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Discuss Real world testing example of Condensing Boiler in older home in the Central Heating Forum area at PlumbersForums.net

JC22024

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Recent install Firebird 26kw Condensing boiler. Space heat only. Installed as replacement boiler in well insulated 80s renovated home. (Silver bead wall cavities, triple glazing 0.6 centre pane U value, 450mm rockwool in loft) This house is now split into 2no zones. Downstairs and Upstairs. Independent time & temperature zone controls.

The system design would have been prior to these new condensing boilers. Therefore rads etc would have originally been sized per 82c supply and 71c return as the old tried and tested favourite say.

Whereas it is said that in order for a condensing boiler to condense the system design should be 65c supply, 45c return ideally. No issue for a new build with under floor heating etc. But for an older property like mine this is what I want to explore and talk about in detail.

Would I be correct in thinking the ideal scenario here would be to review the original radiators, delta temperature calculate the output and most likely have to upsize most all of them by factor of 1.5 etc in order to produce effective heat output in the cooler months, whilst reaping full benefits and efficiency of a modern condensing boiler?

I am NOT a heating engineer. I am a chartered Architectural Technologist by trade and Design Manager in daily job. I require to have sound knowledge in all building elements – and to me this includes heating system design. Today I am conducting an initial experiment on my home. To measure over time:

Run time
Condensate output in ML
Boiler Flow & Return Temps
Flue Temp

I want to see generally that if I run the system temperature in a real world example over the 45c to say 55 & 65c return temps that I need to input useful heat to my home, is the boiler still producing condensate and is the flue gas temp also rising indicating loss of efficiency.

Mods can move this to the Oil / Solid fuel section if they like but it is a gerneral condensing central heating discussion and probably will get more exposure in the central heating section.
 

ShaunCorbs

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Tbh you won’t notice much of a difference as the lower rad output slightly has been dropped due to cavity / loft insulation and triple glazing reducing the heat required
 

JC22024

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I will update this post / chart as the time goes by this evening:

From first heating run of the day on downstairs zone. Granted the first 30 mins was a cold system - look how much condensate was produced. I then empty that at 30min stopwatch, record temps and set for next 30 mins.

Between the 30 - 60 minutes run time the Boiler did NOT produce condensate. Only a 10ml dribble.

So I have now at 60 min run time lowered the boiler stat. I want to make the return temp run at lower temp and see if condensate production restarts.

Boiler Stat setting:Elapsed Boiler Run Time Mins:Flow / Return TempsFlue Temp external as guide:Condenstate produced:
63065/43c45350ml
66081/58c65.810ml / nil
59080/55c56.310ml / nil
4120
3150
2180
1210
 
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ShaunCorbs

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Get the return around / below 50 so flow 65
 

JC22024

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I suppose the other question I have, and I see conflicting information on the Internet: is a white plume from a condensing boiler also an indication it is condensing?

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This was during first 30 min run time. But usually looks about the same.

If you see a plume but don't have any condensate out of the boiler base is it really still operating at full potential?

I suppose it is not reaping full efficiency benefits but still much better than an older non condensing counterpart.

Would the centre flue gas temperature be the real indicator of efficiency?
 

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ShaunCorbs

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All that shows you is there’s water vapour coming out the flue it’s and indication that’s it’s condensing
 

JC22024

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Also set the boiler temperature as low as possible as the really big gain in boiler efficiency is below 50C return temp as it starts to condense then, at 50C return the boiler efficiency is ~ 88% but at 35C return its 96%. Maybe try a boiler temp of 45C initially but bear in mind that this setting, if it results in a return temp of 35C means that your rads will (only) emit 30% of their rated output which will still more than likely give you sufficient heat especially if the heating is on 24/7.

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Taking this over from another recent thread. Very useful diagram! Is the water inlet temperature in this chart actually reffering to the heat exchanger? And therefore the return temperature to the boiler, not the supply / output temp?
 

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Chuck

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All that shows you is there’s water vapour coming out the flue it’s and indication that’s it’s condensing
I think the last few words of your sentence have been cut off somewhere. I guess they would have been "... it's condensing incompletely (at best) and would benefit from a lower return temperature."

When a boiler is condensing nicely, all you see is a 'shimmer' in the air around the flue where the hot flue gas mixes with the colder atmosphere.
 

JC22024

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I think the last few words of your sentence have been cut off somewhere. I guess they would have been "... it's condensing incompletely (at best) and would benefit from a lower return temperature."

