Difficulties in reducing the flow temperature of a condensing gas boiler

[Radian]

I've been trying to reduce the flow temperature of our 30kW condensing system boiler to improve it's efficiency. The lowest I can practically operate it and have HW at 55^oC, is with a flow temperature of 65^oC. Any smaller difference results in extended running and short cycling.

 

As this chart shows for the start of today, the cylinder temperature sensor governing the HW demand fell just below the overnight minimum setback temperature of 40^oC and very briefly fired up the boiler at 4:56AM. The hysteresis is programmed at 2.5^oC so the boiler demand ceased 8 minutes later at 5:04AM as the cylinder sensor reached 42.5^oC

 

 

Notice, however, that the temperature in the HW cylinder continues to rise to almost 50^oC over the next 10 minutes which is how long the boiler overrun is programmed to be. This is another area that could do with more examination as it seems there was still plenty of energy to be transferred from the heat exchanger and pipe work.

 

But the real issue comes at 7AM when the daytime temperature demand of 55^oC kicks in. Initially the temperature rises quite quickly as it did earlier, due to the full 30kW being applied. But on reaching 51^oC the boiler starts modulating and the rate of heating falls (as witnessed by the change in slope). When the cylinder sensor reaches 53.3^oC at 7:33 the boiler switches off completely and commences to cycle on and off at 10 minute intervals at its minimum output of 10kW. This strikes me as being inefficient as it's only at 8:13, 45 minutes later, when 55^oC is finally attained and the boiler demand is removed.

 

From this it seems to me as if a HW temperature setting of 53.3^oC would be about the limit of what the system was comfortable with - as this is where we get to when modulating reaches its limit and the boiler starts cycling off and on. If I drop the flow temperature below 65^oC then this will push down the HW temperature by a similar delta. By inference from the numbers above, a flow temperature of 60^oC would mean something in the region of a maximum HW setpoint of 48^oC for sensible operation. I think this would be too low, yet people talk of boiler flows of 55^oC!

 

At the setpoint of 65^oC the boiler flow itself reaches a self-reported peak of 68^oC just as it clicks off. The actual return temperature at this point is 56^oC also according to the boiler diagnostics menu. My cylinder sensor is typically three or four degrees lower than the return temperature which seems reasonable.

 

Am I wrong to see on/off cycling as undesirable once the turn-down is insufficient to keep the boiler delta happy?

[SteamyTea]

56 minutes ago, Radian said:

Notice, however, that the temperature in the HW cylinder continues to rise to almost 50^oC over the next 10 minutes which is how long the boiler overrun is programmed to be.

When a cylinder is heated from the base, there is a fair amount of turbulence in it.  Take away the heat source (no input to coil at base) and there will still be some turbulence for a few minutes.  That needs to settle before a true 'spot' temperature reading can be taken.

 

What does this mean?

[JohnMo]

Couple of things going on.

 

A cylinder with a coil is designed to have around 75 degree flow temp going into it.  As you reduce the flow temp the heat exchanger coil cannot transfer the heat away very well.  When you get a certain point in the heating cycle the boiler will trip on a high return temp.  Then go in to short cycling.  As you are seeing.  Not efficient.

 

You may use less gas by ramping up the flow temp to 75, your return should be around 55.  The heat cycle should be quite quick.

 

[SimonD]

What boiler make and model have you got? It's worth checking if it can be configured for priority domestic hot water. Some system boilers can't and are dependent on the same setting for both CH and DHW. And some boilers that can be configured that way don't tell you they can be so you have to figure it out. I can tell you that Ideal boilers can and it's very simple, giving 80 deg DHW and modulating CH even though the installation manual doesn't tell you this explicitly.

[Radian]

2 hours ago, SteamyTea said:

What does this mean?

It's an estimate of the gas consumption.

 

1 hour ago, JohnMo said:

You may use less gas by ramping up the flow temp to 75, your return should be around 55.  The heat cycle should be quite quick.

 

If the return was 55 then yes it should be in condensing mode but in practice (my experience) a 75 flow will result in a return over 60 and lose out on efficiency. Obviously there's a trade-off here somewhere but it'd take some finding.

 

1 hour ago, SimonD said:

What boiler make and model have you got?

It's a   Glowworm Flexicom 30hx  The manual is very opaque on HW priority hinting that it all happens using auxiliary eBus control but I don't have that. Not unless I hack something together. 

