Real world testing example of Condensing Boiler in older home

[JC22024]

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.

 

[JC22024]

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.

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

 

[John.g]

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

 

[JC22024]

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

Deeter Boiler Control

Prevent hot water being reheated unnecessarily, cutting your fuel bill by 20%. Product code: GVEdeeter1 24 - 36 month payback 'Fit and forget' 3 year warranty

www.greenvisionenergy.co.uk

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

 

[JC22024]

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

Energy Management Solutions | Vector Group

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.

 

[John.g]

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??.

 

[JC22024]

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??.


View attachment 72892

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.

 

[John.g]

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.,

 

[John.g]

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.

 

[JC22024]

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.

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

 

[John.g]

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.

 

[JC22024]

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.

Thanks for that! I just can't help thinking my boiler Is cycling alot and probably wasting alot of heat up the flue whilst doing so. It's following the stat too closely. Hence why I believe a Boiler Energy Manager will greatly help my system efficiency. They monitor the flow and return temps and calculate the load on the system, controlling the burner.

I can't find any information on that Vector Group item I purchased from fleabay. From what I read they cost about £1600 - £2k from the company but someone on ebay with good feedback etc was selling a new unused one for £30 Inc postage

I'll open it when it arrives and see if anything is labeled and what sensors I would need. But may not be able to use it myself if nothing is labeled inside and its only meant to be fitted by them so looking at other BEM systems for Domestic boilers at present.

Plan is:
Balance all rads precisely and see if I can get return temps down

Meter the burner etc at 26kw setup and run for a few days.

Then downsize nozzle and re-run the same test.

Then fit a BEM (and / or try the Vector super unit) and re test to see what reduction in consumption they may offer if any.

 

[JC22024]

Tonight on upstairs zone only,

72 seconds on time
129 off time
Flow temp goes from 68.3 - 62.3.
Return temp seems to stay around 60c odd.

 

[John.g]

Based on cycle time, boiler output is 9.3kw so below might show why 60C return is valid but assumes ~ 11kw of upstairs rads.

 

[JC22024]

Today I got the twin digital thermometers in the post. Which I wanted for accurate easy comparison of pipe temps.



The first few I checked only had a few C drop.

I tried to guess the order of the rads and it wasn't working out. The first few would either die or have only a couple c difference. Couldn't get them to rise above that.

I did have luck adjusting one though which proves it is possible



So I should carry out the proper procedure. Open all rads, restart check order they heat. Cool again then start balancing in order from start. Will try to do that this Saturday all being well it will likely take ages!!

But I want to get it as balanced as possible. I'll aim for 9 or above I think the 11 or 12 may be out of reach but I'll try. The trick is to watch the flow temperature 🌡 so it doesn't start to drop whilst giving time for the difference to show. Aka easy to go too far and be on a false readout. It'll take me a while to get used to. I hope it's not working at present because I'm not following proper procedure!

The big kitchen rad in particular near boiler seems to want locksheild almost full open before it will heat up. Then drops dead if you turn below that. So some may not want to balance at all it is what it is.

 

[John.g]

What are the dimensions of the kitchen rad and is it a double or single?.

 

[ShaunCorbs]

Try with the pump speed on two instead of one

 

[JC22024]

What are the dimensions of the kitchen rad and is it a double or single?.

The kitchen rad is a double myson 160cm wide x 50cm high panel. The pipe configuration isn't ideal on this one as it was a replacement bigger rad for an older one. Need to sink into floor still, may be able to make good in future:

Try with the pump speed on two instead of one

Thanks I'll try that on Saturday. I'm thinking that since I'm not doing it correctly in order and all other locksheilds open that's not helping. So going by proper process maybe it will balance better.

Regarding the balancing just leave boiler stat at 70 or put up to 80 for the balancing procedure? And leave TRV heads on but full open / max temp?

 

[JC22024]

Also it appears some of the TRVs are installed on the return side. This might make obtaining the desired 11c harder on those particular rads?

They are modern bi directional TRVs but still mixed up.

If thought to be a problem I could drain down and rectify that throughout house.

 

[ShaunCorbs]

Tbh doesn’t really matter temp as that’s not a limiting factor

And tbh if your trying to balance it needs to be on the return as your throttling flow else

 

[JC22024]

Tbh doesn’t really matter temp as that’s not a limiting factor

And tbh if your trying to balance it needs to be on the return as your throttling flow else

OK so yes boiler temp doesn't really matter.

Do you mean the locksheilds should be on the return pipe yes? If so I can rectify that throughout house where it occurs before I attempt a balance.

 

[ShaunCorbs]

Would be best practice

 

[JC22024]

Would be best practice

Yes that's OK I will make that good before I proceed - will try to get that done on Sat and system going again then balance on Sunday.

Since the upstairs and downstairs zones are programmed independently, I will balance them independently too.

This is a good video on the subject FWIW. Not how to balance but the theory of the efficiency and why it is important. A band aid to get a further rad to heat may be to turn pump speed up but as they explain it makes it worse even if that rad heats.

