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Discuss Question about my hot water setup in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

Messages
6
Hi, I'm new to the forum and very confused about my hot water setup.

I recently purchased a house that has an Ideal Logic System S18 Gas Boiler + Megaflo Eco 170l Indirect Unvented Cylinder.
I can control the Ideal Logic System S18 through a Honeywell ST9400C 7-Day 2 Channel Programmer which allows me to program the heating and hot water independently.
I currently have the heating on continuous and set the temperature through a thermostat.
The hot water is set to run for 1 hour in the evening and 1 hour in the morning, but I think the cylinder has it's own heating element that runs on electricity and keeps the water at a certain temperature(which is super hot if I turn on the hot water tap)
Is there any way I can stop the cylinder from toping up the water temperature by itself and just leave it to whatever temperature the 2 hours a day from my Gas boiler gives me?

I can see there is a titanium heating element that is connected to a switch. If I just turn that off will it work as I want it to work?
 

EvilDrPorkChop

Gas Engineer
Messages
769
Hi, I'm new to the forum and very confused about my hot water setup.

I recently purchased a house that has an Ideal Logic System S18 Gas Boiler + Megaflo Eco 170l Indirect Unvented Cylinder.
I can control the Ideal Logic System S18 through a Honeywell ST9400C 7-Day 2 Channel Programmer which allows me to program the heating and hot water independently.
I currently have the heating on continuous and set the temperature through a thermostat.
The hot water is set to run for 1 hour in the evening and 1 hour in the morning, but I think the cylinder has it's own heating element that runs on electricity and keeps the water at a certain temperature(which is super hot if I turn on the hot water tap)
Is there any way I can stop the cylinder from toping up the water temperature by itself and just leave it to whatever temperature the 2 hours a day from my Gas boiler gives me?

I can see there is a titanium heating element that is connected to a switch. If I just turn that off will it work as I want it to work?
Yes you have the immersion heater turned on that'll heat it 24/7 to a set temperature. Turn it off and use the programmer as you've described.
 

Timmy D

Gas Engineer
Subscriber
Messages
297
Turn off the switch (I assume this is connected to the immersion? It’s currently using the electricity to heat the tank throughout the day and just a little top up with the gas boiler. Electric is about 4 times the cost of gas.

The cylinder thermostat will govern the hot water temp and stop asking the boiler for heat once the cylinder stat is satisfied.

The immersion stat and cylinder stat are separate and can be set at separate temps. You need to reduce the stat temps if the water is scolding.

If you’re a practical person and confident using a screw driver without electrocuting yourself, post pics of cylinder and we can advise how to lower it.

Just gonna touch on the subject of timed hot water. In this day and age, the insulation on unvented cylinders is much better than on cylinders 20 years ago.

If you check the data badge on your cylinder it will tell you the standing heat loss. I expect from the size of your cylinder for the heat loss to be around 1.5kw/24 hours.

If we use gas at 4p/kw, it will cost you 6p a day to keep the cylinder at temp 24 hours a day with no draw off.

If you time the cylinder, that heat loss still occurs and the energy loss will need to be replaced during the next timed hot water cycle. It’s still gonna cost you that 6p to reach the set temp.

So for convenience, just leave the hot water on 24/7 unless the boiler is in a location which can be heard when sleeping. The noise could be a nuisance, if so, turn it off for the night period.

You’re saving no noticeable cash, having the cylinder timed.
 
Messages
6
If you’re a practical person and confident using a screw driver without electrocuting yourself, post pics of cylinder and we can advise how to lower it.

Just gonna touch on the subject of timed hot water. In this day and age, the insulation on unvented cylinders is much better than on cylinders 20 years ago.

If you check the data badge on your cylinder it will tell you the standing heat loss. I expect from the size of your cylinder for the heat loss to be around 1.5kw/24 hours.

If we use gas at 4p/kw, it will cost you 6p a day to keep the cylinder at temp 24 hours a day with no draw off.

Thank you Timmy, I've attached a photo of the cylinder and the switch. I will turn it off when I get back home this evening and open the cover highlighted red. I should be safe to operate if the switch is off.

With regards to the heat loss you are correct. It's roughly 1.42kWh/day if the temperature is at 65deg.
I don't mind leaving the gas boiler on continuous for the hot water as it is in the utility room(outside the main house) so can't hear it anyway.

My question is if I switch of that titanium heating element from the switch highlighted in blue will that not invalidate the temperature I set for the hot water cylinder? The temperature is set by turning a screw behind that cover highlighted in red. Or is this like a mechanical setting that doesn't require electricity?
 

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Messages
179
That switch is nothing to do with the gas boiler and it looks like if the cable from it goes straight to the grey panel turning off that switch on the wall is all you have to do. No need to remove cover at all unless temperatures remain too high. One downside of leaving the boiler hot water cycle active overnight might be noise, depending on position of boiler and piping etc.
 
