Boiler and Megalow mistery

[RenzoP]

Hi,
Just moved into a house from a flat and I have a couple of queries.

We have a Worcester gas fired boiler - Greenstar Ri (in the kitchen on ground floor) and a megaflow (on the first floor) plus a small red tank that I do not know what it does.

Heating is set to off on the boiler, while water is set to on (not on a timer, always on).

1) issue/question
I noticed that often the gas fired boiler is turned on, and it would seem to happen when using the hot water eg to do the dishes.
Does it mean as soon as I use some water, the megaflow gets the boiler fired up to replace the water used? Is that efficient from an energy perspective? I.e. I might use the hot water only many hours later..
1) solution
I assume a way to deal with that is that I could just set a timer for the water on the boiler (eg only in the am or at night when the Energy is cheaper), though that would mean we might use the whole hot water in the tank and run out. Would that mean no hot water, or hot water but through boiler (so less pressure)?

2) issue/question
even if the heating is off, I noticed that both the radiator (tower rail) in the bathroom and the radiator in the main entrance (possibly on the way up to the megaflow from the boiler) get warm. Is it just that they are both in the way, so rather than having a pipe run directly from to the boiler to the megaflow, it runs through these 2 radiators?

2) solution
Is the solution to just turn off these two radiators with their valve when not hot enough? So that even if the water flows to the megaflow, it still doesn’t go through them?

Thanks for your help!!

 

[stem]

I'll have a go at some of your questions.

The "small red tank" is an expansion vessel.

When water is heated it expands, and without the expansion vessel the water expansion would split open the Megaflow or connected pipes. The expansion vessel contains air which is compressed as the water expands and prevents damage to your system.

The hot water is stored in the Megaflow. When you use hot water it's drawn off from the top and colder water comes in to replace it at the bottom. About two thirds of the way down the cylinder is a thermostat, it detects when the water inside has cooled, needs heating and then fires up the boiler. The Megaflow is well insulated (and so should be any hot water pipes connected to it - yours aren't) once the Megaflow is hot, heat losses are minimal. Using hot water and allowing cold water to enter at bottom will of course cool it. Your uninsulated pipes especially the on at the top will be loosing heat though.

I'm not aware of any gas supplier that offers cheaper gas overnight, but if you do have that tariff go for it (If you are referring to off peak electricity the boiler & pump will only use such a tiny amount of electricity, I doubt you would notice any saving) If you did heat the Megaflow during the night only, depending upon how much hot water you use during the day you may find only tepid or cold water comes out of the taps later on, especially if you have a bath or a couple of showers. Just pot washing shouldn't be a problem (unless you live in a restaurant ) . The water pressure won't change.

Think of the Megaflow like your phone battery. If your phone is kept fully charged and you don't use it, its ready to go anytime. If you don't keep it charged and use it a lot, you will run out of charge. That's how your Megaflow works it's 'charged' with hot water by the boiler. How long it lasts depends upon how much you use it and how often it's charged.

Don't confuse a combi boiler (instant hot water supplied directly from the boiler) with a system boiler (heats a stored water supply e.g. a Megaflow and is not able to provide hot water directly to the taps)

Regarding the two radiators, it's unusual for this arrangement to be installed as part of a modern gas fired system. So would probably need inspection by a professional to determine why they are heating when just the hot water is in. It could be the way they have been piped up, or there could be a fault somewhere.

 

[cdbe]

I have my megaflo timed - to coincide with significant hot water usage, ie showers. This stops the "nuisance" firing when washing up. In the timed period it doesn't really matter when it fires up as heat loss is minimal. Ours will easily do 2/3 showers outside the timed period - if we need more, eg Saturday morning football I just put the boost/+1hr on.

We have a 15kw boiler and waste water heat recovery units fitted to our showers and with the quick recovery time are probably close to constant hot water.

Quite common for towels rads to be fed from the hot water circuit so they dry towels outside the heating season. Not any other rads - a trv could probably sort it out as a cheap measure.

 

[ianmcd]

Regarding the two radiators, it's unusual for this arrangement to be installed as part of a modern gas fired system. So would probably need inspection by a professional to determine why they are heating when just the hot water is in. It could be the way they have been piped up, or there could be a fault somewhere.

Hi stem
Hope you are well my friend, the OP needs to look at the pipe work installation, the return connection from all rads must be the last connection on the pipework before the boiler connection , it wont be and that is why the rads are heating when not wanted

 

[stem]

I'm good thanks hope you are too. Hmm, the dreaded reverse circulation maybe.

 

[RenzoP]

Thanks to those who replied.

I’ve turned off the hall one and set the timer to come on 2 times a day. Initially it was only 1 hr each time and I realised that it left us with cold water (after 2 people took long showers) so I bumped it up to 2.5 hours each time. I’ll do a bit of trial and error.

