More Solar PV Immersion Questions...

Hobbo2006

Hi All,

Just had a free energy monitor as I signed up to the iplan with Southern Electric. Since it's not compatible with Micro-generation I thought I'd connect it up to the immersion heater to see how much of our electricity bill is made up heating hot water. Well after one "standard" day we use 7.6kWh's out of our total 11-12kWh's. This works out to roughly £350pa!

There are plenty of posts about how to use the excess PV and re-route it to the immersion. Currently I have just timed the immersion for mid-day rather than after bathtime at 7pm-8pm. I know that there will be a couple of devices coming out soon that divert all the excess generated to the existing immersion element and just waiting to buy one of these.

My issue is as follows:
Our toddler has a bath at 6:30pm and then7-8pm is older daughter and then the mrs and I have a shower later.
As soon as the bath starts filling at 6:30pm the cold water starts to re-fill the tank, which brings the temparature down and makes the immersion kick in at 3kW. If we don't heat the water between baths then the water isn't hot enough later on.

Is there a way to "preserve" the hot water in the tank between 6pm and 8:30pm.... ie stop it filling with cold water??

If there is then all the baths would have hot water. The tank would drain down and then fill up after the baths.... we wouldn't need to heat the water at that point and could wait till daylight hours and get some or all of the heating for free.

Any help or pointers you can give would be great!

EricMears

Hobbo2006 wrote: »

Is there a way to "preserve" the hot water in the tank between 6pm and 8:30pm.... ie stop it filling with cold water??

Probably not - sorry !

Hot water cylinders tend to work by having a header tank of cold water. As you open a tap it allows water from the top of the tank along your pipework - driven by the static head of the (cold) header tank. If you don't allow water into the bottom of the tank, nothing will come out of the top.

Several possibilities though :-

You may have your immersion heater high up in the tank in which case you're never going to heat the bottom of the tank properly. If you have an extra 'immersion fitting point' in the bottom of the tank it would be well worth fitting another element there.

If your immersion heater is fitted at the top pointing down, it may be possible to change it for a longer heating element which will heat more of the cylinder.

If you fit a new - and much bigger - tank (preferably incorporating elements at bottom as well as one at top) the hot water would last longer for you.

Hobbo2006

EricMears wrote: »

Probably not - sorry !

Hot water cylinders tend to work by having a header tank of cold water. As you open a tap it allows water from the top of the tank along your pipework - driven by the static head of the (cold) header tank. If you don't allow water into the bottom of the tank, nothing will come out of the top.

Several possibilities though :-

You may have your immersion heater high up in the tank in which case you're never going to heat the bottom of the tank properly. If you have an extra 'immersion fitting point' in the bottom of the tank it would be well worth fitting another element there.

The heater is at the bottom. The cyclinder stat is just over halfway up. The tank is a small one but has enough capacity.... it's just that as soon as the first bath is run the cold water comes into the tank and cools it all down!

If your immersion heater is fitted at the top pointing down, it may be possible to change it for a longer heating element which will heat more of the cylinder.

If you fit a new - and much bigger - tank (preferably incorporating elements at bottom as well as one at top) the hot water would last longer for you.

The heating element is at the bottom. The cyclinder stat is just over halfway up. The tank is a small one but has enough capacity for our uses.... it's just that as soon as the first bath is run the cold water comes into the tank and cools it all down!

Was hoping for a cheaper option if possible but i'll take a look and see what the cost might be.

Kernel_Sanders

Hobbo2006 wrote: »

As soon as the bath starts filling at 6:30pm the cold water starts to re-fill the tank, which brings the temparature down and makes the immersion kick in at 3kW. If we don't heat the water between baths then the water isn't hot enough later on.
Is there a way to "preserve" the hot water in the tank between 6pm and 8:30pm.... ie stop it filling with cold water??!

I too have thought about this and my solution is to thoroughly insulate the header tank, then work out some way of heating its water during the day. You would then turn off the stop tap which supplies it (mine is in the airing cupboard) before drawing the water in the evening.
Safest method would be to use solar thermal to heat the header tank, but I would understand it if you were reluctant to go for this extra expense
when you may already have spare PV at mid-day. So, this leaves the option of using a heating element in some way; not something you would want somebody from Building Regs taking a look at!
Your thoughts on this, anyone?

Cardew

Hobbo2006 wrote: »

Hi All,

Just had a free energy monitor as I signed up to the iplan with Southern Electric. Since it's not compatible with Micro-generation I thought I'd connect it up to the immersion heater to see how much of our electricity bill is made up heating hot water. Well after one "standard" day we use 7.6kWh's out of our total 11-12kWh's. This works out to roughly £350pa!