When a boiler is condensing nicely, all you see is a 'shimmer' in the air around the flue where the hot flue gas mixes with the colder atmosphere.

So you mean to say there should be no white plume from the flue? And in such a case the condensing boiler is therefore at optimum running condition?

I have read this elsewhere too hence my raising the subject. To me however, I have never seen a condensing boiler run without any white plume - have any of you(?) And I do think the white plume is a visual indicator of condensing taking place but just to what extent of the capabilty is the question? If nothing is coming out the bottom of the boiler then what return temp does it take in order for this to change?
 

ShaunCorbs

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It’s one of them your always going to get a plume as the boiler won’t extract 100% and the boiler flue temp is above the outside air temp hence the water vapour, also you have to be careful with oil boilers not to run the return too cold / low as you have issues later on best to keep it above 45 dc return

For no plume vapour you would be around 35dc return and a flue gas recovery system as well
 

JC22024

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Ok so after more testing I realise my temperature readings are not reliable. Therefore I have order pipe thermometers to measure this much better.

AND in addition even at the lowest boiler setting there is still zero condensing coming out of the outlet! It appears the boiler stats lowest setting equates to about 65c outlet. And with heating running for a couple hours the return temp struggles to go below 50c.

Should I look for a replacement boiler stat with slightly lower temp range? Eg 45-75. The original Firebird stat is supposed to be 60 - 80 range.
 

EvilDrPorkChop

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Mines generally running at a 30oc flow temperature, but i've never actually measured the return. I'd be interesting to see what it is. But I get very minimal plume when running at this.
 

ShaunCorbs

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Problem is you need it above 60 for hot water eg legionella
 

ShaunCorbs

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It runs on Opentherm, so when the cylinder needs reheating CH closes off and flow temp increases to 80oc.

But the op is on a simple oil boiler so can’t due to hot water cylinder
 

JC22024

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My hot water cylinder is controlled independently by economy 7 immersion heaters. A Hortsmann controller provides hot water every day. Whilst the oil boiler could heat the water I find it cheaper to run the economy 7 version and for convenience
 
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JC22024

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Mines generally running at a 30oc flow temperature, but i've never actually measured the return. I'd be interesting to see what it is. But I get very minimal plume when running at this.
30c flow temperature? Do you have underfloor heating throughout a modern home to run this low? And you must have the heating on all the time almost to run that low whatever the installation
 

JC22024

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So today's test is put off due to the need for pipe thermometers and the results are not going to be what I anticipated either! 🙃

I figured that by turning my boiler down to its lowest setting it would at least produce some condensate VS where I normally run it at about setting 5. But in reality my boiler will not produce a single drop of condensate out the bottom after the initial warmup phase. Even at lowest setting for several hours running.

Whilst fitting a lower boiler stat would get me into the lowest temps I think I need to run the house on this current setup - lowest boiler setting and await the proper pipe thermometers this week to see what that equals.

That will allow me to re take the readings with accuracy and also understand that say it is at 60c outlet at present and it's quite mild outside is that enough to heat the house up. If it isnt enough output then not much point fitting a lower range stat which would let me see the condensing temps perhaps but end up turned up thereafter anyway.
 

Chuck

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So you mean to say there should be no white plume from the flue? And in such a case the condensing boiler is therefore at optimum running condition?
Whether there is a visible white plume or not depends on atmospheric humidity and temperature as well as the temperature of the products of combustion (POC) being vented from the flue. The visual appearance not a reliable diagnostic for assessing boiler performance.

If you want to do a spot check on the extent to which a boiler is condensing the direct check is to measure the rate at which condensate is discharged along with the gas rate.

To me however, I have never seen a condensing boiler run without any white plume - have any of you(?)

Yes. My own (NG) system, which is carefully balanced to operate at 65 flow and 40 return, often runs without a visible plume but it does depend on the atmospheric conditions on the day.

And I do think the white plume is a visual indicator of condensing taking place but just to what extent of the capabilty is the question? If nothing is coming out the bottom of the boiler then what return temp does it take in order for this to change?

I've re-read @ShaunCorbs' post #6. I now think that a source of confusion in this thread is that the 'condensation' post #6 was referring to is that of the water vapour in the POC mixing with the atmosphere outside the flue giving rise to the visible plume. This is not the type of 'condensation' that I was referring to in post #8, which: improves the efficiency of the system, occurs within the boiler, results in condensate discharge to a drain and reduces the partial pressure of water vapour in in the plume, making it less visible.
 

