radiator output

[phancey]

hi,

I'm trying to get to grips with radiator power ratings. I can't quite get the logic.

If I have a radiator that has flow of 75 and return of 55, that would seem to me to be outputting more heat into the room than one with flow of 75 and return of 65 (since it seems to be losing 20C over the length of the rad as opposed to 10C. Presumably that heat has gone into the room? So this would seem to be more powerful. Yet the calculations say look at the mean temperature and therefore the delta, so this results in the 75/65 combination getting a higher rating..... Please explain to someone with little knowledge of the science!

On the same thought process, losing 10C would be outputting the same power whether that is 75/65 or 60/50. Obviously I assume the flow rate would have to be significantly different in these 2 scenarios but ultimately the output would be the same (so says my simple mind).

And by this stage this question is probably irrelevant (once my previous statements have been blown out of the water) - but why would I not want to lose as much heat as possible into my room (ie as big a gap between flow and return as possible, especially since return temp will affect condensing). Is this all about pipe diameter, max flow rate etc?

thanks for any advice
Phil

 

[grahamderek]

For radiator outputs there are 2 delta T figures.
Water delta t is the difference in temp between flow and return.
The other delta T is the difference between radiator surface temp and room temp.
Radiator manufacturers are supposed to use industry standards, to allow you to compare different makes.
As boiler design changes, so do the standard tests, and there is no guarentee that every manufacturer is in fact using the same standards.
To try and answer your original question;-
flow 75 return 55 gives an average surface temp of 65.
flow 75 return 65 gives an average surface temp of 70, therefore more heat is given off.

 

[phancey]

yes it seems to make sense but if it gives off more heat then why has the other one lost more heat from the water? I'm sure it's obvious but I can't quite see it. Where has this extra 10C been lost to?

Also if a boiler quotes an output using a 20C gradient, what does this mean? Is it more efficient, less efficient, does it matter?

thanks

 

[grahamderek]

The only way to increase delta T across flow and return is to reduce the flow rate. This means that the water looses more heat energy, but over a longer time period, as it takes longer to pass through the radiator

 

[doormouse1]

I know noting about radiator temperatures, but I believe it works this way.

The hotter an object is, the more heat it will emit.
Since the heat capacity of liquid water is constant, the heat emitted will also be proportionate to the temperature drop of the water multiplied by the volume of water flowing through.
Since the temparature drop for the two senarios is the the same, it follows that the 75-65 drop radiator must have a higher flow rate

 

[D_Hailsham]

If I have a radiator that has flow of 75 and return of 55, that would seem to me to be outputting more heat into the room than one with flow of 75 and return of 65 (since it seems to be losing 20C over the length of the rad as opposed to 10C. Presumably that heat has gone into the room? So this would seem to be more powerful. Yet the calculations say look at the mean temperature and therefore the delta, so this results in the 75/65 combination getting a higher rating..... Please explain to someone with little knowledge of the science!

On the same thought process, losing 10C would be outputting the same power whether that is 75/65 or 60/50. Obviously I assume the flow rate would have to be significantly different in these 2 scenarios but ultimately the output would be the same (so says my simple mind).

The heat loss from a radiator depends on three factors: flow temperature, return temperature and room temperature. Manufacturers quote outputs for Flow=75°C, Return=65°C, Room=20°C. This gives a delta T of 50°C, i.e. (75+65)/2 - 20

Many radiator manufacturers have a table of conversion factors for different values of delta T; but they all assume that the only thing which has changed is the room temperature - flow and return temps are unchanged.

If you want to take all factors into account then the maths gets more complicated.

For example, this equation gives the enlargement factor for a radiator in a single pipe situation, where the flow and return temperatures reduce along the pipe run. So, if you have calculated that you need a 1kW radiator based on 75/65/20, but the actual temperatures are 50/35/20, then the equation will give a factor of 2.9. This means that you will have to install a 2.9kW radiator to give the same output as a 1kW one.

t1 = Flow Temp
t2 = Return Temp
tr = Room temperature
ln = log to base e
n = a factor depending on type of radiator ~ 1.3

View media item 2762
There is an online calculator at Engineering Toolbox for calculating the effects of temperature on radiator output.

why would I not want to lose as much heat as possible into my room (ie as big a gap between flow and return as possible, especially since return temp will affect condensing). Is this all about pipe diameter, max flow rate etc?

Its just a case of balancing the variables. If you extract more heat - lower return temp - you can run the pump at lower speed as the flow rate will be lower and the head will be lower; but you will have to use a larger boiler to replace the heat.

The rate at which a radiator loses heat, as shown by the temperature at the output, depends on the flow rate through the radiator; slow flow = high heat loss = low return temp. By adjusting the flow an undersized radiator can give out more heat or an oversized radiator can give out less heat. The balancing mantra of an 11°C drop is only true if the radiator is the correct size for the room.

 

[phancey]

thanks for this - ok I think I almost get it. Bigger temp drop requires lower flow rate which presumably can be done by either adjusting pump speed or balancing system by turning lockshield valves?

Another question then from a beginner - if the system is balanced to give a xC drop with all radiators on, what happens when some TRVs close as some rooms warm up? Does the pump speed change or does it mean that water is now flowing faster through the radiators that are still open resulting in a lower temp drop (or more likely is my understanding flawed!)?
Do variable speed pumps like Grundfos Alpha Pro change speed dynamically or does it just mean you can set the speed manually to a number of different points?

I am looking at Viessmann boilers which quote 20C gradients and are designed to work at 74/54 for example to ensure condensation.

Thanks for the advice.

 

[D_Hailsham]

Bigger temp drop requires lower flow rate which presumably can be done by either adjusting pump speed or balancing system by turning lockshield valves?