[marshian]

On 07/09/2022 at 15:02, Radian said:

It's a   Glowworm Flexicom 30hx  The manual is very opaque on HW priority hinting that it all happens using auxiliary eBus control but I don't have that. Not unless I hack something together. 

 

Hi been reading this thread and your "zombie boiler" thread and found them very interesting

 

Trying to keep this post short so I don't ramble on too much but in a nutshell I also have Glow worm Flexicom 24HX and am in a similar boat so to speak

 

When installed it was running for several years at 75 deg flow temps due to T11 rads (legacy from the original house build) and a gas engineer who left the boiler providing enough heat but not optimised (probably never running in condensing mode) the temp was fine for CH or HW

 

Last winter 4 T11's got swapped out for T22's and I was able to reduce CH flow temps to around 64 deg c and returns of around 45 but I had to heat water separately from CH by manually increasing flow temps to 68 which is a bit of a pain to have to do twice daily (return temps don't exceed 54 deg C) tank temp gets to high 40's which is fine for us as we pretty much use all the HW daily and it minimises losses (Tank gets a Legionaires once a week where the flow temp is set to 82 and the tank gets to just over 60 deg C)

 

This summer a bunch of the remaining rads got swapped out for T22's or larger vertical rads in rooms with space issues (bathrooms and Kitchen)

 

Now in the early autumn when CH is starting to be needed I can heat the house with 45 deg flow temps and 30 deg return temps - it may need to rise to 55 deg in the colder months but the return temps will still be lower and keep the boiler in condensing mode all the time.

 

Controls is WISER with Smart TRV's on all but one rad although it now has an automatic by pass in circuit

 

However HW is going to require the same manual work around again this winter so been searching the web for a solution and although I can't find the post I found a thread where an external weather comp sensor was used and small program / switch ran to change the resistance of the outside temp sensor to fool the boiler into ramping up the flow temp when HW was being called for

 

I've found the savings in gas consumption between flow temps at 68 and several cycles compared to a higher flow temp say 75 and one or two cycles to achieve the same temp is minimal especially when the boiler is range rated down

 

Wondered what your thoughts on the Weather compensation hack would be??

[marshian]

One more thing - the parameters for anti cycle d.2 don't have much impact at higher boiler temps but the pump over run timer d.1 is set to 4 mins as std (mine was actually set to 2 mins for some reason and used to cycle a lot) - I've now increased d.2 to 8 mins and it gives much longer time for the HW tank to extract out all the heat (once the flow and return temps equalise the boiler with fire again if there is still a call for heat regardless of the pump over run setting) it's around 6 - 7 mins before it fires again

[Radian]

On 02/10/2023 at 23:14, marshian said:

I found a thread where an external weather comp sensor was used and small program / switch ran to change the resistance of the outside temp sensor to fool the boiler into ramping up the flow temp when HW was being called for

Sounds unlikely for a Flexicom? I don't think WC is available on the mainboard.

d.2 is certainly worth increasing. Espc. if you've got an energy efficient pump.

[Andehh]

With the greatest of respects, this micro management is probably saving you the cost of a pint a month.

 

Set the dials to 'e' and worry not! Focus on draft proofing and reducing usage!

[marshian]

2 hours ago, Radian said:

Sounds unlikely for a Flexicom? I don't think WC is available on the mainboard.

d.2 is certainly worth increasing. Espc. if you've got an energy efficient pump.

 

Apparently if you have X41 on the PCB you can plug in an external temp sensor for weather comp functionality

 

The instruction leaflet below shows what I mean

 

https://www.glow-worm.co.uk/glow-worm/products-1/outdoor-sensor/0020266792-installation-instructions-1476886.pdf

 

I saw you had another PCB that you were experimenting with and I was interested in if that had X41 on the board

[marshian]

On 05/10/2023 at 18:18, Andehh said:

With the greatest of respects, this micro management is probably saving you the cost of a pint a month.

 

Set the dials to 'e' and worry not! Focus on draft proofing and reducing usage!

 

Flexicom HX don't have a dial or an "e" position You have "mode" "Digital flow temp display" and "-" and "+"

 

In my case I've already reduced drafts to the point where I had to fit a PIV to manage the humidity gain and In 14 mths I've reduced gas usage from 15 - 16,000 kWh to 8,500 kWh per year.