EvilDrPorkChop

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30c flow temperature? Do you have underfloor heating throughout a modern home to run this low? And you must have the heating on all the time almost to run that low whatever the installation
No, it's all Radiators however they are oversized. It's a relatively modern house, and it's well insulated and I have quite an advanced control system.

I'm still in the experimenting stages as to what is most efficient. I was originally running it all day and turning it off a night, i'm now running it 24/7 which appears to use less gas, the only issue you have is it uses more electric as the pump is nearly running 24/7.
 

John.g

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Taking this over from another recent thread. Very useful diagram! Is the water inlet temperature in this chart actually reffering to the heat exchanger? And therefore the return temperature to the boiler, not the supply / output temp?
That's the return temp, a oil fired boiler which fires at 100% output will have a higher dT between the boiler return temp & flue temp, with a higher flue gas temp than a gas fired boiler as its output matches (generally) the heating demand so may be only firing at 20% or so with a lower dT and lower flue gas temp.

You will soon establish if running at 60C will allow sufficient rad output to safisfy your house requirements as as a rad with a dT of 10C with flow/return of 60/50C will still give 63% output based on current 50C rating and 50% if based on the old 60C rating.

Measuring the condensate is the way to get a accurate handle on your gains all right.
That 0.35L/30min means that you are gaining ~ 0.44kwh (/hr) or a efficiency gain of 0.44/20*100, 2.2%.
but probably achieved with boiler firing continuously while getting the system contents up to normal temperature

Boiler Stat setting:Elapsed Boiler Run Time Mins:Flow / Return TempsFlue Temp external as guide:Condenstate produced:
63065/43c45350ml


Overall, IMO, very few oil fired boilers achieve any condensing effect, I prefer to think of all condensing boilers as high efficiency (HE) boilers, the few (oil fired) flue gas analysis printouts that I've seen indicate a flue temperature of ~ 80/110C say 95C average, my 16 year old SE Firebird has a flue temp of ~ 230C so a HE Firebird will give a 6/7% increase in efficiency without even trying.
 
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Chuck

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Overall, IMO, very few oil fired boilers achieve any condensing effect, I prefer to think of all condensing boilers as high efficiency (HE) boilers, the few (oil fired) flue gas analysis printouts that I've seen indicate a flue temperature of ~ 80/110C say 95C average, my 16 year old SE Firebird has a flue temp of ~ 230C so a HE Firebird will give a 6/7% increase in efficiency without even trying.
One piece of O' level chemistry that is relevant here is that (stochiometric) methane combustion produces two molecules of water for every molecule of carbon dioxide whereas for oil it's closer to one-to-one. This roughly halves the amount of water in the POC so condensing it out is harder (lower return temperatures required) and less beneficial (the latent heat recovered is a lower fraction of the total). The figure in post #7 is for natural gas, i.e. methane.
 

John.g

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JC has a 26kw boiler so input ~ 23.5kwh so the saving ~ 0.44kwh (/hr) or a efficiency gain of 0.44/23.5*100, 1.9%.
In my industrial oil and gas fired boiler days we used something like 6% wet gas loss on oil firing and 11% on gas.
 

JC22024

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That's the return temp, a oil fired boiler which fires at 100% output will have a higher dT between the boiler return temp & flue temp, with a higher flue gas temp than a gas fired boiler as its output matches (generally) the heating demand so may be only firing at 20% or so with a lower dT and lower flue gas temp.

You will soon establish if running at 60C will allow sufficient rad output to safisfy your house requirements as as a rad with a dT of 10C with flow/return of 60/50C will still give 63% output based on current 50C rating and 50% if based on the old 60C rating.

Measuring the condensate is the way to get a accurate handle on your gains all right.
That 0.35L/30min means that you are gaining ~ 0.44kwh (/hr) or a efficiency gain of 0.44/20*100, 2.2%.
but probably achieved with boiler firing continuously while getting the system contents up to normal temperature

Boiler Stat setting:Elapsed Boiler Run Time Mins:Flow / Return TempsFlue Temp external as guide:Condenstate produced:
63065/43c45350ml


Overall, IMO, very few oil fired boilers achieve any condensing effect, I prefer to think of all condensing boilers as high efficiency (HE) boilers, the few (oil fired) flue gas analysis printouts that I've seen indicate a flue temperature of ~ 80/110C say 95C average, my 16 year old SE Firebird has a flue temp of ~ 230C so a HE Firebird will give a 6/7% increase in efficiency without even trying.

Thanks for the info! The gain calc on the condensing outlet is less than I would have thought I suppose that is comforting in a way!

