Central heating pump sizing

[John.g]

I don't think it's worth the hassle/expense, I have always been skeptical of the actual condensing benefit achieved.
On oil fired boilers, the flue gas temp on a SE boiler is around 230/250C and 80/100c on a HE so a real gain of ~ 6 to8%, I would like to see typical flue gas temperatures for SE/HE gas boilers, I wouldn't expect a SE gas boiler temperature to be as high as that of a oil fired boiler.

But there are lots of ways of skinning a cat, I ran and maintained relatively large 50MW, 45 bar superheated steam boilers and because of the high water temps involved (259C) regenerative rotary air heaters were used to heat the incoming combustion air with the outgoing flue gas and the boilers had ~ 85% efficiency....your next project.

 

[Reddwarf4ever]

I don't think it's worth the hassle/expense, I have always been skeptical of the actual condensing benefit achieved.
On oil fired boilers, the flue gas temp on a SE boiler is around 230/250C and 80/100c on a HE so a real gain of ~ 6 to8%, I would like to see typical flue gas temperatures for SE/HE gas boilers, I wouldn't expect a SE gas boiler temperature to be as high as that of a oil fired boiler.

But there are lots of ways of skinning a cat, I ran and maintained relatively large 50MW, 45 bar superheated steam boilers and because of the high water temps involved (259C) regenerative rotary air heaters were used to heat the incoming combustion air with the outgoing flue gas and the boilers had ~ 85% efficiency....your next project.

I worked most of my life on refineries, where heat recovery was paramount, especially producing 600# steam from furnaces to drive turbines. I love thinking of ways to be more efffcient, but sometimes the odds are weighed against me. I envisaged a system, where a separate cold water tank ( feeding the DHW cylinder, could be preheated, using an other boiler heat exchanger, just before it vents outside, to extract the remaining latent heat, would only need a very small pump, only running when the boiler is on......the ‘heated’ DHW storage tank would be the size needed for a days use of DHW, if it fell low, it would be made up from the main DHW storage tank, would need to be foam insulated also......just a dream......

the way Energy is going, won’t be long before burning gas is a thing of the past, we will all be on electric heating, so much simpler. There is so much more better ways to use the gas, or at least the oil it was produced from, than just burning it. Burning fossil fuel for energy should be the very last thing we do, when it can’t be re used or recycled anymore IMO

take care

 

[king of pipes]

Products do exist but they are costly. Kop

 

[Reddwarf4ever]

Shaun corbs said “Need to keep it condensing else it will break the boiler in a year or two as the heat exchanger will get blocked as there’s nothing to move the waste”

but dont see why this should be or how 🤔

 

[ShaunCorbs]

You will get particles that will build up in the burner chamber that won’t get flushed through with the condense and will build up to a point where it just blocks the hex and could cause it to split

 

[king of pipes]

Pm him I am sure he will have a explanation

You will get particles that will build up in the burner chamber that won’t get flushed through with the condense and will build up to a point where it just blocks the hex and could cause it to split

Like this

 

[ShaunCorbs]

Pm him I am sure he will have a explanation

Like this

forgot to reply yesterday

 

[Reddwarf4ever]

That does make sense, but wonder how many condensing boilers are actually running in condensing mode ? Maybe apart from new installs 🤔 mine not been condensing since new in 2009, only since January 2021 have I been trying, but still not a great success, minimum return temperature 53-55oC, only thing I can do is replace two lounge radiators with double fin, each would then give approx 30% extra heat output ( 100cm x 50 cm )

 

[king of pipes]

I am sure it will be condensing at certain times of the day, I by no means have John G' s grasp of the figures but I have 80 + boilers on my books which I service and maintain, and where possible I advise my customers to keep the flow temperature setting at 65° c by keeping the heat exchanger clean serviced correctly and that means a full strip and clean every year flushed though electrodes cleaned condense trap removed and fully cleaned or replaced any small leaks addressed gas valve adjusted , every 5 years the system drained flushed and a good quality inhibitor added I rarely get a breakdown from doing a flue gas analysis which we do on each service you can determine efficiency at different stages of firing , modulation, and flue gas temperature I personally think you have probably got your system running the best you can without changing things replace radiators for larger ones by all means this allows you to run the system at a lower temperature and still be comfortable in your living space. Regards kop

 

[John.g]

The boiler will certainly condense very heavily on start up in the morning where it might take a 18kw boiler ~ 10 minutes to heat say 75 litres of water from 20c to 50C, also if the house/room temp is very cold then a certain amount of condensing will also take place.

Its interesting though that if you have a fixed flow temp of 65C and a flow of 0.75m3/hr (fairly normal for a 18/20 boiler) then no matter how oversized the rads are that the return temp cannot be lower than ~ 51C once the rooms are up to temperature (I use 23C) the room temp and the return temp will then start rising.
I spoke to a (long retired) acquaintance of mine earlier on and he said he never remembers measuring a gas fired boiler flue gas temp below 55C, what kind of flue gas temps are you seeing KOP?.

 

[king of pipes]

I am working on one now as we speak

 

[John.g]

Boiler NO 81? should now be be running at 65C/0.39M3/hr (flow) and rad(s) output of 59% OR
55C/0.77M3/hr (flow) and rad(s) output of 48%.
and Shaun's condensate flow at roughly 0.7 lit/hr @ 20 kw boiler output or pro rata.

