Setting Up Auotamtic Bypass Valve and Pump Curves

[heating12numpty12]

Thank you for the flow rate stuff. So is this right?

Boiler running on min power and 10 C differential = 5.7Kw/4.18x10 = 490 litre/hr

So this means the boiler is running outside the minimum specified flow rate, manf quotes 1200 litre/hr

 

[Steelmasons]

Boiler running on min power and 10 C differential = 5.7Kw/4.18x10 = 490 litre/hr

Learn to walk before you run.

 

[D_Hailsham]

So is this right?

Boiler running on min power and 10 C differential = 5.7Kw/4.18x10 = 490 litre/hr

So this means the boiler is running outside the minimum specified flow rate, manf quotes 1200 litre/hr

You calculation is correct, but you are missing the point.

The point of the minimum flow rate is to ensure that the temperature differential across the heat exchanger does not exceed 20°C.

At Max output and 20°C differential, this needs to be 1200 litres/hr. But at Minimum (5.7kW) and a 20°C differential you only need a flow rate of 245 litre/hr.

If you maintained 1200 litres/hr at Min (5.7kW), the temperature differential would be only 4°C.

 

[picasso]

Untitled

• picasso
• 8 Feb 2015
• 1

could not resist

 

[D_Hailsham]

 

[heating12numpty12]

I think I have just had a light bulb moment, based on my now understanding of the mathematics.

With old boilers their heat output was constant, so the temperature differential across the boiler was always the same.

However, with a condensation boiler once the boiler goes into modulation mode, the power output reduces, but this results in the temperature differential across the boiler also reducing.

So how do you balance a heating system with a condensation boiler, it sounds like you can only measure the temperature differential when the boiler is running at full load (before modulation). Have I got this right???

 

[D_Hailsham]

I think I have just had a light bulb moment, based on my now understanding of the mathematics.

Here's a few more - in case you need them!

So how do you balance a heating system with a condensation boiler, it sounds like you can only measure the temperature differential when the boiler is running at full load (before modulation). Have I got this right???

Almost.


The equation given earlier also applies to radiators but, unlike a boiler, the actual temperatures are important. That's why the rad manufacturers' tell you the temperatures used to measure the output. BS EN 442 is the relevant British Standard.

Now the purpose of balancing is to ensure that each rad gets the correct amount of heat, i.e. the flow is shared correctly. That means the temperature differential across all rads is the same.

So it's not a case of the boiler running at full output but at an output which will maintain the required flow temperature (75°C). The flow temperature is set on the boiler, which then modulate to maintain that temperature.

Obviously you don't want the boiler turning on and off unnecessarily, so it's advisable to set the room stat to a high temperature which keeps the boiler running.

 

[heating12numpty12]

Thanks for that. So a differential of 12 C is fine across the system, with the boiler in modulation mode, as long as all the rads have 12 C, right?

What's the best way/procedure for me to balance the system?

I am aware that the return to boiler must be below 54C such that condensation occurs.

 

[D_Hailsham]

Thanks for that. So a differential of 12 C is fine across the system, with the boiler in modulation mode, as long as all the rads have 12 C, right?

All modulation does is to adjust the boiler output so the flow temperature maintains the set value. If you set this to 75C, then a 12C differential at the boiler and across all rads is fine.

I am aware that the return to boiler must be below 54C such that condensation occurs.

True, but it will be difficult for you to achieve that and have the house warm enough.

How to balance the system

1. Close all LS valves; then open them 1/3rd of a turn
2. Remove all TRV heads and set all wheel valves (other end of rad from LS) to fully open
3. Set the boiler temperature to at least 75C
4. Set pump to speed 2
5. Set room stat high (so boiler does not go on and off)
6. Let the system warm up
7. Measure the temperature difference, between flow and return, at the boiler and at each rad. It should be above 10c and less than 20c
8. If difference is much smaller, set pump to speed 1; and vice versa.
9.If no noticeable difference, (±1C) go to step 13
10. Go to the rad with the greatest difference (i.e coldest return) and open the LS valve opened very slightly (1/12th turn or less).
11. Wait 5-10 mins for the system to settle down.
12. Go to step 7
13. Stop
14. Set room stat to usual temperature
15. Replace TRV heads and set wheel valves to required position.