I was thinking of also getting a 240v hour counter gauge. And attach only to the burner feed. So this would also tell me the exact burner run time/ ratio over a given day.

I have read in several places that a condensing boiler should also be undersized to some degree to get the best out of it. I have my own calibrated Testo analyser. This is from it set with 0.65gph nozzle/ 26kw now:

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It was factory set with 0.5gph nozzle. I did note quite alot of extra air flow needed to adjust to the 0.65gph nozzle when I did it.

Is it possible to take the values above with GPH nozzle spec and determine the flow / consumption per hour? (Eg, 9.8bar at 0.5gph)

If I had the hour counter on the burner I would be most interested to run side by side tests through a full day or week going between the nozzle sizes. To see if there is a marked consumption increase with the bigger output or does it balance out.

In addition, when I have the pipe stats for accurate measurements I wonder at what point it would produce condensate with either nozzle. Maybe the larger nozzle and air flow makes it harder for it to produce condensate. In other words the volume of fumes expelled dries off the heat exchanger. Excess air / latent heat.

If the flue Temps drop on the lower nozzle and run times do not increase significantly then it would make sense to keep the 20kw output. I intended to undersize it at 20kw by the way but the circulator pump problems sent me down the wrong path initially thinking the boiler wasn't cranking out the heat where it actually was but it wasn't going anywhere. What I will say is this new Firebird boiler heats up very quickly. I mean I can hear rads clicking warm at the furtherst part of the house in minutes after turning it on.
 

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JC22024

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Using Danfoss Nozzle Calc spreadsheet I got the following values based on specific Firebird settings:

0.5gph / 1.5kg/hr nozzle set at 9.8bar actual pump pressure consumes 1.66 kg/hr / 2.1L/Hr / 19.92kw
0.65gph / 1.95kg/hr nozzle set at 9.3bar actual pump pressure consumes 2.1 kg/hr / 2.66L/Hr / 25.23kw

A test I would like to conduct is the difference in burner run time / consumption. Say between a Saturday & Sunday same time and temp setting ground floor. Take the total run time all day with same settings to compare values more closely. Then change and set to 20kw the Sat night. Wait until same temps and time the next morning and run / measure again. To see is there a marked saving in actual fuel consumption when running the boiler at a lower kw output. To see does it actually make it more efficient making the condensing boiler work harder.

Or could run a week test measuring both the pump and also burner ratio / run times too. Exterior temps will be a variable factor of course and programme and temps would need to be unchanged between test cycles.

What do you guys think? Sound useful or am I barking up the wrong tree here?
 

JC22024

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The OK marked danfoss nozzles indicate Oftec / UK market. Therefore the fuel consumption specified on these nozzles is based on 8 bar pressure. The following link is the Excell sheet I refer to above for anyone interested. Choose the UK Kerosense tab at the bottom of the spreadsheet!

 

John.g

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The OK marked danfoss nozzles indicate Oftec / UK market. Therefore the fuel consumption specified on these nozzles is based on 8 bar pressure. The following link is the Excell sheet I refer to above for anyone interested. Choose the UK Kerosense tab at the bottom of the spreadsheet!

I have that one somewhere.

I suppose if you get a lower flue gas temp with the same flow and return temps then one would have to say that the boiler is more efficient, but you may also require the ambient temperature recorded as well.
The boiler will cycle less so should increase efficiency but I was very surprised how little difference cycling allegedly makes on overall efficiency.
 

JC22024

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I have that one somewhere.

I suppose if you get a lower flue gas temp with the same flow and return temps then one would have to say that the boiler is more efficient, but you may also require the ambient temperature recorded as well.
The boiler will cycle less so should increase efficiency but I was very surprised how little difference cycling allegedly makes on overall efficiency.

Boiler cycling is a good point! Good article here:


Now that article is referring to boilers which can modulate which mine can't. It is either off or on. In my case it turns off and on every couple of minutes roughly.

One of the good points in that article is rhe fact that even my oil boiler runs air for a few seconds before it fires at each cycle. This would contribute to wasted heat up the flue also.

It should almost certainly be more efficient and cycle less if running the smaller 20kw setup. But by exactly how much this translates to in potential fuel consumption saving is what I'll try to measure.
 

JC22024

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OK so it appears that using an IR gun to check temps is pretty darn inaccurate! Despite it being a new Bosch item also not a cheapie etc.

The pipe thermometers arrived today so I affixed them to the flow and return pipes.

