TIME TAKEN TO HEAT HW TANK:

[RONCHAM]

Hi - not sure if my schoolboy physics is letting me down?

If you input, say, 3kw or approx. 10,000 Btu into a 30 gallon HW cylinder, how long would it take to heat the ENTIRE contents to recommended temperature? I make it close on 3 hours?? - although I know in practice you probably wouldn't want entire contents heated.

 

[stem]

I'm rusty too, but from memory here's my recollection of the simple formula to calculate the heat required in calories to raise the temperature of a volume of water. I'm sure someone will put me right if Ive got confused (it is nearly 30 years since I was at school!)

heat = mass × specific heat × temperature change

The specific heat value for water is 1 (I think)

There are other things to be taken into consideration, such as the ambient room temperature, insulation on the cylinder, the real starting temperature of the water (ie cold from the mains, or ambient)

If you are heating the cylinder indirectly via a heating coil fed from a boiler, then a whole load more calculations far too complicated for me become involved relating to the surface area of the coil, temperature differential across the coil, temperature of the water arriving at the coil etc., etc.,

 

[TicklyT]

A 30 gallon cylinder will only heat about 20 or so gallons of water.
You can discount the volume of water below the primary coil from your calculations, as the only heat input to that is by conduction, which water is pretty lousy at. There's nothing unusual in the bottom of the cylinder hardly warming up at all.

One BTU raises the temperature of one pound of water one degree Farenheit.

 

[JohnD]

3kW sounds like you are using an immersion heater. Provided it is well-insulated (remember to insulate the pipes too) your figure of 3 hours sounds quite reasonable to me. It certainly wouldn't be 30 hours 0r 0.3 hours

If you have a modern indirect cylinder with a pumped gas boiler, it should heat from cold in about 20 minutes. Domestic Gas boilers are usually in the 15-25 kW range, which is 5 to 8 times as much power as an immersion heater (but limited by efficiency of cylinder coil).

 

[Agile]

Whats the objective behind the question or the desire to work out how long?

Tony

 

[JohnD]

college homework?

 

[TicklyT]

In old money, raising 300 lbs of water from 50 F to 140 F with an input of 10,000 BTU/hr will take 300 X 90 / 10,000 hours = 2.7 hours

 

[RONCHAM]

The objective was to satisfy my curiosity!!

For the past week or so, becos of a CH prob, I've been using the immersion for HW. It seemed to produce a fair quantity of v. hot water in about 30 minutes but I wasn't sure how much. My calculation was similar to TicklyT - and you guys also said allow 2/3kw for HW in boiler sizing.

Btw, thanks to doitall, Agile, Seco Services, davebgas and others who advised on my CH problem (no HW, no hot d/s rads) - it DOES seem the pump was part of the problem. I was beginning to think it must be so I fitted a new one today and all rads are now hot. BUT still no HW . There's a cheap and nasty crudded-up ball-o-fix type valve on HW circuit, presumably for balancing, which looks suspect - I don't fancy touching it!

Also, I didn't doubt advice given on boiler sizing but I rang W/B's renowned "technical" support today (I'm sure you plumbing guys get put thru to more knowledgeable people). After 10 minutes relentless music, I was moved on elsewhere and after another 5 minutes of music, finally got to speak with someone. I only wanted very general advice but she didn't know her a... from her e... and said I would a need a visit from a gas engineer. Their approx. web-guide suggests 12kw for 1-3 bed semis and 24kw for 4-5 bed semis - I wondered what they did with their 15/18kw which is what I think I need (for my 3-bed semi)??

 

[JohnD]

said I would a need a visit from a gas engineer.

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