Understanding Indirect Hot Water Cylinders

[DIYNovice1979]

Hi,
We've just bought a house and we have a " Unvented Indirect Hot Water Cylinder" in the loft. Having never had one before, I was hoping someone can educate me a bit - I don't know whether it operates of the main boiler or not (I'm assuming the boiler, but it is also plugged into the mains. Does this mean it has an electrical backup?

https://www.heatandplumb.com/media/heatrae-sadia/datasheets/file-45735.pdf

As we're doing the place up, I've turned the heating off with the HIVE system, I'm assuming this automatically turns off the heating in the hot water cylinder (as opposed to it just sitting there using electricity to keep water hot all the time?)

Lastly I'm looking at getting a new energy supplier, with a hot water cylinder and a hive system in place, is there a recommended tariff (e.g. can we set the hot water cylinder to just heat overnight so we use cheaper electricity)?

Thanks in advance (and apologies for any stupid questions!)

 

[Madrab]

If you have a gas boiler then the cylinder will be heated indirectly by the CH. The electric immersion heater is there as a backup should the primary heat source fail.

No matter when you use electricity, it would still be more expensive to heat the cylinder than using the gas central heating. It all depends on your HW usage but typically a heat up in the morning before getting up then another heat up before dinner time will give you 2 full cylinders of HW for showering morning and evening etc.

 

[ericmark]

No matter when you use electricity, it would still be more expensive to heat the cylinder than using the gas central heating.

Not so sure on that, at 8.95p per KWh although more expensive than gas, there are no losses, either from boiler not being 100% efficient or the pipe work losing heat, I know my pipe work goes up three floors and is not lagged, I have three electric rates, on peak 31.31p per KWh, off peak 8.95p per KWh and export 14p per KWh so even when the sun is shining it still costs to heat water.

But my boiler to give us hot water to wash hands, needs to run ½ hour 4 times a week, actually it only runs 20 minutes as it cuts out after that, and boiler is 20 kW. Since only used to wash hands, shower is instant electric and plumbing costs not worth changing, and dish washer and washing machine are cold fill.

The megaflo is better insulated to my tank, so will need less to maintain the water temperature, down side is the annual pressure test.

As to tariff, it depends on how much you want to gamble, with smart meters you can have variable tariffs, but the question is how do you set up your home to use variable tariffs? Setting my battery to charge at 1 am to 5 am and using the delay on washing machine, tumble drier, and dish washer if no solar that day we can use a split tariff, but when you need to keep consulting an app to see what the tariff is, it results in one not being able to pre-plan what you use.

My British Gas supplier has been promising to send us the paperwork to get payments for export for 6 months, so now considering moving to Scottish Power because of the way they are dragging their feet. Our solar panel installers say they will do all the set up with Scottish Power.

You can have different firms to sell to as buying from, but one will normally get more for the export if you use the same firm. There has been some talk about batteries without solar, but batteries have a limited life, so to have enough to run house on off peak only without solar as well not sure it will pay. The solar inverter also does the batteries, so without solar still paying for an inverter.

What got me to install solar was the UPS which is all part of the system, so even with no grid power my central heating and freezers continue to work, still need candles or torches for light. But I remember the winter of discontent and will not rely on electric power to heat home.

Much is down to how much power you use, we live in the country in mid Wales and have freezers so in snow we don't need to leave the house, but also means we will lose money should the freezers de-frost, hence the UPS, if we did not live on a hill, but on the flat, then snow would not be such a problem.

 

[Madrab]

A hot water cylinder will use a certain amount of heat (KW) to heat the water to a set point regardless of the heat source - at the moment, Kw for Kw, gas is cheaper. Therefore by using that simple premiss, where the cylinder will use the same amount of energy regardless, then surely gas is going to be cheaper to heat it to that set point?

Off peak (overnight) is cheaper yes - not sure it's still as cheap as gas? My gas at the mo is 5.12p/kWh still cheaper that the quoted off peak rate?

It all depends on your HW usage

Off course it depends on HW usage, which is information that wasn't given so the presumption was on 2 full cylinders a day for an average family use, for some it would be more and for some it would be less. I always refer to this as a standard use model when discussing clients requirements and not clever use of HW where say only enough water would be heated to wash hands etc. If that was the case then of course I would suggest having a different types of instantaneous HW heating if that was the case.

dish washer and washing machine are cold fill.

They then need to be heated by a 3kw element - I use mixed HW to feed these appliances.

I'm not saying with clever use and fastidious monitoring and control - which I do agree with but most people don't or wont do that - that leccy can be economical and cost effective but alas that's not in the majority yet.

 

[Rodders53]

Don't forget that the unvented will have a heat loss into the loft space so wasted. Mind, it might work the other way in a hot summer?
Mine in the airing cupboard uses (loses) about 2 kWh every day according to the maker (Oso) but is used to dry off washing completely.

Yes a timer-control can be fitted to use off peak electric heating on that tanks immersion element. It will shorten the life of that backup emergency facility and not be cheap to replace if/when it fails. Such electric switch controls can be part of smart heating controls I believe (e.g. Wiser do a 16A switch).

Gas (or Kerosene that I use) at even 80% efficiency will be cheaper than peak rate electric prices currently to heat the water. OP needs to do the sums based on their tariff options.

 

[ericmark]

If to reheat the boiler is running 2 hours, then even with off peak, the gas/oil will be cheaper. But for a run of 20 minutes, one has to consider in my case around a 20 meters at 0.5308 kg per meter (28 mm pipe) feed and return so 10 kg of water is heated and allowed to cool in the pipe work. In 20 minutes the return has got hot enough to turn the boiler off, so 21 pints of water so around a kWh is used up to heat the pipe work.

OK my system is C plan so maybe other systems have smaller pipes, the boiler efficiency rate can reach up to 90-91% at full condensation (80% from fuel burning and 11% from steam condensation). So my 8.95p per kWh = 8 kWh if the boiler is condensing, if not 7.16p per kWh so combine the two together, losses through pipe work and boiler efficiency then the electric v gas or oil costs are very close if using off peak power.

All figures are approximate, can't get spot on figures, but the cost to heat DHW with off peak electric verse gas or oil are very close. The problem is I can measure the electric used very easy, a CT coil can be clipped around a wire very easy, the same is not true with a gas or oil pipe.

I was looking at boiler use in my mothers house, I wanted to know was the boiler actually modulating and condensing, the gas meter was not real time, I could not read the gas meter in kW, I could the electric, but not gas, so we take what the boiler uses on trust. With oil which does not modulate, OK 20 kW boiler only need to record time running, but gas no idea if running at 8 kW or 28 kW the immersion heater I can look at the display

it tells me exactly what is being used,

no info is given with my mothers old gas boiler, so can't really compare, the pictures are from the instruction manual not from my own iboost+, but when the device does not give info, I am suspicious as to what it is really using. I moved from gas to oil, and my bills were a 1/3 less in spite of a larger house. This should not be the case, oil costs more than gas, and bigger house, three floors instead of two and each floor bigger, double glazing was also older, we expected to pay more for heating. But actually pay a lot less, which to my mind questions all the info were were given about gas, since moving to a area with no mains gas, I have talked to a few people who have moved from oil to LPG and every one has said if costs more, even LPG is cheaper than oil, and the boilers are claimed to be more efficient, but bills have gone up.

We must view with some some trepidation claims when we can't see the data in real time. I can sit in my living room and watch the smart meter show what I am using from the grid, normally zero, and can on my PC or phone see what is being generated, imported, stored, exported etc with electric, oil all I can do is dip my tank.