Help balancing all rads in my system - 2 not playing ball.

[mikeyw]

Guys,

I’ve recently had a go at balancing my gas central heating system with mixed results. My house is a 4 bed detached and has around 12 rads in the house. I used a digital thermometer and achieved a drop off of around 10-11C across nearly all the rads.

The boiler is outputting water at around 81C and showing 62C on the return.

Most of the rads are running about 80C on the input and roughly 70C on the output however I have 2 upstairs bedroom ones that are struggling and I don’t understand why. They are getting about 77-78C into the rad but the output falls towards lows 50C’s on both. Even having the flow fully open I cannot get this much higher.

Any ideas what could be causing this and if there is anything I can do ?

Also a plumber has somehow managed to install a couple of the rads with the Thermostatic valves on the wrong side ie on the return and not the input…obviously this screw up the temp control a bit but apart from that would this cause any other problem ?

Thanks in advance,
Mike.

 

[D_Hailsham]

I ... achieved a drop of around 10-11C across nearly all the rads.

The boiler is outputting water at around 81C and showing 62C on the return.

Which boiler do you have - make and model number?

Most of the rads are running about 80C on the input and roughly 70C on the output however I have 2 upstairs bedroom ones that are struggling and I don’t understand why. They are getting about 77-78C into the rad but the output falls towards lows 50C’s on both. Even having the flow fully open I cannot get this much higher.

Why are you balancing to 11C when the boiler differential is 19C?

Also a plumber has somehow managed to install a couple of the rads with the Thermostatic valves on the wrong side ie on the return and not the input…obviously this screw up the temp control a bit but apart from that would this cause any other problem?

It will not screw up the temp control.

Do you have TRVs on most rads?
Did you balance with the TRV heads on?

 

[mikeyw]

Thanks for the reply D_Halisham

The boiler is a Vokera Compact but i dont' have the model number to hand.

I was told by a neighbour that the optimum was 11C across a rad....i didn't appreciate it should be linked to the boiler differential.

Does this mean I should be working on an 19c differential on the rads on trying to somehow reduce the differential on the boiler ?

Won't the TRV's that are fitted on the wrong side be working on a lower temp setting ? ie the colder return pipe ?

Yes all the Rads bar 1 or 2 have TRV's and they were all set fully open when balancing.

Thanks again,
Mike.

 

[D_Hailsham]

The boiler is a Vokera Compact but i dont' have the model number to hand.

I was told by a neighbour that the optimum was 11C across a rad....i didn't appreciate it should be linked to the boiler differential.

Both the Compact SE and Compact HE are designed for a 20C differential - 19C is near enough, it's not that critical!

Does this mean I should be working on an 19c differential on the rads on trying to somehow reduce the differential on the boiler ?

Yes you should use the boiler differential when adjusting the rads. But as you will not be able to reduce the boiler differential, adjusting the LS valves to reduce flow is the only way to go.

However a radiator running with a 19C differential will give out about 10% less heat than one with an 11C differential. But, as most rads are oversized, this should not be noticeable.

Won't the TRV's that are fitted on the wrong side be working on a lower temp setting ? ie the colder return pipe?

A TRV works on the air temperature, not the water temperature.

Yes all the Rads bar 1 or 2 have TRV's and they were all set fully open when balancing.

That's good; but it's better to remove the heads when you balance to prevent any possibility of a TRV closing during the process.

Here is my Balancing Procedure

 

[mikeyw]

Thanks again you've been very helpful

Can I ask a couple more questions please :-

1) What dictates the boiler flow differential ? - is it pre-determined by the boiler manufacturer ?...will it always reamin the same regardless of the flow temperature setting on the boiler ?....ie if I drop the output temp to 65C would the return be roughly 45C.....are boilers that have a 10C differential better / more efficient ?

2) So if i've got a differential of 10C across most of my rads on a system designed for 20C am i simply wasting energy ? - or is it a case that i'll get very hot rads in the main but the last couple will never get up to full temp as i'm seeing ? - effectively is the low diff on the rads causing the last 2 to struggle ?...i imight try a 15C diff see if that evens things up a bit but if you think it'll make no difference over the 20C drop i'll do that instead.