Don't aim for perfection, Provided the temperatures differences are all approximately the same at the boiler and each rad it doesn't matter what the exact difference is. However, the the lower the temperatures and greater the difference the more the rad output reduces. A nominal 1kW rad will produce 850W with a 75C flow and 55C return and only 750W with a 65C flow and 55C return.

 

[heating12numpty12]

Just to say thanks for all your help. I now have the information about the system and to balance the system.

 

[heating12numpty12]

Been doing some setting up and wanted to check my settings.

Pump speed set to: 40 Watts 4.4 M Head and from the curves 1.5m3/h

Boiler set to 25KW. (23KW Radiators and 2KW DHW)

I have recalculated my radiators to 23KW (didn't notice the difference between type 21 and 22.

The boiler on switch on this morning was a differential of 18C for the first 20 minutes.

Therefore the Flow calculates at: 25/(4.18x18C) = 1196 L /Hr

The manufacturers spec for the boiler is 1200 L/Hr, therefore should I increase the pump speed another 5 Watts/0.5 M Head to be on the save side?

I still have to make some minor adjustments to the radiators and close down some radiators and I have 1 radiator that is not as hot as the other 16.

 

[Agile]

Dont touch the pump! Otherwise you will reduce the differential and lower the efficiency.

The minimum flow rate quoted is based on the maximum output power and the requirement to not exceed a differential of 20 C.

The 25 kW is the MAXIMUM power the boiler will provide.

When the flow temp approaches the set figure then the power output will reduce as the boiler modulates back.

Tony

 

[D_Hailsham]

Pump speed set to: 40 Watts 4.4 M Head and from the curves 1.5m3/h

Is that with the pump set to Constant?

I have recalculated my radiators to 23KW (didn't notice the difference between type 21 and 22.

Boiler set to 25KW. (23KW Radiators and 2KW DHW)

That should be OK

The boiler on switch on this morning was a differential of 18C for the first 20 minutes.

What were the actual flow and return temperatures when the differential was 18C?

Therefore the Flow calculates at: 25/(4.18x18C) = 1196 L/Hr

You are assuming that the boiler was producing 25kW when you read the temperatures. You can't make that assumption. Furthermore, that is at odds with your estimate that the pump is delivering 1500 litres/hr.

If the pump is delivering a constant pressure of 4m and the ABV is set to 2m, the ABV will be permanently open.

The manufacturers spec for the boiler is 1200 L/Hr, therefore should I increase the pump speed another 5 Watts/0.5 M Head to be on the save side?

No! I thought I had explained the significance of the flow rate.

Your boiler now produces a maximum output of 25kW. So, with a 20C maximum differential the flow rate must not be less than 1076 l/hr. You are above that, so no worries.

 

[heating12numpty12]

Hi

The flow temperatures for the 18 C differential were 60 C and 42 C.

Based on previous comments the pump is now in constant mode.

The ABV is currently set to 0.3, and I am fairly certain that its closed.

You are assuming that the boiler was producing 25kW when you read the temperatures. You can't make that assumption. Furthermore, that is at odds with your estimate that the pump is delivering 1500 litres/hr.

Just to confirm, with the boiler set to 25Kw does that not mean that the maximum output of the boiler is 25KW?

 

[D_Hailsham]

The flow temperatures for the 18 C differential were 60 C and 42 C.

With those temperatures your 23kW of rads will be producing only 12kW. The boiler will have modulated down to produce the required output so the flow rate will be approx 575 litres/min.

You say the pump is using 40W with a 4.4m head. Was that when you measured the temperatures?

You estimated the flow rate as 1500 litre/min. With an 18C differential this would mean a boiler output of 31.35kW, which is more than the boiler can produce! At 25kW with an 18C differential the flow rate is only 1200 litre/min

The ABV is currently set to 0.3, and I am fairly certain that its closed.

What happens if it is on the highest setting?

Just to confirm, with the boiler set to 25Kw does that not mean that the maximum output of the boiler is 25KW?

Yes, that is the maximum, but you can't assume it is producing the maximum.

Have you tried balancing the system? You must have a flow temp of about 75C when you do