Increasing radiator output and improving CH performance

[ajrobb]

The purpose of balancing is not reduce the flow rate and lower the return temp but to ensure that the correct flows rate and return temperatures are obtained.

But, if you start with all the LSVs open (as in this case), and the pump turned up to try and get cold radiators hot, balancing will reduce the boiler return temperature and allow flow rate to be reduced. I can see that boilers can need a minimum flow rate, so DIYers shouldn't take it too far and should be wary of turning down the pump speed. I suspect that boiler manufacturers' instructions would include a maximum temperature differential across the boiler that corresponds to minimum specified flow rate at full chat.

Quarter turns is a large amount.

Those were the instructions in one TRV commissioning guide (another thread).

That will only be true if the flow rate decreases as the feed temperature increases.

I disagree. As the boiler is turned up to cope with cold weather, the heat output from radiators not on TRVs will increase. Assuming constant flow rate, increased heat lost from the water must increase temperature drop.

I'm getting the impression that 'proper' commissioning is more complicated than balancing radiators to have 11°C temperature drops if you need to ensure a minimum flow through the boiler. On a small system, this flow should be spread between all the radiators according to their nominal outputs. This might require 'higher than designed' radiator flow rates and, consequently, reduced temperature drops. Otherwise, an ABV will have to supply the extra flow rate, even with all the TRVs open.

However, even crude DIY balancing can fix the problem of larger and/or more remote radiators not heating up properly. The main domestic requirement is to get all the radiators hot as quickly as possible. To this end, I suggest at least one radiator (most difficult to heat) should have its LSV wide open. All the other radiators should have their LSVs adjusted to match the temperature drop of this 'most difficult' radiator. Finally, if the temperature drop across the most difficult radiator is more than 11°C, the pump speed can be increased.

 

[MJN]

I can see that boilers can need a minimum flow rate, so DIYers shouldn't take it too far and should be wary of turning down the pump speed.

Whether you're being paid for it or not is irrelevent. Whoever is configuring the setup ought to know what they're doing.

I suspect that boiler manufacturers' instructions would include a maximum temperature differential across the boiler that corresponds to minimum specified flow rate at full chat.

Yes, an example being the Ideal Icos which aims for a target differential of 11°C when balanced (and TRVs open), and a maximum 20°C differential with all TRVs, where fitted, closed.

I'm getting the impression that 'proper' commissioning is more complicated than balancing radiators to have 11°C temperature drops if you need to ensure a minimum flow through the boiler.

They're not mutually exclusive, and indeed their are benefits even if you don't reach the target drops. The goal is more ensuring balance whilst aiming for this drop and, whatever else, ensuring you end up below the maximum permissable at the boiler.

On a small system, this flow should be spread between all the radiators according to their nominal outputs.

Yes, that's exactly what balancing achieves. The require flow is proportional to the specified heat output.

This might require 'higher than designed' radiator flow rates and, consequently, reduced temperature drops.

I don't follow this, at least assuming your boiler is correctly sized for the system.

Otherwise, an ABV will have to supply the extra flow rate, even with all the TRVs open.

If that's the case then the boiler is over-sized and will operate inefficiently.

However, even crude DIY balancing can fix the problem of larger and/or more remote radiators not heating up properly.

Absolutely, and I dare say that's most people's definition of balancing i.e. closing off a couple of rads near the pump by an arbitrary amount until those further away start warming up.

Mathew

 

[ajrobb]

If that's the case then the boiler is over-sized and will operate inefficiently.

At the risk of going off topic, combination boilers in small homes are usually over-sized for central heating. Those boilers still run at full chat for a few minutes to get the feed temperature up. The total flow rate through the radiators should be increased to allow this.

 

[MJN]

At the risk of going off topic, combination boilers in small homes are usually over-sized for central heating.

That's an inevitable consequence of the compromised design with a basic combi - the power needed to provide instantaneously-heated water is signficantly higher than that required to satisfy the central heating requirements.

Those boilers still run at full chat for a few minutes to get the feed temperature up.

They ought to start from a low output and modulate up as required. Whilst they may have a high maximum output they usually have a wide modulation range hence can at least start low enough.

The total flow rate through the radiators should be increased to allow this.

You balance the system in exactly the same way as any other boiler and aim for the drop specified by the manufacturers. The exception to this being that you usually cannot adjust the pump speed as this is often mandated by them to be left at maximum.

Mathew

 

[D_Hailsham]

But, if you start with all the LSVs open (as in this case), and the pump turned up to try and get cold radiators hot, balancing will reduce the boiler return temperature and allow flow rate to be reduced.

I agree with that. The problem is that you are starting from the wrong place! You should balance from a know position, i.e all LSV's closed. THe problem being that, due to design, most LSVs are fully open when they are one to one and a half turns open.

I can see that boilers can need a minimum flow rate, so DIYers shouldn't take it too far and should be wary of turning down the pump speed. I suspect that boiler manufacturers' instructions would include a maximum temperature differential across the boiler that corresponds to minimum specified flow rate at full chat.

That's why you initially adjust the pump with all rads full open to make sure you can.

Yes, the flow rate and temp diff are mathematically related.

That will only be true if the flow rate decreases as the feed temperature increases.

I disagree. As the boiler is turned up to cope with cold weather, the heat output from radiators not on TRVs will increase. Assuming constant flow rate, increased heat lost from the water must increase temperature drop.

What you have forgotten is that the heat output of a radiator is not constant; it varies with flow, return and room temperature.

I'm getting the impression that 'proper' commissioning is more complicated than balancing radiators to have 11°C temperature drops if you need to ensure a minimum flow through the boiler. On a small system, this flow should be spread between all the radiators according to their nominal outputs. This might require 'higher than designed' radiator flow rates and, consequently, reduced temperature drops. Otherwise, an ABV will have to supply the extra flow rate, even with all the TRVs open.

You don't balance to meet a set temperature differential, but so that the differentials across each rad is the same as that across the boiler. If the boiler is designed for a 20C differential, this has to be taken into account when sizing the radiators.

 

[ajrobb]

What you have forgotten is that the heat output of a radiator is not constant; it varies with flow, return and room temperature.

No, that is the point. Once warm, room temperature is supposed to be constant. As a first approximation, with constant flow rate and room temperature, radiator heat output is proportional to the difference between flow temperature and room temperature. It is also proportional to the difference between flow temperature and radiator return temperature. Therefore, increasing flow temperature increases temperature difference between radiator flow and return.

 

[D_Hailsham]

Once warm, room temperature is supposed to be constant. As a first approximation, with constant flow rate and room temperature, radiator heat output is proportional to the difference between flow temperature and room temperature. It is also proportional to the difference between flow temperature and radiator return temperature. Therefore, increasing flow temperature increases temperature difference between radiator flow and return.

I would agree that the output is approximately proportional to the difference between the room temperature and the mean temperature of the water, i.e. (Flow + Return)/2.

We have now arrived at the interesting fact that, if the purpose of balancing is to obtain the correct flow through each radiator, we can't just say that the differential has to be 11C, we have to take the flow temperature into account. So you get the following, which shows how the required temperature differential changes with flow temperatures to achieve the same flow rate.

[code:1]
Temperature
Flow Diff
75 11.0
70 9.73
65 8.5
60 7.3
55 6.15
50 5.04
45 3.97
40 2.98
35 2.05
30 1.21
[/code:1]