Real world testing example of Condensing Boiler in older home

John.g said:

I'm not sure re Firebird condensate trap but some traps, to avoid the waste pipe freezing up, fill up the trap with condensate and then blows this down and refills again so on the off chance that this is occuring keep a eye on it but I wouldn't hold my breath as firebird supplied a plastic trap originally which used to melt.

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I can edit my post above but I suppose mybtwst conditions there were incorrect. Restricting the return means the boiler isn't actually firing anywhere near what it would normally be. The radiator temps dropped inside the house.

So when I just reopened the ball valve there whilst return temp was below 40c approx and the burner firing there was some condensate being produced albeit only about 50ml. I suppose I would need a Lower c stat in order to conduct that test properly.

I am collecting all condensate during the tests with a very advanced setup- a coke bottle with a hole cut in the side

Its the wrong container, you require a still water bottle.

I am still also exploring boiler compensators. Or Boiler Energy Managers as they appear to the called. This one is also interesting:


I will email them and ask if all required parts are included in the price. And for a wiring diagram to see if it's compatible with my controls. This would be further down the line not in the next few weeks

I spotted a great deal on one of these and bought it last night




It's brand new unused. I believe my boiler is switching on and off too much. I think this device is aimed to reduce boiler cycling by learning the boiler demand and firing cycles and reducing the latter.

This is a boiler energy manager not a Weather Compensator but I suppose you have have both if you wanted. I think it goes between the boiler stat and burner.

In any case this would only go on once I get the system balanced and boiler jetting complete. Then the simple meters will show what savings it is bringing. Reading up on it they appear to offer savings of 15 - 30% on boiler firing time.

They appear to guard how exactly it operates so I would imagine I there will be no instructions supplied and I'll have to study it and see what inputs it requires.

I am going to carry out a test on just one rad soon because as you say, the only sort of fit is if the flow temperature falls as well, see below. I'm quite happy that I had a flow rate of 5LPM both by throttling the 5M pump setting to replicate yours and then at a pump head of ~ 3m unthrottled to give me the same 5LPM. As the boiler output was fairly constant (cycling) at 45 to 50% then the only scenarios that sort of fit are the last two below, ie a flow temp of 64C & return of 35C for 50% output and 45% output at 60C/34C, I didn't see my return going below 42/43C, the only explanation I can give for a apparent low flow temp is that a oil fired boiler acts as a 20/25 litre buffer tank and with the burner off perhaps cold return water flows from the flow side until the burner "catches up" and reheats the buffer??.

John.g said:

I am going to carry out a test on just one rad soon because as you say, the only sort of fit is if the flow temperature falls as well, see below. I'm quite happy that I had a flow rate of 5LPM both by throttling the 5M pump setting to replicate yours and then at a pump head of ~ 3m unthrottled to give me the same 5LPM. As the boiler output was fairly constant (cycling) at 45 to 50% then the only scenarios that sort of fit are the last two below, ie a flow temp of 64C & return of 35C for 50% output and 45% output at 60C/34C, I didn't see my return going below 42/43C, the only explanation I can give for a apparent low flow temp is that a oil fired boiler acts as a 20/25 litre buffer tank and with the burner off perhaps cold return water flows from the flow side until the burner "catches up" and reheats the buffer??.


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Are you saying that on your radiator readings the return pipe was 34 / 36c but at the boiler pipe you are measuring no less than 42/43c?

I suppose the return temperature will be a mix of all of the radiators as they flow back to the boiler. IE the first rad in ciruit will likely have the hottest flow and return temp - so maybe those rads you measured are further into the house?

The thing that bugs me with my Firebird is that it is trying too hard to maintain the 70c stat. It would be better if there was a lag of sorts there so it drops back a few C first then re-starts. Which would make it run longer when it does and less cycling. This other BEM mentioned in my other post above should address that cycling issue hopefully. I will need to open it up when it arrives and see what inputs it requires. I would imagine pipe stats will be needed on the boiler and it will connect between the boiler stat and burner, therefore learing the burn times and heating cycles / demand and being smarter than the stat to better control the burner. I think so anyway but time will tell on that one.

What I am saying is that the (theoretically) hottest return with a 72C flow temp is 37C, with perfect distribution through all the (6 rads), the lower the flow through any of the rads will result in a still lower return temp from that rad or rads which shuld result in a still lower boiler return temp.
I'm beginning to wonder if you think about TRVs and how they operate, they just keep throttling the flow until they get their set temperature but looking at the calcs below and your boiler/rad required output of 8.9kw based on a on/off times of 68secs/130secs then the TRV would have to shut off completely for some periods to allow the whole rad to cool down in one instance to allow a average flow temp of 60C as the return temp cannot be lower than the sensing/room temperature say a minimum of 24C, see below. The only way to achieve that measured return temp of ~ 41C is to have a average flow temp of 43C so perhaps a TRV is just opening/closing to achieve this and maybe this where evohome and others got their ideas from, where they use constant on/off boiler firing times to control the mean rad temperature necessary for any particular heat demand?.. The only other way is to have the flow temperature set to 43C with a very high flow rate, this, basically is how outside temperature compensation works I would think, also UFH.

That's why I a going to carry out a few tests soon on one rad and then maybe two/three to see what is actually going on. I can also use the TRVs as all my rads have them installed, all working perfectly.,

Carried out the TRV test on one X 1.7kw rad and as expected, the TRV was just copening/closing but keeping fairly tight control of the mean rad temp at ~ 42/44C which gave the required rad output of 0.6kw to maintain a room temperature of 20/22C. The boiler had very long cycling times of around 2.5 mins on and 35 minutes or more off. I then progressively introduced the remaining rads (all TRVd) and their mean rad temps reflected the outputs required, the combined return temp didn't vary hugely ~ 39/45C.

John.g said:

Carried out the TRV test on one X 1.7kw rad and as expected, the TRV was just copening/closing but keeping fairly tight control of the mean rad temp at ~ 42/44C which gave the required rad output of 0.6kw to maintain a room temperature of 20/22C. The boiler had very long cycling times of around 2.5 mins on and 35 minutes or more off. I then progressively introduced the remaining rads (all TRVd) and their mean rad temps reflected the outputs required, the combined return temp didn't vary hugely ~ 39/45C.

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That's interesting on the off time of your boiler too- what return temp drop do you get before it fires up again? Eg - 4.5c drop before it re-fires

Its very difficult to get a good reading in the heat pac due to restricted space but initially it actually seems to rise a little but I suspect this is because the boiler flow pipe turns down immediately where as the boiler return goes out the side of the boiler and then very gradually rises so I think the boiler contents are inclined to "circulate" back through the return. The cycling time matches ~ the rad heat demand but its not that simple as the boiler once it cuts back in when the stat reaches ~ 60/65C has to reheat say 20/25 litres of boiler water from ~ 40c to its cut out of 70C so requires ~ 0.9kwh which takes ~ 2.7 minutes from a 20kw boiler. In general, once other rads are brought back on line then the % firing time does reflect the heat demand.