.

Kevin

thenudeone

I think people are confusing tanks and cisterns.

A tank is a sealed container with just inlet and outlet pipes.

A cistern is an open-topped rectangular store, usually with a ball or float valve to automatically top it up when the water level lowers.

Traditional plumbing systems always have two cisterns in the loft as explained in post 3

Although heating water with electricity is more expensive than oil, an electric instantaneous heater could be cheaper overall, if water demand is low, because there would be no heat losses due to storage, pipework etc.
When you initially turn the hot tap on and have to waste water until the water gets hot, that gives an idea of how much heat is lost using a stored hot water method.