The pump speed is set to give the required temperature drop across the boiler. The lockshield valves are used to adjust the drop across the radiator. Adjustment of the pump is usually done with all radiator valves fully open, including the lockshields.

if the system is balanced to give a xC drop with all radiators on, what happens when some TRVs close as some rooms warm up? Does the pump speed change or does it mean that water is now flowing faster through the radiators that are still open resulting in a lower temp drop (or more likely is my understanding flawed!)?

When a TRV closes the head (pressure) increases. If the pump is a uncontrolled type, such as the Grundfos Selectric, the flow rate will reduce.

Do variable speed pumps like Grundfos Alpha Pro change speed dynamically or does it just mean you can set the speed manually to a number of different points?

It all depends on how you set them! The Grundfos Alpha can be either set to one of three fixed speeds (as on the Selectric) or set to adjust the speed dynamically according to the conditions met.

If you go to Grundfos and open the Alpha2 Sales leaflet pdf file about halfway down the page, you will get a good idea how the latest pump works.

I am looking at Viessmann boilers which quote 20C gradients and are designed to work at 74/54 for example to ensure condensation.

According to the online calculator at Engineering Tool book, radiators set to work at 74/54 will only produce about 84% of their rated output. In other words you will have to install rads about 20% larger than that calculated by a heatloss calculator.

 

[phancey]

I understand most of that

Not sure about the "uncontrolled type" - what are the alternatives/differences?

ok, speed does not equal flow rate...? As trvs close, the pressure increases so it is harder for the pump to push the water around so the flow rate reduces.... which means the temperature drop across open radiators becomes bigger as trvs close down on others?

Only an adaptable one like Alpha2 will increase the speed to keep the same flow rate? (why would you not set this to adaptable? What would be the advantage of not doing this?) And does all this mean that since in most cases the latest adaptable speed pumps are not in place, then temperature drops increase as TRVs close making the whole output rating thing very difficult?

Or is this what an automatic bypass valve does - to keep the flow rate the same however many trvs shut down? In which case, why would you want to adapt your pump speed by increasing it and therefore presumably increasing the power usage?

yours slightly confused!

 

[D_Hailsham]

I understand most of that

Not sure about the "uncontrolled type" - what are the alternatives/differences?

Did you read the Grundfos Alpha2 Sales Leaflet. It explains everything in reasonably well.

ok, speed does not equal flow rate...? As trvs close, the pressure increases so it is harder for the pump to push the water around so the flow rate reduces

That's correct

which means the temperature drop across open radiators becomes bigger as trvs close down on others?

That is not necessarily true. If you look at a TRV valve you will find that the opening through which the water flows is considerably smaller than a 15mm pipe. The lockshield valve will probably be open about a ¼ or ½ turn. This means that the pressure drop across the radiator, which determines the flow rate, is virtually constant, so the temperature drop does not change significantly.

Only an adaptable one like Alpha2 will increase the speed to keep the same flow rate? (why would you not set this to adaptable? What would be the advantage of not doing this?)

In most cases the pump will work best when set to adjust automatically. But there are always exceptions.

And does all this mean that since in most cases the latest adaptable speed pumps are not in place, then temperature drops increase as TRVs close making the whole output rating thing very difficult?

No. See above

Or is this what an automatic bypass valve does - to keep the flow rate the same however many trvs shut down?

The purpose of an ABV is to ensure that there is a minimum flow through the boiler at all times. It does not ensure that the flow through the radiators is constant.

In which case, why would you want to adapt your pump speed by increasing it and therefore presumably increasing the power usage?

The effect is exactly the opposite. The power usage over a year will be about a quarter of that with a fixed speed pump.

 

[phancey]

Did you read the Grundfos Alpha2 Sales Leaflet. It explains everything in reasonably well.

Yes but I'm still an idiot so it doesn't necessarily make it clear to me! There is some technical stuff there that assumes a certain understanding.

That is not necessarily true. If you look at a TRV valve you will find that the opening through which the water flows is considerably smaller than a 15mm pipe. The lockshield valve will probably be open about a ¼ or ½ turn. This means that the pressure drop across the radiator, which determines the flow rate, is virtually constant, so the temperature drop does not change significantly.

Back to basics for me. If the pressure drop remains constant then where does the increase in pressure on the pump come from (which we already established earlier)?

The effect is exactly the opposite. The power usage over a year will be about a quarter of that with a fixed speed pump.

This bit I can't figure out. If you set the pump to pump at a certain flow rate with all the trvs open (and presumably that is the speed you would set it at for a fixed speed too), then when the pressure increases and the flow rate reduces, doesn't the pump need to speed up to increase the flow rate?

almost there now!

thanks for your time. It is invaluable.

 

[D_Hailsham]

There is some technical stuff [in the Grundfos Alpha2 Sales Leaflet] that assumes a certain understanding.

Which bits?

If the pressure drop remains constant then where does the increase in pressure on the pump come from (which we already established earlier)?

The increase in pressure at the pump is caused by a TRV closing. However the differential pressure across a radiator stays almost constant; and it's this which determines the flow rate through the rad.

If you set the pump to pump at a certain flow rate with all the trvs open (and presumably that is the speed you would set it at for a fixed speed too), then when the pressure increases and the flow rate reduces, doesn't the pump need to speed up to increase the flow rate?

You are right in thinking that the automatic pump will achieve the same flow rate as a fixed speed one when all TRVs are open.

When a TRV closes down there is an increased pressure but there is also a reduction in the amount of heat required - one rad is temporarily out of use - so the flow rate can also reduce. If you have a boiler which requires a minimum flow rate, i.e low water content boilers, the ABV will open to provide a route back to the boiler and the pump will see no change in the pressure. Modulating boilers will adjust their output to match the requirements.