Bathtub capacity, combi sizing, Ecobollocks

Me too on weekday mornings, but I like a long, hot shower at the weekends.

In the past I'd have had a relaxing bath instead on one day of the weekend.

SteveW

Some have flow regulators built in that will limit the flow rate so that the water does not run below a minimum temperature point. However most don't and the flow rate will be inversely proportional to the temperature once you have reached maximum burner output.

You can't alter the basic laws of physics. So that suggests that your mains cold flow rate was relatively poor (i.e. < 20 lpm) and the boiler was fairly high powered (i.e. >30kW), or it was a storage combi that maintained a larger store of pre-tempered water, that could meet a high peak demand for a period of time.

(Something like the Trianco Evo Accu range go up to 40kW power output, but also include a 130L unvented cylinder in the box with the boiler - so they will do both high volume and temperature for at least 150L)

My bath measures 0.5 x 1.2 = 180 lites max.

That goes will with the 180 L HW tank

twice as big as reality.

If I get in it it isn't 0.3m deep before I do..

To heat 180 liters of water with an immersion heater from say 5C to 55C is

or a shaade over 1kwh - so it costs about 20p max and takes 20 minutes on a 3Kwh immersion heater.

It happens that PeterC formulated :

My electric shower is able to continuously vary its Kw and water flow input, to tightly control its temperature output to what you dial in on its panel. Most modern gas boilers can also modulate there Kw to meet there heating demands and HW temperature.

So Kw input cannot be determined.

Thanks, John.

Sage advice as usual.

Cheers

Dave R

This was a Grant boiler - not sure what model or power rating. It did have a small storage tank in the boiler, but that only serves to "cushion" the start of calling for hot water so there will be some as soon as the hot tap is turned on (excluding the time it takes for the hot water to displace the cold in the pipe between the boiler and tap); before the boiler has heated incoming water from cold. A storage tank doesn't affect the flow rate once the boiler is up to temperature.

I was always surprised that the temperature of hot water didn't decrease as the tap was turned on more: evidently we just had a very powerful boiler that could heat the water sufficiently, right up to the maximum flow rate of the incoming water. The flow rate for and cold was fairly high: it certainly wasn't a trickle by any means. The kitchen tap could fill a washing up bowl with hot water under a minute.

Ah - a fancy on. Mine has 3 settings: 3.75, 4.75, 8.5kW. I bought it because of the low setting - last summer, 2kW would have been enough!

The most powerful external combi they seem to have at the moment is The Vortex Blue which is available in a 36kW version.

So you can make a quick estimate of the maximum delivery rate of DHW...

If we say the incoming water is at 5 degrees, and you want that delivered at 45, that's a 40 degree lift.

So if we take that 36,0000 J/sec the boiler can muster, it will give

Divide that by the temp lift and the SHC of water and we get

So not a torrent, but adequate for one very good shower or a pair of moderate ones, and perhaps a bath in 10 mins or so.

Note that in the summer, with warmer incoming water, you could get 17 lpm

The actual rate will be limited by the cold main delivery capacity. However that is likely to be more than the useful delivery rate, which will likely require a slower rate in order to get a high enough temperature. If you have a large storage tank, then you can deliver at the maximum incoming mains rate until the tank is depleted. This is how the storage combis can punch "above their weight" - basically behaving like a system boiler and unvented cylinder combo.

Yup, that's possible. In my last house the incoming water main was 1/2" copper, and could only deliver around 17 lpm. In the summer at least the

A washing up bowl is usually about 12 litres... so even in winter a 35kW combi will fill that in a minute.

Not really a shower then is it though! :-)

TBH it was a quote from David Derbyshire from MAILONLINE