Improving HW Temperature (Regular Boiler, Vented Gravity-fed System)

[gabx]

I'm after some advice for improving HW temperature in a semi-detached property. The property is running a 24kw regular boiler (boiler is less than 2 years old) supplying both CH and HW in a low-pressure vented arrangement.

I don't have a high demand of HW as there is only one shower bath in the house. Primary HW usage is for taking 1 or 2 showers in the afternoon/evening. I use a Tado system to control CH and HW. Every morning I turn on HW for about 30 minutes (aim is to replenish hot water consumed the night before), and HW is switched on again for 30 minutes before someone takes a shower in the afternoon/evening.

During shower, I barely need any cold water input from the non-thermostatic mixer tap as the HW is just enough hot (though my cylinder's thermostat is set at 60C). This makes me wonder if the cylinder is storing HW at a temperature hot enough to reduce health risk from Legionella.

The questions that I have are:
- should I drastically increase the boiler's flow temperature in order to make HW hotter? Currently I use the boiler's Auto function. I can see the boiler adjusting the flow temperature on its own and oftentime it is running at around 60C. Problem is the boiler doesn't seem to allow me to control what temperature is to be used for CH and HW separately.
- is my HW schedule not long enough to heat up the 120l cylinder? I tried heating the water for a longer time before shower but I didn't seem to get much hotter HW output.
- is it necessary at all to use the immersion heater?

Thanks in advance!

 

[JohnD]

What colour is the cylinder?

You do not need to use the electric immersion heater unless the boiler breaks down. Energy from electricity costs about four times as much as energy from gas.

The boiler thermostat must be set higher than the cylinder thermostat.

How hot is the water from the taps?

Is your shower a thermostatic mixer?

 

[Johntheo5]

I'm after some advice for improving HW temperature in a semi-detached property. The property is running a 24kw regular boiler (boiler is less than 2 years old) supplying both CH and HW in a low-pressure vented arrangement.

I don't have a high demand of HW as there is only one shower bath in the house. Primary HW usage is for taking 1 or 2 showers in the afternoon/evening. I use a Tado system to control CH and HW. Every morning I turn on HW for about 30 minutes (aim is to replenish hot water consumed the night before), and HW is switched on again for 30 minutes before someone takes a shower in the afternoon/evening.

During shower, I barely need any cold water input from the non-thermostatic mixer tap as the HW is just enough hot (though my cylinder's thermostat is set at 60C). This makes me wonder if the cylinder is storing HW at a temperature hot enough to reduce health risk from Legionella.

The questions that I have are:
- should I drastically increase the boiler's flow temperature in order to make HW hotter? Currently I use the boiler's Auto function. I can see the boiler adjusting the flow temperature on its own and oftentime it is running at around 60C. Problem is the boiler doesn't seem to allow me to control what temperature is to be used for CH and HW separately.
- is my HW schedule not long enough to heat up the 120l cylinder? I tried heating the water for a longer time before shower but I didn't seem to get much hotter HW output.
- is it necessary at all to use the immersion heater?

Thanks in advance!

As above, boiler stat should be set to at least 65C if you want the cylinder stat to control the HW temp at 60C. 30 minutes seems very short to reheat the cylinder, it depends on the coil rating, probably not much more than 8kw which will only heat (assuming a 120L cylinder) the water from 10C to 39C. I would heat it for at least a hour, you won't wast any energy as the cylinder stat will shut the motorized zone valve.
What is the volume of your HW cylinder?

 

[gabx]

The outer coat of the cylinder is brownish/yellowish foam, definitely not modern looking.

I don't have a thermostatic mixer. When I turn the hot water valve of my manual mixer to max flow, it is not scalding hot. I believe the temperature is just around low 40ish C. I get the same temperature from the wash basin and the kitchen tap as well.

I will try cranking up the boiler's flow temperature manually to see if that will bring improvement.

Will follow your advice regarding immersion heater. Thanks

What colour is the cylinder?

You do not need to use the electric immersion heater unless the boiler breaks down. Energy from electricity costs about four times as much as energy from gas.

The boiler thermostat must be set higher than the cylinder thermostat.

How hot is the water from the taps?

Is your shower a thermostatic mixer?

 

[JonathanM]

Currently I use the boiler's Auto function. I can see the boiler adjusting the flow temperature on its own and oftentime it is running at around 60C

What boiler is it, make/model?

What do you mean by Auto function? Do you have weather compensation or load compensation?

 

[gabx]

What boiler is it, make/model?

What do you mean by Auto function? Do you have weather compensation or load compensation?

it's an Ariston Clas One R

Ariston Group

www.ariston.com

the user manual is not quite informative. i suspect the Auto function is more related to CH than to HW.

 

[gabx]

As above, boiler stat should be set to at least 65C if you want the cylinder stat to control the HW temp at 60C. 30 minutes seems very short to reheat the cylinder, it depends on the coil rating, probably not much more than 8kw which will only heat (assuming a 120L cylinder) the water from 10C to 39C. I would heat it for at least a hour, you won't wast any energy as the cylinder stat will shut the motorized zone valve.
What is the volume of your HW cylinder?

Thanks for the advice. It's a 120l cylinder. I will try lengthening the heating time to an hour in the morning.

 

[gabx]

One additional question:
the radiator inside my bathroom is connected to DHW instead of CH. I presume that means instead of using the boiler's full available power to heat the water inside the cylinder, some energy is diverted to heating the bathroom. is this setup common in older houses? thanks.

 

[JohnD]

The outer coat of the cylinder is brownish/yellowish foam, definitely not modern looking.

