Gas Rate boiler spec sheet

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Hello,
I am researching for a residential boiler. I have seen specs sheet for various boilers and they have written average gas rate as 1.8 m3/hr for 18 kw boiler.
I am interpreting that if the boiler runs for 1 hour at full load it will consume 1.8 m3/hr? Is it correct.

It is important for me to understand. I gas rated my existing boiler (non condensing, low pressure system, Central heating only) for 2 minutes and gas meter consumed 0.082 m3; multiply 0.082x30 = 2.46 m3/hr. I have logged its actual gas consumption on gas meter and with a total load of 8 kw it consumes 1.1m3/hr almost half of gas rated figure. I understand that with current load of just 8kw the burner is lit 3 times in one hour; 8 minutes on and 10 minutes off; it cycles on off roughly 3 times in one hour to consume 1.1m3/hr. It doesn't have any modulation controls, when setpoint (a sensor which is connected to the return line of the boiler) is achieved the burner turns off. I run the boiler at optimum settings and over the last two winters, I have managed to optimize it to its maximum in terms of gas usage (I think).

If I were to get a new boiler with spec sheet of 1.8m3/hr and if I take this figure as actual then I will not be saving any money? How can I be sure that new boiler will consume less gas than my existing gas boiler othern than that it will modulate and will be a condensing boiler. My plan is to get a system boiler and install it first for CH only and once I observe its gas savings in CH, I plan to install a Hot water cylinder to it as I have to supply water to 3 bathrooms and kitchen. I wouldn't mind a good condition used system boiler as it will be better than mine anyway.

Any ideas or thoughts.

Thanks.
 
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You may well not save a huge amount unless your radiators are large enough to maintain comfortable temperatures at a lower flow temperature (50° or so rather than your probable current flow temp of 70°).
If you are currently heating the dhw with electricity you'll notice an immediate significant drop in your overall bills (gas per kWh is about 1/3 the cost of electricity per kWh).
 
Thanks. I am still not clear on gas rate specs of proposed boiler and how can I be sure that it will consume less gas than my existing one?

I have large radiators in my rooms; 4 of them and I don't run boiler on high temperatures either, just enough to heat the room at comfortable temperatures; I don't have room thermostats but rads due to large surface area gets the job done and are nice and warm to touch. I have balanced them and flow pipes are nice and warm (i can hold them for 10 secs).

I have two floors and each floor has its own separate DHW; ground floor is heated through immersion heater; upper floor's DHW is heated by gas (not CH boiler).

If I get savings in my CH usage then I plan to optimize DHW by having one 50 gallon cylinder with immersion heaters (indirect with system boiler) serving both floors.
 
Gas rates in our country have been doubled for people using CH systems. My avg usage in Jan is 500m3 and if I can bring it down to between 3-400 I would be paying almost 50% less due to slab rates for moderate use.
 
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If you've only got 4 radiators then 18kW is way too much for your needs
 
If you've only got 4 radiators then 18kW is way too much for your needs
Sorry, I have 13 radiators in total; ones highlighted in green are used frequently. Rest I keep them off and put into use when I have a need for them. A large fan coil unit is occasionally in use as well. I would be grateful for any advice on an efficient system boiler (will add DHW to it later).

80% of my gas consumption is for CH in winters.
 

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I have attached a picture of my locally made boiler (CH only) which was installed when I knew nothing about boilers (still don't know much!). It is rated as 35kw/120k btu but its efficiency will be much lower even if it is true. It has 13 radiators attached to it; 4 (1.5kw rads), a fan coil unit (3-5kw), 5 medium and 4 small bathroom rads). It does a decent job of heating the house running for 10-12 hours daily. (6 hours in morning and 6 in evening). I have logged its gas usage so I know how much it consumes for central heating; 1.1m3/hr with 8kw load.

What worries me that even after changing an old boiler (outdated and inefficient in my view) I may not be able to gain in terms of gas usage. I would be grateful for any guidance on efficient boilers in the market that can outperform my existing one.
 

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I have seen specs sheet for various boilers and they have written average gas rate as 1.8 m3/hr for 18 kw boiler.
I am interpreting that if the boiler runs for 1 hour at full load it will consume 1.8 m3/hr? Is it correct.
That's about right. If boiler efficiency = 92%, gas CV = 10.92 kWh/m3, gas flow = 1//0.92/1.02 ~ 1.8 m3/h.
It is important for me to understand. I gas rated my existing boiler (non condensing, low pressure system, Central heating only) for 2 minutes and gas meter consumed 0.082 m3; multiply 0.082x30 = 2.46 m3/hr. I have logged its actual gas consumption on gas meter and with a total load of 8 kw it consumes 1.1m3/hr almost half of gas rated figure.
You say in #7 your existing boiler is 35 kW, but 2.46 m3/h gives about 25 kW. 1.1 m3/h would be about 11 kW, but I assume your 8 kW is a guesstimate.

On your general point, if a new boiler is more efficient than the existing, which it probably will be, you will use less gas. But the new one needs to be correctly sized, and how long before you're in pocket depends on how bad the current one is.
 
You say in #7 your existing boiler is 35 kW, but 2.46 m3/h gives about 25 kW. 1.1 m3/h would be about 11 kW, but I assume your 8 kW is a guesstimate.
8 kw is the minimum load that is on this boiler on regular basis. From the attached spreadsheet, I have calculated kw of radiators; 2kw for double panel larger ones and 1.5 kw for large single panels.

I have gas rated it for 2 minutes on more than one occasion and figure of 2.46m3/hr is what I get on each occasion.

I have been monitoring gas consumption when boiler is in operation (with other gas appliances off) very diligently. I have logged it every hour and during boiler's on - off cycles and it consumes 1.1m3/hr with 8kw load (on a very cold evening it may consume more) but this number is reliable.

Since it is locally made, it is rated as 35kw but I am not sure. With my figures of 8kw output with 11.88kw (1.1x10.8) of input, it is giving an efficiency of 67%. Have I calculated it correctly?

Based on my calculations of mean water temp of 50deg C my requirement is an 18kw boiler for all rads? I hardly ever use all rads but 8kw is minimum load that will be on this boiler.

Am I right in thinking that any modern condensing boiler will save me on gas as compared to my existing one but I need member's view on it to make this call.

Thanks in advance.
 

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