Unvented Hot Water Cylinders.

Belenus

We may need to replace our Potterton Powermax HE 150 integrated boiler and unvented pressurised hot water cylinder sometime soon.

We are investigating Megaflo and other makes of unvented cylinders.

Do any of these cylinders come with a temperature display of the current hot water temperature?

I know that boilers can be set to heat the hot water to maybe between 45 and 65 degrees but, unless the hot water is always set to on, the hot water will cool through usage or heat losses over time.

It would be useful to know the actual temperature in order to judge if there is enough for a shower or bath etc.

Another question: Is there an increased risk of legionnaires disease if the hot water is allowed to cool to below 40 degrees before being reheated to 60/65 degrees?

We heat our hot water to about 60 degrees every couple of days but, at times, it has cooled to below 40 degrees through usage and heat losses. Should we heat it to 60 degrees more often?

Thanks

RedFraggle

On an unvented cylinder like a megaflo you're heating incoming mains cold feed directly by a heat exhange with a stored heated reservoir so there's no legionnaires risk as the stored water never comes out of your tank. 
Do you have high demand for hot water or could you just fit a gas combi?

tacpot12

Even if you are using a thermal store with an heat exchanger to instantaneously heat water for the shower, bath and sinks, you would still benefit from having an indication of the temperature of the water in the tank. I'm not aware of any cylinder that comes with a display of the temperature of the water in them, but it would be very easy to add a remote display. 

I would use one of these: AQUARIUM LCD DIGITAL THERMOMETER FOR FISH TANK WATER, FRIDGE FREEZER TEMPERATURE | eBay

You just push the sensor capsule under the foam insulation of the tank. The one in the link comes with 1m of wire, which is enough to mount the sensor on the outside of a cupboard containing the cylinder, but which might not be enough if the cylinder is in the loft. You can get them with 2, 3 or 5m cables, which is plenty. 

ComicGeek

RedFraggle said:

On an unvented cylinder like a megaflo you're heating incoming mains cold feed directly by a heat the change with a stored heated reservoir so there's no legionnaires risk as the stored water never comes out of your tank. 
Do you have high demand for hot water or could you just fit a gas combi?

No, the unvented cylinder is filled from the mains water supply, and is heated by the closed heating system through the coil. So the stored volume is drawn off to taps etc. Megaflo is just an expensive type of unvented cylinder - good quality, but priced more for the high end market. Lots of cheaper options available that are fine for most people. 

The heat loss through modern cylinder casings is extremely small, around 1-2 kWh per 24 hrs - that's about 3 mins per day of a normal boiler operation to replenish, so not significant. Much, much lower than your current Powermax unit.

Legionnaires is a risk when you have warm, stagnant water. Heating the water above 60 degrees is a simple way of minimising the risk, but the risk is also minimised if you are using hot water each day and preventing the water from sitting stagnant for long periods. Personally I have our cylinder at 55 degrees, and heat it above 60 degrees only after being away for more than a week. If I had a shower room that wasn't used much, I would run the shower for a few minutes each week to clean it out.

If you have a well insulated modern cylinder the best thing is to heat up the cylinder in the evening, set up on the heating programmer for a couple of hours - if your cylinder is sized correctly then it should last you for 24 hrs until it's reheated the next evening. If it does start to run out, then normally there's a boost function on the programmer that will get the boiler heat diverted to the cylinder, which should get you hot water in about 30 minutes. To be safe, you can always have an evening heating period and a late morning heating period - the boiler stops once the cylinder is at temperature so you're not really wasting any energy from doing this. 

Surely you just make sure that you have sufficient stored hot water to have a bath/shower, rather than waiting until you do to have one. That just doesn't make sense to me - very easy to control the storage temperature of the cylinder through the programmer and cylinder stat so that it's always available.

Belenus

RedFraggle said:

Do you have high demand for hot water or could you just fit a gas combi?

We are a retired couple with nobody else living in the house so our water demands are lower than a family.

We have considered a combi but we do prefer to have stored hot water that is still available even in the event of a power cut or boiler failure so we are likely to stick to stored hot water.

Browntoa

I have a megaflo

It's set to heat only when the boiler is on for heating and I never run out of hot water . It's also an immersion heater so in the event of a boiler fault I can manually switch to immersion for hot water . 

