combi-boiler - condensing mode?

[Arthurx564]

i have a worcester bosch junior 24i combi boiler fitted with a honeywell DU145 automatic by-pass valve. What i wish to know is when is the boiler operating most efficiently? I believe that this occurs when the differential central heating water is above 20degC, ie the difference between the feed and return temps. I have taken temp measurements on my system and I only see a maximum difference of 15degC (see attached plot). Is this what I should expect or should the differential temp be higher and for a longer period?

How do I know that the by-pass valve is set up correctly, as it seems to allow hot feed water seep through the valve at all time, ie when all the TRV are fully open (the radiator in the hallway has no TRV)!

Untitled

• Arthurx564
• 5 Jan 2011
• 2

 

[Agile]

Whilst you do some nice plots, I dont know where you get those odd ideas from.

Merely reading a few postings on here would quickly tell you that a boiler starts condensing when the RETURN goes below about 55 C.

Dont get too excited as the increase in efficiency is only about 3-6% under normally achieved parameters.

Tony Glazier

 

[GaytonTonner]

The lower the return temperature of a condensing boiler the greater the efficiency. If you look at a gas condensing boiler graph, the boiler starts to condense at around 53C return water temperature - it could be lower. That is the key point. Just above and below doesn't make that much difference in efficiency. Once the return temperature drops to 30C the efficiency rises sharply.

At 65C the efficiency is 87%
At 53C (dew-point) the efficiency is 89%
At 30C the efficiency is 97%
At 25C the efficiency is 98-99%
At 20C the efficiency is near 100%

So 10% more efficient using lower return temperatures.

Look at this:
http://www.keston.co.uk/downloads/pdf/cel25-b.pdf
There is a graph here about half way down (the boiler is now discontinued)

The boiler's heat exchanger has a makers maximum for the delta T. Stick inside it or you could have a cracked heat X.

The auto by-pass valve should not be passing when all TRVs are open. It may have crud on the seating (very common). If you are replacing the valve also install a Magnabooster filter on the return pipe as this will grab crud.

 

[bengasman]

As Tony said, only point of interest is the return temp.
System will work best with bypass closed.
Ignore advice from GT, he is a troll, not a pro and has been banned many times

 

[mysteryman]

Condensation BEGINS when the return is below 53C; it is fully condensing when the flow is below 53C. Weather compensation will achieve this automatically for as much of the heating season as possible - about 95% of it. A bypass will raise the return temperature; you should have one radiator on lockshield valves as the bypass.

 

[GaytonTonner]

Condensation BEGINS when the return is below 53C; it is fully condensing when the flow is below 53C.

It doesn't. That is the flue gas temperature dew point. The return water temp is higher.

Weather compensation will achieve this automatically for as much of the heating season as possible - about 95% of it. A bypass will raise the return temperature; you should have one radiator on lockshield valves as the bypass.

Yep.

 

[GaytonTonner]

As Tony said, only point of interest is the return temp.
System will work best with bypass closed.
Ignore advice from GT, he is a troll, not a pro and has been banned many times

I am no troll, despite what some uninformed people say. I gave the data.

 

[mysteryman]

The 'flue gas temperature dew point' is the maximum temperature at which condensation occurs. The flow needs to be below this temperature for full condensation to occur.

 

[GaytonTonner]

The 'flue gas temperature dew point' is the maximum temperature at which condensation occurs. The flow needs to be below this temperature for full condensation to occur.

Sorry, you are right. I had it reversed.

 

[Agile]

I am no troll, despite what some uninformed people say. I gave the data.

If you were a genuine person then why is it that no one from this site has ever met you?

You also fell for the mistake given above when you quoted it and supported what it said!

All radiators should now have TRVs and all systems an auto bypass valve. The days of a rad with no TRV to act as a bypass have gone!

Tony

 

[GaytonTonner]

If you were a genuine person

I must be! I typed all this! AI didn't do it.

 

[joinerjohn]

I must be! I typed all this! AI didn't do it.

Probably a sheer fluke that none of it was cut'n pasted then??

 

[bengasman]

As Tony said, only point of interest is the return temp.
System will work best with bypass closed.
Ignore advice from GT, he is a troll, not a pro and has been banned many times

I am no troll, despite what some uninformed people say. I gave the data.

Any fool can spoon up data, as is daily proven my countless council"workers" that don't have a clue what they are on about.
You should know that bigwaterdrivel.

 

[Arthurx564]

Thanks for the replies and I apologise for the delay in my response.

I have inserted two more images; one shows a schematic of the boiler measurement positions, and the other shows the return temp (before the bypass valve) with the "mixed" return temperature (after the by pass valve). This plot clearly shows that there is a constant increase in the return temp past the BPV, which indicates that the BPV is allowing hot water through at all times. Am I therefore correct in assuming that the Gas Safe engineer did not set the BPV correctly? He took it straight out of the box without adjusting it. He said that it is "pre-set!"

The plot also shows that the return temp goes above 55C after 30 minutes, thus the boiler will not be in condensing mode after this point. Is it best to set the boiler to a lower heat setting to ensure that the return temp stays below 53C?

Your comments/advice will be much appreciated.

Thanks.

Return Temprature

• Arthurx564
• 22 Jan 2011
• 1

Measurement Schematic

• Arthurx564
• 22 Jan 2011
• 1

 

[bengasman]

A condensing boiler should always be set as low as possible to optimise efficiency.
Have a look in the manual to see if this boiler needs an external bypass; it would be the first WB combi to do so that I have ever seen.