Is electricity the only method of heating water? Only using a total of 11-12kWh a day suggests you have gas/oil/LPG or you are on economy 7 with storage heating.

It seems to reach a total of £350 you have taken 365 x 7.6kWh and are costing each kWh @12.6p?

Kernel_Sanders

Kernel_Sanders wrote: »

this leaves the option of using a heating element in some way; not something you would want somebody from Building Regs taking a look at!

I've been thinking about this all day, and I've just come up with a better solution that could possibly comply with Building Regs.
Instead of heating the header tank, let it instead feed a second cylinder in the loft. This would be vented, so the immersion heater would have to be at the bottom, which is exactly where you would want it; it is from the bottom that the pipe will emerge in order to feed the airing cupboard cylinder. Crucially, the header tank would have to be raised so that it's water level would be the same as the top of the cylinder, with a horizontal pipe feeding it at the top Thus, the normal locations for entry and egress of water would be reversed.
I think an inspector from Building Control would primarily be concerned that the element in the secondary cylinder could be heated when not submerged, so a solution would need to be found to obviate that possibility.

EricMears

Kernel_Sanders wrote: »

Crucially, the header tank would have to be raised so that it's water level would be the same as the top of the cylinder, with a horizontal pipe feeding it at the top Thus, the normal locations for entry and egress of water would be reversed.

It might be better to have your second cylinder more conventionally arranged with water drawn from its top then fed to 'old' cylinder. Wherever the element is in a tank, the hot water will tend towards the top so extracting water from base won't be using the hottest water. You'll need a few more bends in the pipework to do this.

The header tank always needs to be higher than the top of the next tank - 'level with' isn't really good enough although 'slightly higher' would be fine.

If your old tank is on the floor of an 'airing cupboard' with space above it, best possible place to fit 'new' tank might be at top of same cupboard (but brace yourself for fight with OH !).

Kernel_Sanders

I don't have an OH ATM, but this wouldn't be an issue as I don't see why you would want to lose airing space when the cylinder could easily be accommodated in the loft.
The only reason I am suggesting the venting of the cylinder is that the OP didn't want cold mixing with his heated water as he needs the volume. However, your suggestion may acually be preferable on low generation days, as the smaller volume of heated water would indeed be concentrated at the top of the secondary cylinder.

EricMears

Kernel_Sanders wrote: »

I don't see why you would want to lose airing space when the cylinder could easily be accommodated in the loft.

A lot of people have restricted headroom in the loft and raising a storage tank by several feet isn't always an option.

If a DHW cylinder is well insulated, an 'airing cupboard' really doesn't perform very well. Temperature in the cupboard will be warmer than the room outside but it would probably take a long time to achieve that.

grahamc2003

Hobbo2006 wrote: »

My issue is as follows:
Our toddler has a bath at 6:30pm and then7-8pm is older daughter and then the mrs and I have a shower later.
As soon as the bath starts filling at 6:30pm the cold water starts to re-fill the tank, which brings the temparature down and makes the immersion kick in at 3kW. If we don't heat the water between baths then the water isn't hot enough later on.

Is there a way to "preserve" the hot water in the tank between 6pm and 8:30pm.... ie stop it filling with cold water??

If there is then all the baths would have hot water. The tank would drain down and then fill up after the baths.... we wouldn't need to heat the water at that point and could wait till daylight hours and get some or all of the heating for free.

Any help or pointers you can give would be great!

Before answering, I'd like to ask if there are any compelling reasons for everyone having a bath everyday?

While I wouldn't advocate going back to my childhood and a weekly bath, perhaps cutting the baths by 50% is perfectly feasible (and even socially desirable with our current water and energy situation).

The water in your tank when hot has a varying temperature - hottest at the top and coolest at the bottom, and the thermostat set temperature at the thermostat. As you draw off hot water, the hottest is drawn off first, and gradually the water drawn becomes cooler, even ignoring the effects of cold water entering the bottom of the tank (incidentally, the design is such that little mixing occurs when the water enters, precisely to minimise the new cold water cooling the warm water in the bottom of the tank).

To get more heat stored in your water, you could turn your thermostat up - but be careful that it's not so hot to be a danger of bad scolding to you or your kids. Usually, for cost reasons, it's better to turn the thermostat temp down.

It should cost about 1.5kWh for a hot bath, making reasonable assumptions. Do you turn the immersion off before taking the last bath?

Hobbo2006

Cardew wrote: »

Is electricity the only method of heating water? Only using a total of 11-12kWh a day suggests you have gas/oil/LPG or you are on economy 7 with storage heating.

It seems to reach a total of £350 you have taken 365 x 7.6kWh and are costing each kWh @12.6p?

yes that's our only way of heating water, we have warm air heating. Yes our elec costs 12.67p with southern electric.