New heating system - 130m^2 with 2x power showers
- Thread starter markocosic
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usually this level of rgi disdain comes from those who as a result have no issues working on their own gas appliances.
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No low opinion of heating engineers Tony, nor those who chose vocational rather than academic. Its the RGIs (especially the fast-track course/van with name brand on the side type) who think that their gastec cert qualifies them to design heating systems that wind me up.
The big question I had in the original post (and still have; more below) was "to what degree is cycling acceptable/how short is too short?"
Didn't know that about the wall bracket. I'll have to dry fit it then leave it on the floor then, for the RGI to put the screws back in and nip up the compressions.
Have found RGIs willing to replace the existing gas pipe (as the system stands) with a correctly sized one (300 quid) and commission a boiler or two at the same time (add 200/300 quid) which is reasonable for a day's work BUT it looks like I won't get a boiler warranty with that. Approved installer? 2200 for the boiler/500 to fit. Non-approved? 1200/500 to fit. No extended warranty with the guy who charges 500 to fit. Extended warranty with the approved installer, but you're then paying 1500 to fit. At that kind of price you may as well buy the boiler once and buy the parts later as they fail, rather than pre-paying for all the failures upfront. Will look further north/east as there were just the two official Vokera installers in Sutton Coldfield.
Anyhow... ...back to cycling. (expertise required here Breesey; happily confessing what I don't know here...)
You have a house. Say you wish to keep it at 20C +/-0.5C with outside temperatures from 0C to 20C.
If you wanted the boiler to run continuously, then it would need 20:1 modulation/turndown. Say 20kW when 0C outside and 1kW when 19C outside for a house. 100 kW and 5 kW for a commerical application.
The commercial lot with their 100 kW demands using five 20kW boilers with 4:1 modulation/turndown and a boiler management system.
In residential applications the boiler must cycle on and off, as nobody makes a boiler with 20:1 modulation/turndown.
Increasing the hysteresis to +/-1C would drop the modulation/turndown required, but folks notice that temperature swing, and it won't drop it far enough to avoid the need to cycle. I fyou want heating down to -5C the problem is worse.
You can control the cycle length with the water content of the system, hysteresis and setpoints on the flow/return temperatures, and radiator sizing. You can cycle on the room stat, the boiler stat, or indeed both.
What are the acceptable time constants? How short is too short? This is a question a heating engineer (which I am not) will know the answer to.
(At some point the benefits disappear, so "as long as possible" isn't an acceptable answer)
Having done some reading around I'm thinking 10-15 minutes is about as short a time as you'd want the boiler to fire for.
The EU boiler efficiency tests at part load are 10 minutes long and the manufacturers are optimising for these/to cheat these. See PP31 of this document:
http://www.ecoboiler.org/public/ecoboiler_task1_final.pdf
I'm now fairly sure that the "10:1 modulation" Vokera boiler has "cheat mode" built in. The efficiency at 30% load is probably* rubbish, so they deliberately make it fire at 75% (probably the optimum efficiency point...) for the first 15 minutes then modulate down. The EU still gives them an "efficiency at 30% modulation" but it will be from firing for a shorter time at 75% then turning off, rather than the actual efficiency observed at 30% modulation.
You can turn off "cheat mode" in the field using the settings on the boiler as the technical centre described.
On some (Vaillants?) you can't and unless you've got radiators that'll dissipate the full output it'll sit there bouncing off the overheat/anti-cycling control. That'll be an issue with combis with massive outputs, or where radiators have TRVs and the house is close to the setpoint.
SEDBUK is hopeless for choosing between the boilers, as it takes the lab test data then process it using all sorts of assumptions about the heating system and its control that renders the output useless.
The Dutch system is better as is described in that paper. Am now looking for HR-107 rated boilers that really do modulate down and retain their efficiency (those tested under operating mode No 1) and will size everything such that cycle times are >10 minutes and return temps are <45C.
Still curious what a heating engineer would do.