3) Is it best to have the boiler running at the boilers max temp and control the rads on TRV or is it more energy effecient to have the rads operating at lower temp but left on longer - obviously the with the rip-off gas prices at the moment i'd like to think i'm using the most cost effective method.

Thanks again for your informative and helpful answers.

 

[mikeyw]

Any thoughts on the last post guys ?

 

[Ltnman]

I always thought the big thing for combi efficiency was a return temperature of 57C or below into the boiler so the boiler actually condenses. With that in mind is it not irrelevant whether the drop is 11C or 19C?

 

[mikeyw]

Thanks - how do you go about getting the temp to drop to 57C ?

 

[D_Hailsham]

Sorry I wasn't around yesterday to reply to this.

1) What dictates the boiler flow differential ? - is it pre-determined by the boiler manufacturer ?...will it always remain the same regardless of the flow temperature setting on the boiler ?....ie if I drop the output temp to 65°C would the return be roughly 45°C.....are boilers that have a 10°C differential better / more efficient ?

The manufacturer will determine the parameters. One of the factors which determines the differential is the water flow rate, which is determined by the pump speed. The heat generated by a boiler is given by the equation:

kW= flow x temp difference x 4.2.

Where flow is in litres/second and temp difference in degrees Celsius.

You can see from this that it is not the actual flow and return temperatures which are important but the difference between them. Some manufacturers still produce boilers which are designed for an 11°C differential, presumably because that was the design temperature of most older pre-existing installations. However, as ltnman says, the return temperature has to be about 57°C - it's not a fixed temperature as it depends on many factors - for condensing to occur. Assuming that the flow temperature is 75°C, a differential of 20°C would give a return of 55°C, which should guarantee condensing when the boiler was running flat out.

However, while boiler depends on temperature difference, the output of a radiator is dependent on the actual flow and return temperatures and on the temperature of the air surrounding the radiator.

Radiators are specified to meet a worst-case condition with outside temperature of -1°C and a room temperature of, say, 21°C. But that does not mean that a 2kW radiator has to give out 2kW all the time. If the outside temperature is 10°C the radiator will only have to give out 1kW to keep the internal temperature at 21°C.

There are several ways of doing this. The boiler could be kept at max output and a TRV on the rad would open and shut to keep the room temperature constant. But the boiler would be cycling on-off all the time as the return temperature would be very close to the flow temperature.

Modern boilers will modulate - vary their output to meet the current requirements - the do not continually cycle on and off. Some boilers have weather compensation, so they can measure the inside and outside temperatures and adjust the boiler output to give the required heat.

If I've got a differential of 10C across most of my rads on a system designed for 20C am i simply wasting energy ? - or is it a case that i'll get very hot rads in the main but the last couple will never get up to full temp as I'm seeing ? - effectively is the low diff on the rads causing the last 2 to struggle ?...I might try a 15C diff see if that evens things up a bit but if you think it'll make no difference over the 20C drop I'll do that instead.

Because the temperature difference is only 11°C, each radiator is giving out about 15% more heat than one running with a 20°C differential. Now the maximum output of the boiler is fixed so, if early radiator are using up 15% more heat, there will be nothing left for the rads at the end of the line.

You need to increase the differential, a few degrees at a time, until all rads are working with the same differential. If the flow temperature drops significantly between the first and last rads, due to heat losses in the pipe work, you could reduce the differential slightly, say so it is 20°C at the beginning and 17°C or 18°C at the end rads. If you want to see how rad outputs vary with temperature go to: Heat emission from radiators

Is it best to have the boiler running at the boilers max temp and control the rads on TRV or is it more energy efficient to have the rads operating at lower temp but left on longer

I prefer to run the boiler at max temperature and let the controls do their work.

I hope you are not relying on TRVs by themselves but have a room thermostat. A TRV will not turn the boiler off when the house is up to temperature.

 

[D_Hailsham]

Thanks - how do you go about getting the temp to drop to 57C ?

Missed this one!