 

[king of pipes]

I am working on one now as we speak

Bit cleaner now 😉

 

[John.g]

That's some service.

 

[Reddwarf4ever]

I thought the same thing as John mentioned, when the boiler first fires up in the morning, return temperature is typically 20-25oC & after it cuts out on the room stat, may not restart for 1-2 hours, then again very low return temperature for some time ( will try and monitor return temperature next time it restarts, be interesting to see the climb rate and how long it ‘ should be’ condensing for. So going back to deposits on the heat exchanger wouldn't these periods of heavy condensing keep the exchanger clean ? I am running my boiler outlet at 64oC, i tried 62oC, but even 2oC seemed to make noticeable difference to radiator heat output. The return temperature when steady and modulating is 53-56oC. As I said previously I was considering replacing 2 lounge radiators with double fin, but looks like the best I might get is a 45oC return temperature.....only an educated guess.....which would give a 3% saving, would take around 20-25 years to pay back the cost..so very likely going to abandon that idea.

I know I may get shouted at, but I dont suppose I could clean the heat exchanger myself, I think it will need a Gas Safe person most likely.......

there is a move to make manufacturers add an efficiency display to their boilers and show when it’s in condensing mode, I think they will fight hard against this, as people will soon realise they are not getting the 97% efficiency that they expect..

take care all & keep safe 😎

 

[Reddwarf4ever]

Products do exist but they are costly. Kop

Says “cost effective “ at £630 I wonder how long it would take to pay back, also for combi boilers only, so no good for me...🤯

 

[John.g]

Not shouting but not whispering either, NO you cannot clean the heat exchanger yourself.

 

[ShaunCorbs]

I maybe thinking wrong but how can 2dc make a room feel colder when it’s well above the room temperature

I could understand if the flow temp was 40 but not 60dc you sure you have a flow temp at 62dc at all your trvs ?

 

[Reddwarf4ever]

I maybe thinking wrong but how can 2dc make a room feel colder when it’s well above the room temperature

I could understand if the flow temp was 40 but not 60dc you sure you have a flow temp at 62dc at all your trvs ?

Hello, there must come a flow temperature that just doesn’t release enough heat to be comfortable, not sure if flow temperature and radiator heat release is linear. Just seeemed to make a difference when I lowered it to 62oC.....no idea if its the same temperature at the TRVs all rads do get hot, so assume that must be the case.....

also at 62oC the boiler often cuts out at 65oC as it can’t reduce below 5KW......

very happy with how it’s running now, have had some excellent advice and learnt lots in the process.....

thanks

 

[John.g]

This thread has thrown up some extraordinary "facts" re flowtemps/flowrates and their effect on return temps etc.

Any temperature change in either flow or room temp will have a effect on rad output but its not linear, it works with a exponent of 1.3 where the rad output is based on the ((meanradtemp)-requiredroomtemp))^1.3.

Using your observations above, initially I calculated that the 4/5 rads that you have running at a fixed flow of 0.76M3/hr......
flow/return/rad output 65C/59C/5.21kw. Now reduce the flowtemp to 63C results in........
63C/57.4C/4.9kw (which is why your boiler is cycling), you are running right at the switch point because you require the rads to run at ~ 68% of their rated output so running very close to the minimum boiler output.
If you require a rad output of 5.21kw with that 63C flowtemp then the flowrate would have to be increased from 0.76M3/hr to 2.24M3/hr, (impossible) and you would achieve 63C/61C/5.21kw.

If you had TRVs on these rads and increased the flowtemp to 75C then they would have throttled in to give 75C/48.8C/5.21kw at a very reduced flowrate of 0.17M3/hr. rad&boiler deltaT of 26.2C, OK if boiler is running but flow too low for the 1 minute fire up requirement of 9.6kw.

The above might look a bit complicated but very easy to calculate in a spreadsheet I made.

 

[John.g]

Should also add another bit of info.
If your room(s) should suddenly fall to 10C then the flow/return/output will be
65C/56.6C/7.35kw. 5C roomtemp = 65C/55.6C/8.21kw & 0C = 65C/54.4C/10.6kw.
The most interesting part of all this is that the return temp doesn't change much in any of the above so it looks like any system with fixed flow temp and fixed flowrate will never condense under any conditions except at start up

 

[Reddwarf4ever]

Very interesting John, would love to see the spreadsheet, always making these for all sorts of things......thanks

Should also add another bit of info.
If your room(s) should suddenly fall to 10C then the flow/return/output will be
65C/56.6C/7.35kw. 5C roomtemp = 65C/55.6C/8.21kw & 0C = 65C/54.4C/10.6kw.
The most interesting part of all this is that the return temp doesn't change much in any of the above so it looks like any system with fixed flow temp and fixed flowrate will never condense under any conditions except at start up

interesting stuff 👍 would love to se your spreadsheet, can you attach ?

 

[John.g]

I will try once again but for the past few months havn't been able to attach any, they used to attach no problem previously in zipped form.

 

[Reddwarf4ever]

I will try once again but for the past few months havn't been able to attach any, they used to attach no problem previously in zipped form.

Can give you a dummy email address that I keep for such purposes.......👍

 

[John.g]

Yes, do that, no problem.