*Assuming that the manufacturer is dishonest. pp92 of that document says that this is a reasonable assumption - all but one of the 70 boilers that the old DEFRA tested by 2007 were found to have grossly overstated their actual efficiencies whilst the so called independent certifying labs turned a blind eye. You also get lower total CO and CH4 emissions if you get the boiler stinking hot quickly then modulate down than if you warm it up slowly too.
The big question I had in the original post (and still have; more below) was "to what degree is cycling acceptable/how short is too short?"
Didn't know that about the wall bracket. I'll have to dry fit it then leave it on the floor then, for the RGI to put the screws back in and nip up the compressions.
Have found RGIs willing to replace the existing gas pipe (as the system stands) with a correctly sized one (300 quid) and commission a boiler or two at the same time (add 200/300 quid) which is reasonable for a day's work BUT it looks like I won't get a boiler warranty with that. Approved installer? 2200 for the boiler/500 to fit. Non-approved? 1200/500 to fit. No extended warranty with the guy who charges 500 to fit. Extended warranty with the approved installer, but you're then paying 1500 to fit. At that kind of price you may as well buy the boiler once and buy the parts later as they fail, rather than pre-paying for all the failures upfront. Will look further north/east as there were just the two official Vokera installers in Sutton Coldfield.
Anyhow... ...back to cycling. (expertise required here Breesey; happily confessing what I don't know here...)
You have a house. Say you wish to keep it at 20C +/-0.5C with outside temperatures from 0C to 20C.
If you wanted the boiler to run continuously, then it would need 20:1 modulation/turndown. Say 20kW when 0C outside and 1kW when 19C outside for a house. 100 kW and 5 kW for a commerical application.
The commercial lot with their 100 kW demands using five 20kW boilers with 4:1 modulation/turndown and a boiler management system.
In residential applications the boiler must cycle on and off, as nobody makes a boiler with 20:1 modulation/turndown.
Increasing the hysteresis to +/-1C would drop the modulation/turndown required, but folks notice that temperature swing, and it won't drop it far enough to avoid the need to cycle. I fyou want heating down to -5C the problem is worse.
You can control the cycle length with the water content of the system, hysteresis and setpoints on the flow/return temperatures, and radiator sizing. You can cycle on the room stat, the boiler stat, or indeed both.
What are the acceptable time constants? How short is too short? This is a question a heating engineer (which I am not) will know the answer to.
(At some point the benefits disappear, so "as long as possible" isn't an acceptable answer)
Having done some reading around I'm thinking 10-15 minutes is about as short a time as you'd want the boiler to fire for.
The EU boiler efficiency tests at part load are 10 minutes long and the manufacturers are optimising for these/to cheat these. See PP31 of this document:
http://www.ecoboiler.org/public/ecoboiler_task1_final.pdf
I'm now fairly sure that the "10:1 modulation" Vokera boiler has "cheat mode" built in. The efficiency at 30% load is probably* rubbish, so they deliberately make it fire at 75% (probably the optimum efficiency point...) for the first 15 minutes then modulate down. The EU still gives them an "efficiency at 30% modulation" but it will be from firing for a shorter time at 75% then turning off, rather than the actual efficiency observed at 30% modulation.
You can turn off "cheat mode" in the field using the settings on the boiler as the technical centre described.
On some (Vaillants?) you can't and unless you've got radiators that'll dissipate the full output it'll sit there bouncing off the overheat/anti-cycling control. That'll be an issue with combis with massive outputs, or where radiators have TRVs and the house is close to the setpoint.
SEDBUK is hopeless for choosing between the boilers, as it takes the lab test data then process it using all sorts of assumptions about the heating system and its control that renders the output useless.
The Dutch system is better as is described in that paper. Am now looking for HR-107 rated boilers that really do modulate down and retain their efficiency (those tested under operating mode No 1) and will size everything such that cycle times are >10 minutes and return temps are <45C.
Still curious what a heating engineer would do.