I did have the boiler down on lowest setting for a few days there and whilst it was technically heating the house it was much too slow and low for my liking. Eg, stat would be at 18c only after many hours of running.

So this is the min setting I can run and get good heat input into the house.

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This photo is the flow and return temps (flow on top) with GF on for a few hours:

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And this is the upstairs after a few hours:

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Still waiting on the 2no hour counters to start logging the boiler duty ratio over time.

I have only about 3 - 8c difference in flow temp. How would I bring that up to the ideal 20c? 🤔 Running a 25/80 180mm Triton pump on lowest speed setting. (Per other recent threads the house never heated properly before until I upgraded the pump size)

Would dropping the boiler to 20kw, which I am highly likely to do in any eventuality increase the temp drop on the flows? Or is it a fluid slowing / balancing exercise needed? Gate valve etc
 

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ShaunCorbs

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Throttle the rads / balance your looking for around dt10 on each rad
 

John.g

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The flow (and return, maybe to a lesser degree) temp should rise and fall with the boiler cycling, yours is set to ~ 70C and your (flow readings) should vary between say 68C and 58C. If the temp is a constant 60C then it means that the boiler is firing flat out and either can't meet the heating demand (very unlikely) or is still heating up the system contents, even then it should keep rising gradually until the boiler cuts out at its 70C setpoint.
Have you used your IR gun at the same pipe thermometer locations?.
 

JC22024

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The flow (and return, maybe to a lesser degree) temp should rise and fall with the boiler cycling, yours is set to ~ 70C and your (flow readings) should vary between say 68C and 58C. If the temp is a constant 60C then it means that the boiler is firing flat out and either can't meet the heating demand (very unlikely) or is still heating up the system contents, even then it should keep rising gradually until the boiler cuts out at its 70C setpoint.
Have you used your IR gun at the same pipe thermometer locations?.

I didn't monitor the Pipe stats over time will take a look later and see does the temperature raise and lower. The boiler is definitely cutting out and then back on again - cycling. Not running full bore by any means.

I think the difficulty with the IR gun accuracy is trying to take readings from metallic surfaces such as these pipes. The reading can vary wildly. But I will see what comparative reading I get near the pipe thermometers. But I would say the pipe thermometers are much more accurate
 

JC22024

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Throttle the rads / balance your looking for around dt10 on each rad

Thanks for that suggestion, I never did properly balance these rads. Just by feel and all of them get hot. Taking IR gun readings - this was taken a couple of weeks back:

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But correct that I should re check the lock shields and obtain the desired 10c drop
 

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ShaunCorbs

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Increase the pump speed up one also to help the ir temp reader use some black insulation tape where your taking the readings
 

JC22024

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Undecided about the pipe stats, getting mixed results.

But had a better approach:

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As I watch the flow temps after shutdown they drop to low of 64.1c. Then as soon as the boiler fires they raise to 70.0c. And so forth. These are flow temps only.

Interestingly that is exactly the 70c where the stat is set on boiler.

Running upstairs zone only.

Return Temps also can be seen slowly going back and forth between 60.3 to 62.2c.

Burner firing time 1min 8 secs. Off time 2 min 10 secs during this time.
 

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JC22024

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FWIW upstairs zone has 2no fairly new double Myson rads: about 4.5 foot.

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  • +2no older 6 foot double rads no fins.
  • +1no tall vertical bathroom radiator
 

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John.g

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The boiler averaged 8.9kw and the rads were producing 85% of rated output.

Choices to reduce return temp for greater boiler efficiency.

Existing: Boiler temp 70C (67C avg) return 61C output 85% of rated output.

1. Leave boiler temp at 70C and throttle the return(s) to give a return temp of 45C (16C reduction) but with a reduced output of 65% of rated output (76% of existing output)

2. Increase boiler temp to 80C and throttle the return(s) to give a return temp of 51C (10C reduction) with the same output (85%).
 

JC22024

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The boiler averaged 8.9kw and the rads were producing 85% of rated output.

Choices to reduce return temp for greater boiler efficiency.

Existing: Boiler temp 70C (67C avg) return 61C output 85% of rated output.

1. Leave boiler temp at 70C and throttle the return(s) to give a return temp of 45C (16C reduction) but with a reduced output of 65% of rated output (76% of existing output)

2. Increase boiler temp to 80C and throttle the return(s) to give a return temp of 51C (10C reduction) with the same output (85%) but return temp of 51C (10C reduction)

Thanks for the figures, very helpful

What does "throttle the returns" refer to please?
 

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