Question/comment: when the megaflow had run out of hot water I expected the hot water to come straight from the boiler “on demand”, allowing me to take a hot shower. Instead only cold water came through. Is it because the “new” new water from the boiler was going into the tank - ie not directly to the shower? If that’s the case, what’s the advantage of having the megaflow tank? A standard boiler would just fire up and heat water whenever required on demand… I sort of miss that!

 

[RenzoP]

I'll have a go at some of your questions.

The "small red tank" is an expansion vessel.

View attachment 315412

When water is heated it expands, and without the expansion vessel the water expansion would split open the Megaflow or connected pipes. The expansion vessel contains air which is compressed as the water expands and prevents damage to your system.

The hot water is stored in the Megaflow. When you use hot water it's drawn off from the top and colder water comes in to replace it at the bottom. About two thirds of the way down the cylinder is a thermostat, it detects when the water inside has cooled, needs heating and then fires up the boiler. The Megaflow is well insulated (and so should be any hot water pipes connected to it - yours aren't) once the Megaflow is hot, heat losses are minimal. Using hot water and allowing cold water to enter at bottom will of course cool it. Your uninsulated pipes especially the on at the top will be loosing heat though.

I'm not aware of any gas supplier that offers cheaper gas overnight, but if you do have that tariff go for it (If you are referring to off peak electricity the boiler & pump will only use such a tiny amount of electricity, I doubt you would notice any saving) If you did heat the Megaflow during the night only, depending upon how much hot water you use during the day you may find only tepid or cold water comes out of the taps later on, especially if you have a bath or a couple of showers. Just pot washing shouldn't be a problem (unless you live in a restaurant ) . The water pressure won't change.

Think of the Megaflow like your phone battery. If your phone is kept fully charged and you don't use it, its ready to go anytime. If you don't keep it charged and use it a lot, you will run out of charge. That's how your Megaflow works it's 'charged' with hot water by the boiler. How long it lasts depends upon how much you use it and how often it's charged.

Don't confuse a combi boiler (instant hot water supplied directly from the boiler) with a system boiler (heats a stored water supply e.g. a Megaflow and is not able to provide hot water directly to the taps)

Regarding the two radiators, it's unusual for this arrangement to be installed as part of a modern gas fired system. So would probably need inspection by a professional to determine why they are heating when just the hot water is in. It could be the way they have been piped up, or there could be a fault somewhere.

Thanks so much for your detailed answer. People like you make these forums an amazing source of information.

1) with regard to “Your uninsulated pipes especially the on at the top will be loosing heat though”, how easy/expensive is to get those insulated? Can I do it myself?

2) you mentioned inspection by a professional. Who would you call? Plumber? Or another professional?

 

[stem]

You can buy insulation from any DIY outlet, usually it's made from expanded foam and can easily be cut with scissors. It will come in different sizes most copper pipe is 15mm or 22mm make sure the pipe is covered without gaps, joints can be taped.

A plumber would be a good idea, but anyone competent that understands how central heating systems work should be able to assist. It sounds like the system may have been designed and plumbed so that the two radiators work with the hot water, or you might have a case of reverse circulation. There's a good explanation of what that is here. In this example, an additional radiator has been incorrectly added to an existing system and introduced reverse circulation.

 

[Lower]

I'll have a go at some of your questions.

The "small red tank" is an expansion vessel.

View attachment 315412

When water is heated it expands, and without the expansion vessel the water expansion would split open the Megaflow or connected pipes. The expansion vessel contains air which is compressed as the water expands and prevents damage to your system.

The hot water is stored in the Megaflow. When you use hot water it's drawn off from the top and colder water comes in to replace it at the bottom. About two thirds of the way down the cylinder is a thermostat, it detects when the water inside has cooled, needs heating and then fires up the boiler. The Megaflow is well insulated (and so should be any hot water pipes connected to it - yours aren't) once the Megaflow is hot, heat losses are minimal. Using hot water and allowing cold water to enter at bottom will of course cool it. Your uninsulated pipes especially the on at the top will be loosing heat though.

I'm not aware of any gas supplier that offers cheaper gas overnight, but if you do have that tariff go for it (If you are referring to off peak electricity the boiler & pump will only use such a tiny amount of electricity, I doubt you would notice any saving) If you did heat the Megaflow during the night only, depending upon how much hot water you use during the day you may find only tepid or cold water comes out of the taps later on, especially if you have a bath or a couple of showers. Just pot washing shouldn't be a problem (unless you live in a restaurant ) . The water pressure won't change.

Think of the Megaflow like your phone battery. If your phone is kept fully charged and you don't use it, its ready to go anytime. If you don't keep it charged and use it a lot, you will run out of charge. That's how your Megaflow works it's 'charged' with hot water by the boiler. How long it lasts depends upon how much you use it and how often it's charged.