*Assuming that the manufacturer is dishonest. pp92 of that document says that this is a reasonable assumption - all but one of the 70 boilers that the old DEFRA tested by 2007 were found to have grossly overstated their actual efficiencies whilst the so called independent certifying labs turned a blind eye. You also get lower total CO and CH4 emissions if you get the boiler stinking hot quickly then modulate down than if you warm it up slowly too.
You have a requirement for 14 kW yet you want to fit a 25 kW boiler ( perhaps even two of them! )
Then having seriously oversized the boiler you wonder why its going to cycle on/off inefficiently!
Have you not totalled the heat output of all the rads?
Tony
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Not to mention vokera are ****e and miserable Scottish ****s
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Online calculators are set to maintain the temperature when -1C outside.
I'd like the house warm even at -5C external temperature (+20% oversize) and some headroom for warming the house up in an acceptable (<1hr) timeframe. (+25% again)
A 14kW boiler will also cycle - everything will at domestic scale as in the post above.
Is cycling actually inefficient?
At what point does it cease to have a material impact on efficiency?
10 secs - sure.
100 secs - probably.
1,000 secs - I'd be surprised.
10,000 secs - almost certainly not.
Genuine question.
Current rads total ~15 kW nominal @ 90/70, are full of open-vent system sludge, and have seized lockshields.
They match the current boiler but can't keep the house warm in current conditions even on paper, and are hopeless at raising the temperature from any night setback.
The calcs already have an allowance for heat up! That is usually achieved well within an hour depending on how much heat has been lost since it was last heated.
To get faster heatup you need larger rads which will also increase efficiency in the boiler.
The effect of cycling will depend on the exact boiler and to a lesser degree on the system.
I would say that any boiler on for less than 70% of the time will be rather less efficient. Its the ratio you need to be looking at.
But a properly designed system would only cycle when the outside temp is close to the set internal temp!
Tony
To get faster heatup you need larger rads which will also increase efficiency in the boiler.
The effect of cycling will depend on the exact boiler and to a lesser degree on the system.
I would say that any boiler on for less than 70% of the time will be rather less efficient. Its the ratio you need to be looking at.
But a properly designed system would only cycle when the outside temp is close to the set internal temp!
Tony
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BRE's whole house boiler sizing does indeed allow 10%, but I'd done the heat loss calcs only - and then added the warmup allowance afterwards.
10% (1.7kW) is pretty mean and the warmup rate in cold weather is pretty damn marginal. 20ish % is still fairly mean, but makes all the difference - I've tried. (external temperatures such that existing system running flat-out to maintain 18C, then crack on a 3kW fan heater on 1/2/3 setting)
With you all the way on that one. Rads will be sized to boiler max output @ 45C return temperature - assuming that there's room.
Else as large as physically possible whilst retaining balance between rooms and limit boiler output such that it can still run at full chat if asked, albeit with a higher (non condensing) return temp.
Can you clarify that?
"70% of the time..."
...where "time" is the total time that the boiler is operating? Ie, 70% of the gas will be burned at max efficiency and 30% at some dubious cycling efficiency?
Possibly. These are the numbers I'd like to run. I've a suspicion that it won't matter one jot on space heating, but the tradeoff between cylinder losses vs the naff combi efficiency in hot water mode will be a tough call to make.
The jury is out on this one pending some numbers. (where the cycling isn't of material detriment to the efficiency of the system it isn't a problem)
In the limit:
-assume that all gas during ignition and shutdown sequences is wasted
-assume that the entire boiler chills to ambient temperature between runs
You know the nominal gas input, you know the mass of the boiler internals, and you can work out the losses at various cycle rates.
Quick ignition sequence helps. Lower boiler internal mass helps. Lower flow and return temps/higher ambient temp both help. Fan assisted flues (that don't allow the gales in from outside) help. Pump overrun helps.
SEDBUK assumes 2% of gas (2% of total annual demand) is wasted on startup/shutdown. They do say that they assume on-off controls and cycling but they haven't published how this number was arrived at. I suspect that this is excessive.
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