 

[John.g]

Any (if) build up of sludge in the rad itself has a huge effect on rad dT, I find that to make any appreciable difference in flowrate and dT that I would have to throttle in (either) one valve to only ~ 1/4 turn open, it seems, on my system anyway that I will only start increasing the dT by throttling from say 1/2 turn open, down.. Even with both valves fully open my rads have a dT of ~ 7C to 10C with a pump head of ~ 3.5M.

 

[JC22024]

This is a domestic Boiler Energy Manager I also got on ebay. They are about £200 from supplier but a guy on ebay has a few new ones for £50 so worth a try at that. At £200 probably wouldn't have bothered but interested to see if / how it may reduce oil consumption/ boiler cycling.

Energy Minder Auto | Total Energy Controls

 

[JC22024]

I checked the radiator valves and only a couple have the TRV on the return side. They are bi directional TRVs but all the same I will move them.

But thinking more about the 11c ideal temperature drop - how obtainable is this really? The Delta radiator temperature drop would be directly linked to the heat disapated into the room.

So the room temperature / weather would also be important.

It's very mild at present with temperatures outside at or near 7-10c. So in that case I think turning the boiler stat up for the balancing will provide better results unless the weather drops when I get to it.

Do you guys agree with what I'm saying in principle? If my rooms are 18c now, 7c outside and I am seeing 4 or 5 c drop max across a radiator then perhaps that is all it would achieve unless the room temperature dropped.

Maybe open the windows for balancing

 

[John.g]

All my TRVs are on the returns and I can certainly balance/reduce rad outputs by throttling the lockshields, but as I said I reckon I would find it extremely difficult to get all rads to a dT of say 10C or 15c or whatever, having TRVs on all rads, once they throttle in will give a variable dT depending on the room temperature(s). I have watched my boiler return over the past few days and it never goes below 42/43C and probably averages more like 45/46C. I really think the best way to get a relatively constant low (but not below 38/ 40C for oil firing) is to use flow temperature control. You could get limited return temperature control by controlling the flow with a smart circ pump but this only allows a minimum rad output of ~ 58% whereas flow temperature control (if achievable) allows rad outputs as low as 30% with return temps not falling below ~36/38C.

 

[JC22024]

All my TRVs are on the returns and I can certainly balance/reduce rad outputs by throttling the lockshields, but as I said I reckon I would find it extremely difficult to get all rads to a dT of say 10C or 15c or whatever, having TRVs on all rads, once they throttle in will give a variable dT depending on the room temperature(s). I have watched my boiler return over the past few days and it never goes below 42/43C and probably averages more like 45/46C. I really think the best way to get a relatively constant low (but not below 38/ 40C for oil firing) is to use flow temperature control. You could get limited return temperature control by controlling the flow with a smart circ pump but this only allows a minimum rad output of ~ 58% whereas flow temperature control (if achievable) allows rad outputs as low as 30% with return temps not falling below ~36/38C.

View attachment 73023

Controlling the return temperature would be good- but I am not a huge fan of smart pumps. On my house at least. Because some radiators drop out altogether. The pump now fitted does a great job.

But what I was thinking is wouldn't it be great to have a thermostatic device on the return pipe to the boiler which could sense the 50c return temp etc and maintain it. Eg throttle down automatically if the flow temp tries to climb above 50c but in all other instances normal flow. (Since I like 70c flow setting then 50c is what my system would be good at)

There must be something available along these lines. At a quick glance maybe something like this that can be set to thermostatically control a flow.

MTCV-CCR2

Danfoss MTCV thermal balancing valves – reduce the risk of legionella, long waiting times and high energy consumption in drinking water applications.

www.danfoss.com

Get radiators balanced as precisely as possible first then something like this to provide precise control on return.

 

[John.g]

Rad stats give individual room control so why throttle the main if the room stats are set properly and operating properly?. The boiler return temp will then be at its lowest possible but individual rad returns will vary from say 50/55C downwards, consistent with your requirement for a 70C constant flow temp, my rad stats give excellent individual room control.

 

[JC22024]

Rad stats give individual room control so why throttle the main if the room stats are set properly and operating properly?. The boiler return temp will then be at its lowest possible but individual rad returns will vary from say 50/55C downwards, consistent with your requirement for a 70C constant flow temp, my rad stats give excellent individual room control.

Thanks yes not a great idea then was just throwing it out there as an alternative means to obtain the magical 20c drop if radiator balancing alone can achieve it.

Probably a case of do what you can with rad balancing and it is what it is. Eg 10c delta on boiler, if that's what you get then no more to reduce it by. But on colder days and warmup it will in realty be a greater delta.

 

[John.g]

Just read in a UPS 2 8M pump replacement post that a Dab Evoplus & a Grundfoss Magna 3 are suggested, you might be interested from the point of view that the also do temperature control, might be worth a look?.