Messages
6
So I turned off the switch and voila the electricity consumption dropped by nearly 5kWh/day!
The only issue is the boiler doesn't seem to listen to the 2 hours a day I've programmed it to heat the water.
I turned the thermostat way down while the boiler was running(outside those 2 hours a day I set it to heat the water) and it didn't stop. Plus my hot water is still very hot, so it's obvious the boiler is now heating the water back to temperature continuously, regardless of the timer.
My gas usage is not too bad, my only question is: will this not shorten the life of the boiler? It probably cycles 30-40 times/day between heating the water and heating the house.
 

EvilDrPorkChop

Gas Engineer
Messages
769
It looks like you've got an old stat wired in there? Have you got a closer picture of the motorised valve (Silver box on pipework to cylinder) and the white box next to it on the left?

Someones obviously swapped the cylinder previously.***
 

Ric2013

Plumber
Messages
2,939
You’re saving no noticeable cash, having the cylinder timed.
During winter months, I would generally agree with you as the boiler and system will be hot to some extent anyway and any 'wasted' heat goes into the house.

In winter months, having the DHW demand as a constant will mean that the boiler and associated pipework will be warming up regularly to top up the cylinder and hence the heat transfer to the water in the cylinder will be inefficient and hence the boiler may not run efficiently due to the low heat requirement. Also, any heat escaping into the house from the boiler and primary pipework will add to the possibility of the house over-heating in warm weather.

It would be interesting to compare gas usage by reading the meter for 2 similar time periods out of the heating season, but I would imagine the difference in gas consumption might be noticeable even for this modern installation.
 

Timmy D

Gas Engineer
Subscriber
Messages
297
During winter months, I would generally agree with you as the boiler and system will be hot to some extent anyway and any 'wasted' heat goes into the house.

In winter months, having the DHW demand as a constant will mean that the boiler and associated pipework will be warming up regularly to top up the cylinder and hence the heat transfer to the water in the cylinder will be inefficient and hence the boiler may not run efficiently due to the low heat requirement. Also, any heat escaping into the house from the boiler and primary pipework will add to the possibility of the house over-heating in warm weather.

It would be interesting to compare gas usage by reading the meter for 2 similar time periods out of the heating season, but I would imagine the difference in gas consumption might be noticeable even for this modern installation.

I appreciate your reply Ric. As per your last paragraph, id also like to compare. I’ll see if I can set up a test.

I don’t disagree with what you’ve said but rather disregard the additional heat loss from the boiler/pipework as minimal and for the sake of convenience, pay for that heat loss.

If we covert the standing heat loss from kw to C. We get a loss of roughly 6c.

On average a stat has to drop 2c before it’ll demand.

So over 24 hours, with the hot water on constant, you’d expect the boiler to fire 3 times to replace the cylinders heat losses.

If you time your hot water for once in the morning and once in the evening, our 3 reheats is only once more than 2 timed. In my opinion, it’s minimal.

However, I’m not arguing that I’m correct, just laying out my thought process. I’m happy to be corrected.

I’ll get my brother to test it on his unvented over the next few days.

How do you summon someone? Where’s John with his calcs?

Edit: just thinking about it, if the cyclometer was sitting with no demand, the boiler wouldn’t fire often but if there was demand, the boiler would fire every time.
 
Messages
6
It looks like you've got an old stat wired in there? Have you got a closer picture of the motorised valve (Silver box on pipework to cylinder) and the white box next to it on the left?

Someones obviously swapped the cylinder previously.***

It's probably easier to understand if shown in a video:

What do you mean someone swapped he cylinder?
The boiler and cylinder were installed at the same time.

I've also added a close up of the silver box
 

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EvilDrPorkChop

Gas Engineer
Messages
769
It's probably easier to understand if shown in a video:

What do you mean someone swapped he cylinder?
The boiler and cylinder were installed at the same time.

I've also added a close up of the silver box
What I meant was is that your cylinder probably wasn’t originally, as in there was another cylinder beforehand.

That cylinder stat is incorrect for the MegaFlo. It should be wired into the stat behind the grey panel not an external stat. This is probably why you are finding the HW is never switching off as it’s not reading the temperature correctly.
The white box is used on other Unvented cylinders with pockets specifically design for it. The MegaFlo doesn’t have this.
 
Messages
6
What I meant was is that your cylinder probably wasn’t originally, as in there was another cylinder beforehand.

That cylinder stat is incorrect for the MegaFlo. It should be wired into the stat behind the grey panel not an external stat. This is probably why you are finding the HW is never switching off as it’s not reading the temperature correctly.
The white box is used on other Unvented cylinders with pockets specifically design for it. The MegaFlo doesn’t have this.

Ok, I understand.
It's not very expensive to leave it like this for now and I will have it corrected during next boiler service.
Thank you for all your help!
 

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