That's a very old one, maybe 30 years or more. The insulation is relatively thin, and the heat exchanger coil quite small.

Half an hour will be too short. To find out the reheat time, let it get fully cold, with the boiler off, then turn on the boiler to HW only.

When the cylinder reaches target temperature, it will turn the pump and the boiler off. Depending on the coil, around an hour, possibly more, especially in winter. The boiler may turn the flames on and off before then, if it puts out more heat than the cylinder can absorb.

Edit
You can add a red insulating jacket to the cylinder, and also foam lagging to all hot pipes. Yours are probably 22mm diameter. "Bylaws" grade is extra thick, and better if you have room for it, especially in lofts and unheated areas. The felt stuff is not much good, except for expansion creaks and ticks.

Search Lagging | Wickes.co.uk

Search results for Lagging on Wickes.co.uk

www.wickes.co.uk

 

[JohnD]

One additional question:
the radiator inside my bathroom is connected to DHW instead of CH. I presume that means instead of using the boiler's full available power to heat the water inside the cylinder, some energy is diverted to heating the bathroom. is this setup common in older houses? thanks.

It's not a problem. The boiler will have more than enough power for one radiator plus the cylinder. It's a good arrangement, because it will warm the radiator during your shower. Handy on cold mornings and drying your towels even if the CH is not on.

 

[ericmark]

My boiler DHW is not thermostatically controlled, no way to turn off, independent to CH, so in winter it is always piping hot. Summer however clearly don't want boiler cycling throughout the day, so would set it for ½ hour 4 times a week which was enough to have warm water to wash ones hands, but although set for ½ hour it would only run for 20 minutes as the hot coil is simply not good enough to sink the 20 kW from the boiler. Once pipe work is hot, it will start cycling around 50% on and 50% off slowly decreasing the on time as the cylinder warms up.

As to cost oil, gas, electric, the electric is the odd one out, and it can have a variable tariff, so although at 30p per kWh electric is a lot more expensive to oil or gas, at 9p per kWh it gets harder to work out which is the cheaper, as gas or oil also heats up pipework, and the boiler.

I now use solar to heat the DHW in the summer, I am not sure that's the cheapest, likely I get more for exports during the day than I pay during the night, but not much in it. The immersion heater only heats top of tank, so heating it during the day may be required, not done a summer yet.

As to oil v gas, most oil boilers have a fixed output, most gas have a variable output, so a gas boiler may only be heating using 6kW, depends on how much it can modulate, so to have showers likely it needs to run an hour a day, with an immersion heater your not heating any pipe work, so turning on at midnight when cheap rate starts and off at 8 am when it ends is a reasonable approach, but with a boiler you don't want the pipe work to stay hot for too long. Depends on length of pipe work as to losses running boiler in summer, winter any losses from pipe work is heating house so does not matter.

It would be an interesting study to find the losses from the pipe work and boiler, I know my pipework is not lagged, but in summer more interested in not over heating house, so would want to heat over night so house not being heated during the day, but with electric easier to work out how much energy is used, I get a report and at 9p a kWh is it really worth worrying about? I simply leave the iboost+ to auto turn on when ever there is spare solar, it looks after its self. It will vary what is used, but not really work worrying about.

With oil to dip tank to see how much oil is used in a week is not really accurate enough to work out what is costs. With gas one can actually see how much gas used in a week, or month, but if using for a shower it would seem likely it needs to run for a couple of hours a day, it will not be running flat out, so running for 2 hours does not mean using 4 times as much as running for an hour, it takes time for the heat to transfer from the hot coil.

 

[JonathanM]

the user manual is not quite informative. i suspect the Auto function is more related to CH than to HW.

I have read the manual. The boiler seems to have a built in weather compensation/load compensation mode called AUTO. How it works depends on which controls and sensors are fitted to it.

When you have the HW on by itself, without the CH, is the boiler temperature higher?

If you turn off AUTO, you can set the flow from the boiler by pressing the +/- buttons on the right of the display.

 

[gabx]

first thing in the morning i set the cylinder thermostat to 60C and let the boiler ran DHW on for a full hour (with CH switched off and boiler's Auto function on). after an hour, my boiler was still running at 70C and the cylinder thermostat did not seem to have kicked in.

water coming out from the tap was hotter than before (my gut feel was maybe 10C hotter) after an hour of boiler operation.

I then continued to run DHW on for another half an hour to see if the thermostat would kick in. After another 20 minutes the boiler finally stopped running.

Considering the cylinder was not emptied the night before, is 1 hour and 20 minutes of reheating time indicative of any problem in my system? I start wondering if my cylinder/boiler/insulation is too inefficient for DHW.

 

[Johntheo5]

So after say 1.5 hrs the cylinder stat is reaching 60C, if so and if the whole say effective 110L was heated from 25C to 60C then the cylinder coil is emitting < 2.0kw, something very wrong there, was the boiler cycling on and off very frequently?.
Check the cylinder coil return (bottom) pipe and see if it has a valve on it, either a wheel valve or a valve with a lever on it.

I think your boiler's minimum output is around 3.5kw so may have been cycling or borderline.

 

[JohnD]

Is any heat reaching the other radiators? If the 3-port valve is old and worn out, this can happen.

I don't know the limits of the heat exchanger coil in a yellow cylinder. If it was only 3kW, it would take about 2 hours to heat 120 litres. There might also be sludge in the pipes. There will be losses from poor insulation. Presumably it is fully pumped? Please show some photos of the pipes, cables and devices around the cylinder.

Have a look in the loft and see if the feed and expansion tank is hot.

P.s.
Buy some thermometers so you can accurately test the water.