Although I said it is probably best to run the boiler at Max temp, there is nothing to stop you running at a lower temperature. The Vokera Compact has a temperature control, but it is hidden. You have to remove the casing to access it. Info can be found in the installation manuals at http://www.vokera.co.uk/productinfo...ers&id={7F682E9E-0C3C-4153-BBE7-45B06195D769}

There is an "auto" position, which is supposed to alter the flow temperature to meet the requirements. I suspect that your installer has left the dial set to max. If you want to experiment, reduce it so it is only 70-75°C or in the auto range.

 

[Ltnman]

The problem I found with this magic 20C drop across a radiator is that to achieve it my radiators have to be set to pass almost no water, which means they take ages to warm up. As a compromise my boiler is set for 68C which returns water into the boiler between 48C and 54C.

Because the temperature difference is only 11°C, each radiator is giving out about 15% more heat than one running with a 20°C differential. Now the maximum output of the boiler is fixed so, if early radiator are using up 15% more heat, there will be nothing left for the rads at the end of the line.

Is that not assuming that the boiler has nothing in reserve. I thought most boilers are oversized for the job in hand.

 

[curium]

Because the temperature difference is only 11°C, each radiator is giving out about 15% more heat than one running with a 20°C differential. Now the maximum output of the boiler is fixed so, if early radiator are using up 15% more heat, there will be nothing left for the rads at the end of the line.

I'm no expert so don't interpret this as a criticism I'm merely asking to clarify.

If a radiator drops 11°C then it dissapates less heat energy across its surface than a radiator which drops 20°C. Am I correct?

If so, then the rad which drops 20°C uses up more heat making it very difficult for the the rads at the end of the line to achieve the same differential.

I'm asking because i had difficulty agreeing with your quote.

 

[Dan Robinson]

If so, then the rad which drops 20°C uses up more heat making it very difficult for the the rads at the end of the line to achieve the same differential.

Parallel circuit dear boy. It doesn't work that way.

 

[D_Hailsham]

The problem I found with this magic 20C drop across a radiator is that to achieve it my radiators have to be set to pass almost no water, which means they take ages to warm up. As a compromise my boiler is set for 68C which returns water into the boiler between 48C and 54C.

It's all theoretical!

The real problem is that the temperature drop across a radiator will vary with conditions. Let's assume that the flow temperature is constant and the radiator has been properly sized at 2kW to give a 21C room temperature when the outside temp is -1C and that the rad has been balanced to give a return temp is 20C below flow. Standard stuff, so far.

Now lets take the situation where the outside temperature is 10C (mid point between -1C and 21C). To achieve a 21C room temperature we only need an output from the 2kW radiator of 1kW. To get a nominal 2kW radiator to give out only 1kW heat would require a flow/return differential of about 50C. We could also achieve the 1kW output by reducing the flow temperature by about 20C, in which case the differential would still be 20C. Modulating boiler would automatically adjust their flow temperature to keep the differential constant - within their modulation range.

Balancing is therefore not an exact science and is hard to do properly - which is why its often not done at all. Balancing to achieve the same temperature drop across all radiators as across the flow and return pipes of the boiler should be good enough, irrespective of what the temperature is.

One final thought on balancing. Because radiator output partly depends on the differential, smaller differential = greater output and vice versa, if we have a radiator which is oversized for the room we can reduce the radiators output by increasing the differential, and vice versa. There is, however, a limit to this. Reducing the differential to 1C will increase the radiator output by about 30%; and increasing the differential so the return is 1C above room temperature will reduce the output by 80%. So you can reduce output more than you can increase it.

Because the temperature difference is only 11°C, each radiator is giving out about 15% more heat than one running with a 20°C differential. Now the maximum output of the boiler is fixed so, if early radiator are using up 15% more heat, there will be nothing left for the rads at the end of the line.

Is that not assuming that the boiler has nothing in reserve. I thought most boilers are oversized for the job in hand.

Yes but, even with something theoretically in reserve, there could come a time when that reserve has been used up; and the same problem will arise.

 

[Ltnman]

Sorry for hijacking the original post but I found your replies very interesting and informative. I was going to have another go at trying to archive a 20C drop across more of my radiators but it took me weeks to get where I am today so I am staying as I am.

The whole point of me originally having a go at balancing my system was the fact that for 19 years I lived with a lounge radiator that only ever went luke warm at the top. Thanks to this forum it really kicks out the heat now so thank you everyone who contributes to this great forum.