How to work out usage?

Juleszoemay

Hi,

We have just had a new 35kw gas boiler installed, we use Propane as we are not on mains gas. Our last 3 year old Daikin hybrid gas boiler/heat pump died, the heat exchange filled up with water and it was out of warranty, running in emergency mode and THE most complicated heating system known to man. 

I'm interested to know, roughly how many litres of propane per hour this boiler will use when on. We are with Flogas and our telemetry hasnt worked for about 9 months (have raised this about 10 times with Flogas, but they're awaiting consumable parts - still). Apparently there is a dial on the tank, although this wouldnt be particularly easy to read/accurate at all. The telemetry used to give the litres left total, and would read it daily. But as I dont have this available anymore. I was trying to work it out. 
Does anyone know?!

Of course we were using a heap of electric with the last set up, and heating our hot water with electric because the system never worked well enough to heat the hot water.... but I assume we have flipped those costs onto gas. We are currently paying 40ppl with Flogas, but our contract runs out in Jan!! So lord knows what the new cost will be. Anyway, thats another story. 

Thanks,

Julie 

molerat

Propane gives 7 kWh per litre.  Your boiler when running flat out can use therefore up to 5 litres per hour.  Your boiler will not be running flat out all the time though and a modern boiler will likely modulate itself so that it is not using the full rated amount when running.  The best guess then is around 1 litre for every 12 minutes of actual burning time.

Juleszoemay

Eek Molerat - I'll be having monthly deliveries then!  The only upside I see is that the house gets warmer a lot quicker than before, and we have the room stats fairly conservatively set at 18 upstairs and 20 down, plus also only on for an hour in the morning, and between 5-8pm in the eve, so by those calculations if its on for 5 hours a day, it'll use about £9 of gas. Until we get a price hike in Feb!  

Juleszoemay

Our last set up was a Daikin EHYKOMB33AA, which I believe is 26.6KW output, so would have cost less to run, but you'd have to run it for longer to have any effect. We have underfloor heating in the kitchen/dining/family space, and that never felt warm until the new boiler. 

molerat

Juleszoemay said:

Eek Molerat - I'll be having monthly deliveries then!  The only upside I see is that the house gets warmer a lot quicker than before, and we have the room stats fairly conservatively set at 18 upstairs and 20 down, plus also only on for an hour in the morning, and between 5-8pm in the eve, so by those calculations if its on for 5 hours a day, it'll use about £9 of gas. Until we get a price hike in Feb!  

Your £9 is based on the boiler burning for the full 5 hours.  It won't be, it only uses gas whilst it is actually burning which will be on average probably less than 50% of the time the heating is on. 

QrizB

molerat said:

Propane gives 7 kWh per litre.  Your boiler when running flat out can use therefore up to 5 litres per hour.  Your boiler will not be running flat out all the time though ...

Expanding on this point, the amount of time your boiler will be running flat out will (or at least shound) be tiny.

I don't know your house but my three-bed semi has a 24kW mains gas boiler but only has about 10kW-worth of radiators. What this means is that once the water in the central heating system is up to temperature, my radiators can only deliver 10kW to the house - maximum. So even in the coldest weather my boiler won't be asked to deliver more than 10kW of space heating.

10kW of space heating will need around 1.5 litres of LPG per hour.

(I don't know if your boiler is conventional or a combi. My boiler is conventional, so there's also the water tank to think about. Heating a full tank of water might take another 10kWh but once it's hot, it's hot.)

Juleszoemay

Thanks for the further info  

In answer to your comments, we are in a 2500 sq ft, was a 50's 3 bed detached bungalow, and we converted the loft to add 2 bedrooms and 2 bathrooms. Its pretty well insulated because that was done in the last 3 years. How would I work out how many kw worth of rad's we have? We also have some underfloor, and the water is heated in a tank, its not a combi, it is conventional I believe. 

We were using electric to heat the hot water (pretty unsuccesfully tbh as it was never as hot as it is now with the boiler heating it) and our electric bills were £250 a month, so i'm hoping that what we are saving on electric will balance out the extra spend on gas. 

We have managed to replace the manual tank gauge so we can now read it, its currently at 65% (was filled 1st Oct) correct me if i'm wrong, but it never actually fills to 100% I think 85% is the highest I've seen - when the telemetry was working.

thozza

Juleszoemay said:

Thanks for the further info  

In answer to your comments, we are in a 2500 sq ft, was a 50's 3 bed detached bungalow, and we converted the loft to add 2 bedrooms and 2 bathrooms. Its pretty well insulated because that was done in the last 3 years. How would I work out how many kw worth of rad's we have? We also have some underfloor, and the water is heated in a tank, its not a combi, it is conventional I believe. 

We were using electric to heat the hot water (pretty unsuccesfully tbh as it was never as hot as it is now with the boiler heating it) and our electric bills were £250 a month, so i'm hoping that what we are saving on electric will balance out the extra spend on gas. 

We have managed to replace the manual tank gauge so we can now read it, its currently at 65% (was filled 1st Oct) correct me if i'm wrong, but it never actually fills to 100% I think 85% is the highest I've seen - when the telemetry was working.

Flogas usually fill to 85% or so, another supplier we used filled to 80%, they never fill higher as space is needed for the expansion phase of liquid to gas.

QrizB

Juleszoemay said:

How would I work out how many kw worth of rad's we have? We also have some underfloor, and the water is heated in a tank, its not a combi, it is conventional I believe.

For the rads, measure them and find ones the same size at eg. Screwfix.com. Every listing there includes the nominal heat output of the radiator, in watts and BTU/hr.

For wet underfloor heating, measure the area heated (square metres) and multiply it by the design power; if you don't know the design power, use 100 watts/sq.m. as a rough guide.

If you want to factor in the HW tank too, look up the specs of an equivalent tank. It might take a couple of tries but the manufacturer's info should include the power drawn by the coil. (But again, once the tank is hot, it won't use any extra power.)