Belenus

tacpot12 said:

Even if you are using a thermal store with an heat exchanger to instantaneously heat water for the shower, bath and sinks, you would still benefit from having an indication of the temperature of the water in the tank. I'm not aware of any cylinder that comes with a display of the temperature of the water in them, but it would be very easy to add a remote display. 

I would use one of these: AQUARIUM LCD DIGITAL THERMOMETER FOR FISH TANK WATER, FRIDGE FREEZER TEMPERATURE | eBay

You just push the sensor capsule under the foam insulation of the tank. The one in the link comes with 1m of wire, which is enough to mount the sensor on the outside of a cupboard containing the cylinder, but which might not be enough if the cylinder is in the loft. You can get them with 2, 3 or 5m cables, which is plenty. 

I fitted that same sensor to my current Powermax tank. It cost me under £3 on eBay and works very well. I fitted it about a quarter of the way down from the top.

It would appear that I could use it or something similar on a Megaflo as I got this reply from the manufacturers this morning.

Good afternoon, Thank you for your email enquiry. 

Current cylinders don’t have temperature displays on them. 

Current cylinders do, however, have a temperature probe pocket, which you could insert a temperature probe into and it would be this that will tell you the temperature from just above the central heating coil, but nothing higher up on the cylinder than this is available. This isn’t something we provide as an accessory to the unit.

 Some smart devices/controllers are supplied with temperature probes which can go into this pocket, but the device should still be wired in the normal to operate via our controls supplied with the cylinder.

Belenus

Browntoa said:

I have a megaflo

It's set to heat only when the boiler is on for heating and I never run out of hot water . It's also an immersion heater so in the event of a boiler fault I can manually switch to immersion for hot water . 

Our current Powermax is hot water priority. The diverter valve is 2 way only, heating or hot water but not both together. If both are calling for heat, the hot water is heated first before it switches to the heating.

We also have an immersion heater which has proved very useful a few times when the boiler has failed.

ThisIsWeird

Nice idea!
Hard to know what that temp probe's accuracy will be - I suspect it'll read low as I presume it would be fully immersed when used in an aquarium? Whereas here, for neatness, you may wish to just 'drill' a snug hole through the insulation until you reach the tank skin, clean the contact point as best as possible, and then perhaps smear a thermal paste ( I'd have thought even copper-ease, silicone grease, or similar would work?) on the tip of the probe before pushing it in. If this is done at the same height as the tank thermostat, then it should be easy to work out its accuracy, and add/subtract a few degrees as required. I doubt they are  electronically adjustable?

Belenus

ComicGeek said:

 Megaflo is just an expensive type of unvented cylinder - good quality, but priced more for the high end market. Lots of cheaper options available that are fine for most people. 

Megaflo is just an expensive type of unvented cylinder - good quality, but priced more for the high end market. Lots of cheaper options available that are fine for most people.

We will investigate other options.

One thing that puts us off the Megaflo is the need to service or reset the internal air gap device annually. Other alternatives have an external expansion tank.

We have plenty of room in the boiler cupboard so we have no space issue with an external expansion tank, There is one there already for the Powermax although we would replace that with new as it is 17 years old and counting.

Do you have your Megaflo serviced annually or is the air gap thing a DIY job?

Belenus

ThisIsWeird said:

Nice idea!
Hard to know what that temp probe's accuracy will be - I suspect it'll read low as I presume it would be fully immersed when used in an aquarium? Whereas here, for neatness, you may wish to just 'drill' a snug hole through the insulation until you reach the tank skin, clean the contact point as best as possible, and then perhaps smear a thermal paste ( I'd have thought even copper-ease, silicone grease, or similar would work?) on the tip of the probe before pushing it in. If this is done at the same height as the tank thermostat, then it should be easy to work out its accuracy, and add/subtract a few degrees as required. I doubt they are  electronically adjustable?

The probe claims to be accurate to plus or minus 2 degrees. With the boiler set to heat the hot water to 60 degrees, it shows 59.7 degrees and that is close enough.

Regarding installing the probe, the tank has a pressure relief valve about a quarter of the way down from the top. I pushed the probe into the insulation around that so not only is the tip of the probe touching the tank, the body is touching the copper pipe emerging from the tank.

I will post a few pictures that show that better than I can describe.