Huge hot water cylinder heating more water than needed

PaulMonty

I have a 4 bed house with 2 bathrooms. As a result we have a large hot water cylinder. There is just my wife and I in the house.

I’m heating about 3 times the volume of hot water we need each day due to the cylinder size. I can’t switch to combi or smaller cylinder as apparently it’d put off people when I sell as the hot water available would be too small for the house.

My cylinder could do with replacing as it’s old. Does anyone know if it’s possible or have experience with.
1. Having two smaller hot water cylinders, using only one, but having a switch to make both active for when more people are staying.

or

2. is there such a thing as a hot water cylinder with adjustable capacity. It only feels up half way with hot water. When we have guests or if a bigger family move in at a later date, they can set the volume heated and stored to a greater volume.

would welcome thoughts on how to avoid heating so much water.

QrizB

PaulMonty said:

I’m heating about 3 times the volume of hot water we need each day due to the cylinder size. I can’t switch to combi or smaller cylinder as apparently it’d put off people when I sell as the hot water available would be too small for the house.

When are you planning to sell the house?

• If this year, you can probably put up with the big cylinder.
• If in a decade or more, you could swap it now and then swap back when you sell (you could even keep the old one in your garage or shed)
  

PaulMonty said:

Is there such a thing as a hot water cylinder with adjustable capacity. It only feels up half way with hot water. When we have guests or if a bigger family move in at a later date, they can set the volume heated and stored to a greater volume.

You can get HW tanks with dual coils. These are generally intended for people with two heat sources (eg. a gas boiler plus solar thermal panels) but I'm sure you could set a system up where it can switch between the two.

Example dual-coil cylinder:
https://www.gledhill.net/products/alternative-energy/sunspeed/

lohr500

Agree with Qriz B. I also think it depends how long you intend to stay at the property.

If more than a few years and your cylinder is an old one without bonded on insulation I would just replace the old one with a new one of the same size.

I can't find any definitive figures for the heat loss from a modern cylinder, but various sites suggests between 1.2kWh and 2kWh loss per day. If we assumed 1.5kh per day and gas at £0.09, that would cost around £50 a year to cover the heat loss.

Unless you plan to stay in the house for a very long time, I suspect the cost of putting in a fancy twin coil system, or a small tank and then replacing it with a larger tank when you move would not recover the £50 a year on cost to keep a large modern well insulated tank hot.

Shedman

Based on the last replacement cost of our 160 litre cylinder incl cost of plumber it will probably be £600 or more (and that was 3 years ago so probably a lot higher now).   That would be quite a long payback period based on lohr500's figure.   There's two of us most of the time in our house, the HW goes on twice a day for 1/2 hour a time and, depending on temp of cold water filling it and that's being mixed in the shower, we use between 10kWh and 15kWh of gas a day to heat it to 60°(so £1-£1.50 a day). Not sure a smaller one would save a huge amount.

Obviously if you have to do it soonish anyway if its old (how old? ours was over 30 years old and the coil got pinholes and turns out the cylinder was bulging badly at the back as well so it was a no brainer in our case as didn't fancy it going and having hundreds of litres of water pouring through the ceiling 😅) then the economics change .

Section62

PaulMonty said:

I’m heating about 3 times the volume of hot water we need each day due to the cylinder size. I can’t switch to combi or smaller cylinder as apparently it’d put off people when I sell as the hot water available would be too small for the house.

If the cylinder is well insulated then you won't be heating the whole amount of water each day - you'll just be adding heat to make up the heat loss, and to heat the volume of water you've drawn off.  If you are concerned about the heat loss (which is probably doing something useful like keeping an airing cupboard warm) then the best investment is in additional insulation on the cylinder.

One advantage of a larger cylinder is greater volumetric separation between the hot drawoff and the cold feed - meaning the hot water you draw off stays hotter for longer.  A smaller cylinder which is closer in size to the volume of hot water you actually use would mean the cold water having a greater chilling effect on the stored hot volume.  In other words you may need more heat input (/more regularly) to keep sufficiently hot water coming out of the taps (rather than it going lukewarm).

In theory you could have a system with more than one hot water cylinder, but if the intention is to have one cylinder used only occasionally then you'd need to take precautions against legionnaires.  Unless the existing cylinder actually needs replacing I'd be doubtful you will achieve payback on the cost of setting up a dual-cylinder arrangement and maintaining it in a legionnaires-safe state.

Reed_Richards

You are not heating more water than is needed, the hot water that you use is replaced by cold water in the cylinder and this cold water is what gets heated; that would be true whatever size of cylinder you have.  The rest of the water is still hot and so does not need to be heated.  BUT your cylinder will lose heat even if you don't use any hot water at all and it is this lost heat that costs you money.  A badly-insulated cylinder will lose more heat than a well-insulated one, although I think a lot of heat is lost down the pipes to and from the cylinder so those need to be well-insulated also.  And a large cylinder, with a larger exterior surface area, will lose more heat than a smaller one.  But I do not believe the difference between large and small is a big as you think.  

ProDave

My cylinder could do with replacing as it’s old. Does anyone know if it’s possible or have experience with.
1. Having two smaller hot water cylinders, using only one, but having a switch to make both active for when more people are staying.

WHY do you think it needs replacing?  Photo of the old tank perhaps?

And HOW do you heat the water in it?  Boiler?  Immersion heaters?

Olinda99

If your boiler is fairly old, then if it were me I would pay to switch to a combi boiler.

Energy prices are only going one way, and you may even find when you come to sell you 'get your money back' as the house will have a fairly new modern combi boiler system.

If your boiler is really old, you will also save a lot on gas when CH is on, as the new boilers are more efficient at heating your radiators.

Obviously it would be different advice if your existing  boiler is new-ish.

Section62

Olinda99 said:

If your boiler is fairly old, then if it were me I would pay to switch to a combi boiler.

Energy prices are only going one way, and you may even find when you come to sell you 'get your money back' as the house will have a fairly new modern combi boiler system.

Gas boilers are being phased out.  Whatever energy source people use in the future, it will almost certainly involve some amount of storage of hot water.

Stripping out the existing stored hot water system in a largish house that a combi will probably struggle to adequately provide heat/hot water for isn't likely to make sense, and assuming the OP isn't selling in the very near future, having a fairly new gas combi boiler system may not be much of a selling point when they do sell.

[Deleted User]

We have a 250L unvented cylinder with just two occupants in the home. We use on average about 3.5kWh of electricity a day to re-heat the cylinder. The timer is set for 2 hours a day. We never run out of hot water and my wife prefers to have a bath rather than shower. She is also not a great fan of dishwashers. I suspect that a dual immersion heater might save us a bit of money but it is not worth the cost of fitting a new HW cylinder.

FreeBear

I'd say it is possible to have two hot water tanks, but.... You would need zone valves to switch one or both tanks in to the heating system (fairly simple). Thermostats on both tanks (trivial), Valves on the output of each tank to feed the pipework to the taps (more zone valves). Then you will need a control system to monitor the temperature of each tank and control the zone valves & signal the boiler for heat - It won't be an "off the shelf" solution, so you would need to look for a suitable PLC and then find someone capable & qualified to install & maintain - That isn't going to be cheap (in a previous life, I would be charging a minimum of £1K just to evaluate).

An electric shower plus a kettle will reduce your need to heat a large tank of water - Perhaps look at a small (5l ?) water heater for the kitchen.