[sbsmiths]

@JoeSoap. Can we switch immersion on mega flow off? If we did, would we get hot water from boiler or am I being stupid? We had megaflow installed years ago when we had an extension and an additional shower (not electric) installed. Builder said we needed megaflow as insufficient pressure so we always assumed it just pumped the water. Didn’t realise it heated it 🙈

[retrotecchie]

@sbsmiths

You can switch the immersion off but you'd end up with a tepid water store. What it may be possible to do is turn down the thermostat so it doesn't heat the water up so much. 65°C should be plenty.

[sbsmiths]

@retrotecchie. Thank you

[JoeSoap]

Originally Posted by sbsmiths

@JoeSoap. Can we switch immersion on mega flow off? If we did, would we get hot water from boiler or am I being stupid? We had megaflow installed years ago when we had an extension and an additional shower (not electric) installed. Builder said we needed megaflow as insufficient pressure so we always assumed it just pumped the water. Didn’t realise it heated it 🙈

Can we assume from your post that you have confirmed you have an immersion heater and that you have found how to switch it off?

Heating the Megaflo Cylinder water from your gas boiler will be cheaper than using the immersion heater. The boiler is classed as indirect heat to the cylinder and the immersion is direct heat. The gas boiler just heats water that either flows through you radiators to heat the house or to flow through a coil inside the Megaflo to heat the water stored in there. Also, the boiler can do both at the same time... although perhaps not as fast as if it was just doing one thing.

I used to have the hot water set on the gas boiler control to 'All Day' so that it came on early morning and went off at night so that we had very hot water all through the day. I found it more economical to have it set on the timer to come on for an hour three times a day. Yes, the water may become tepid at the taps (depending on how much you use) before the next timed-heating from the boiler but if more hot water is needed then you can just switch the boiler on for a bit or you can switch the immersion on for a bit. Also, you can switch both on to heat the water faster. It is up to you and you will find out through trial and error how much you need to heat the water through the day.

One thing you need to watch out for is if you have the cylinder thermostat set higher than the boiler thermostat (if it has one). If the boiler thermostat is set lower than the cylinder thermostat then it can never satisfy the heat demand of the cylinder thermostat. I found this out the hard way as my pump was running all the time. It should already be set to a sensible temperature, as should the immersion heater thermostat. With me so far? So, I keep my boiler thermostat nearly at maximum (around 70-80 degrees from memory) and I set my Cylinder thermostats to give me around 60 degrees at the taps as measured with a meat thermometer. So the cylinder thermostats will be set to about 65 degrees.

In short, I heat my water in the Megaflo through the indirect heat of the gas boiler and only ever switch on the immersion to get some hot water quicker if someone wants a bath in the middle of the day.

Hope this all makes sense but please ask away if it doesn't and we will try to help.

[JoeSoap]

@sbsmiths

I have just edited my last post to add a little bit more clarity about the Megaflo cylinder thermostat settings and heating the water in the cylinder from the two different sources of the boiler and the immersion heater.. You really shouldn't have to adjust them but who knows?

[geoffers]

Originally Posted by JoeSoap

Heating the ... water from your gas boiler will be cheaper than using the immersion heater...The boiler is classed as indirect heat to the cylinder and the immersion is direct heat.

@JoeSoap - although this is generally true it's not necessarily always the case

Indirect heating of the cylinder is quite inefficient as there's waste heat from the boiler going up the flue; losses in the boiler heat exchanger; losses in the pipe work running to the tank heat exchanger etc etc. Whilst with the direct heating immersion-heater element all the heat energy is transferred to the water.

I did some comparative tests in the summer (when there were no other gas sources running), with the tank starting from cold and running the gas boiler for 15 minutes (so the thermostat never reached the off temperature). A few days later I did the same with the immersion heater.

Surprisingly ¼hr of running the gas boiler used 3.08kWh, and not surprisingly the 3kW immersion used 0.86kWh

So at 7.39p the gas cost 22.76p, and at 32.8p the immersion cost 28.21p

However on the Next Drive tariff running the immersion on a timer before 7am @ 9.5p it only costs 8.17p

Guess which heat source I now use for my hot water? 🤓

[JoeSoap]

@geoffers

You may well be correct and I have no such concrete stats to back up my claim. That said, when an immersion heater is switched on 24/7, as in this case, I would put money on it being cheaper instead to use the gas boiler in appropriate time slots to satisfy the hot water requirement. Just my hunch, of course. Perhaps if the immersion was switched on and off throughout the day it would be cheaper still… but I won’t be trying it.

[geoffers]

Originally Posted by JoeSoap

@geoffers

You may well be correct and I have no such concrete stats to back up my claim. That said, when an immersion heater is switched on 24/7, as in this case, I would put money on it being cheaper instead to use the gas boiler in appropriate time slots to satisfy the hot water requirement. Just my hunch, of course. Perhaps if the immersion was switched on and off throughout the day it would be cheaper still… but I won’t be trying it.

That's all spot on 👍 - however I only have the immersion on for a relatively short time in the morning on the timer, and the tank stays hot enough all day for occasional hand/dish washing without needing any further input.

On the odd occasion I feel like a relaxing hot bath, I just bung both heat sources on for about 15 mins to get the tank up to bath temperature 🤓

[meldrewreborn]

While there are heat losses from a boiler and pipe work , I think one has to take account of where those losses go. So flue losses are truly lost, while pipe work losses go into the home, which in winter just reduce the heating required . In the summer the pipe work losses could cause excessive heating in the home.

as to comparative testing one would need to ensure the that the commencement and final temperatures Jin the store matched over a number of tests before any definitive conclusions were reached. I doubt that controlled testing would favour electrical water heatinI do though think that we pay too little attention to the use of hot water and the costs of procuring it compared to general home heating.

[geoffers]

Originally Posted by meldrewreborn

.... I do though think that we pay too little attention to the use of hot water and costs of procuring it compared to general home heating.

👍 - agree with all of the above

However I think your final statement is the most important - it strikes me that even using gas as the heat source, running the boiler all day to maintain the tank temperature is wasteful. Unless you're a high h/w usage household, just a single heating to 60⁰C (in my case at the lowest cost electricity) ensures that you'll have plenty of hot water all day, whilst ensuring that you keep those pesky legionella at bay 🤓

Very interesting and informative article on hot water and legionella here...
https://www.heatgeek.com/hot-water-temperature-scalding-and-legionella/

(Note - my points are probs only really relevant to people with hot water storage tanks not combi, and not running lots of baths/showers from the h/w tank)