Don't confuse a combi boiler (instant hot water supplied directly from the boiler) with a system boiler (heats a stored water supply e.g. a Megaflow and is not able to provide hot water directly to the taps)

Regarding the two radiators, it's unusual for this arrangement to be installed as part of a modern gas fired system. So would probably need inspection by a professional to determine why they are heating when just the hot water is in. It could be the way they have been piped up, or there could be a fault somewhere.

If the expansion vessel is red, its the expansion vessel for the central heating system. Megaflos have an internal expansion air gap that needs recharging periodically (instructions printed on the outside of the megaflo)

 

[Lower]

Thanks to those who replied.

I’ve turned off the hall one and set the timer to come on 2 times a day. Initially it was only 1 hr each time and I realised that it left us with cold water (after 2 people took long showers) so I bumped it up to 2.5 hours each time. I’ll do a bit of trial and error.

Question/comment: when the megaflow had run out of hot water I expected the hot water to come straight from the boiler “on demand”, allowing me to take a hot shower. Instead only cold water came through. Is it because the “new” new water from the boiler was going into the tank - ie not directly to the shower? If that’s the case, what’s the advantage of having the megaflow tank? A standard boiler would just fire up and heat water whenever required on demand… I sort of miss that!

Hot water tanks allow you to have a more economical boiler that is correctly sized for your central heating system but can also heat up your hot water in the cylinder and also allow you to heat the water electrically by immersion heater. A combi boiler (what you call a standard boiler) has an oversized heat exchanger to allow production of sufficient hot water when you turn on a tap as well as doing the central heating. A combi will cope with one tap or shower running at the same time, but can't flow enough hot water (unless its massive) to supply more than one tap or shower.

 

[RenzoP]

With the situation below, this morning we had 4 showers, and the forth person had cold water.. I turned on an extra hour on the boiler at that point, but the water was still cold… (I really wish the system was smart enough to send water to the shower first.. and then fill the cylinder).

i guess 170liters is just not enough to take 4 showers one after the other….?



- Cylinder is 170liters
- boiler turns on 2.5 hrs in the am and in the afternoon..

 

[Madrab]

i guess 170liters is just not enough to take 4 showers one after the other…

Depends on how long the showers are and how hot they are. A 7min shower @ 38deg will use about 80-100L @ 14L/Min - 50-60L of that is HW.

If you know there will be 4 showers in a row then why not over-ride the HW before they start?

 

[RenzoP]

Thanks that makes sense. I think my 6 year old had a super long shower….
do you think that if I hit the extra hr button before starting the first shower I’d produce enough additional hot water to have that additional shower that today I couldn’t have?

 

[Rodders53]

Madrab said you will use 60 litres of hot per 7 min shower. Or a bit less if shorter and a bit more if longer...
My estimate is that a 170 litres tank will service 2 showers at that rate 120 litres leaving 50 litres of hot mixed with 120 cold incoming. I'd expect a third shower to go lukewarm to cool....

Megaflo 170i reheat time is a claimed 22 minutes (from 15 C incoming cold to 60 C hot). It'll be a little longer in winter if cold in is 5 C (say). Also IF the towel rail is on the HW reheat circuit it may take a longer to reheat, too as some energy is lost to those?

Ensure the HW is timed to come on shortly before (20 mins?) anyone showers and continues on for 20-30 minutes after the fourth is due out for the best chance of all 4 being happy. After that the HW stored should be enough for hand washing, dish washing and general household use until the next day. Or if you all four shower twice a day, set another time period to suit that as well.

Longer ON periods will do nothing useful as once the tank thermostat is satisfied (the water is at 60 C, ow whatever it's set to) then no more heat is put in from the boiler. {On the other hand relatively little is lost from excess ON times.}

Turning on the 1 hour boost feature will likely do the job, but why risk forgetting?

We are only two and have ours set to once a day for about an hour. I use the boost feature when returning from holiday to reheat the tank as the heat losses are not insignificant over a week or even a few days away with all heating/HW off.

 

[Johntheo5]

With the situation below, this morning we had 4 showers, and the forth person had cold water.. I turned on an extra hour on the boiler at that point, but the water was still cold… (I really wish the system was smart enough to send water to the shower first.. and then fill the cylinder).

i guess 170liters is just not enough to take 4 showers one after the other….?



- Cylinder is 170liters
- boiler turns on 2.5 hrs in the am and in the afternoon..

Any HW cylinder with a air bubble will not have its full capacity of HW available.
Assuming the hot cylinder pressure is not allowed to reach more that say 5 bar then the HW dip tube in a 170L cylinder must extend "30L" downwards into the cylinder, there will be 165L of hot water at 5bar but only 140L of this is available as HW as the cold mains will start entering the dip tube end once the 140L is used up. Effectively you are losing 30L at 60C or = ~ 50L at 40C.
However, if you left the boiler switched on with a "full" cylinder of HW then assuming the coil can input a fairly modest 15kw one could then draw off the equivalent of 7.2LPM at 40C (from mains at 10C) continuously.