I've been in house where just washing my hands in the downstairs loo cause howls of protest from the teenage tart upstairs 'the waters all gone cold'
Frankly one or two people is the max ANY combi can handle.
And they had better talk to each other.
Although having said that my sisters family in Germany had a combi in the basement. About the size of a small car. That worked for 4-6 people
It would fill the entire room I use for the boiler now, which also has a sink, drying rack and upright fridge freezer in it and room for lots of muddy boots.
As I said. 10kw boiler and a pressurised hot water tank gives me better hot water than 120Kw combi boiler would, takes up less space and costs less. It doesn't make sense to have a 120KW boiler when winter average needs are about 10KW.
This is where drivel comes in and says 'yes but two combis and a heat bank would do just as well' yeah, right, even more space and cost..
Demand is the harder of the two to assess. Mixer showers typically need
5 to 8 lpm (litres per minute) although big soaker heads, or body jets may need more (up to 30 lpm in some cases). Basins etc far less. Baths will take whatever you can give em generally in terms of rate, and will need typically 120 - 150 L of water[1]. So work out the most likely simultaneous demands, and you get a figure in lpm. If you want to produce that hot water "instantly" then you can relate a flow rate in LPM to a power requirement.
Boiler size (or any other instantaneous heater for that matter) is the easy bit:
Work out what temperature rise you need... so say you want water at 45 degrees delivered by the heater, and the ground water during the winter is at 5 deg, then that would be a 40 deg rise.
Heater Power = Flow rate x Tdelta x SHC / Time in secs
Where SHC is the specific heat capacity of water, or approx 4200.
Heater power is in watts. (Since a watt is a joule per second, we scale the answer by 60 to deal in joules per minute).
So to have 15 lpm of water with a 40 deg temperature rise, you will need:
Heater Power = 15 x 40 x 4200 / 60 = 42,000 W
To go the other way - i.e from heater power to lpm, just rearrange thus:
Flow rate = heater power x Time in sec / Tdelta / SHC
So a heat bank with 120kW plate heat exchanger on it, and a 50 degree lift in water temperature would give:
Flow rate = 120000 x 60 / 50 / 4200 = 34 lpm of water at 55 deg.
[1] That's water at final "mix" temperature - which with a combi will basically be the temperature of the hot water with very little cold added. With a storage system there may be significant cold added. See:
There are no free pallets round our way any more. At one time I could get a truckload delivered free.
However a big tree fell down across the road a week back. The council workers turned up with hand saws. I kindly volunteered to assist with my chain saw. I even took it away for them. Heh Heh!
I am busily reducing it down to size this last few days. I also have an electric chainsaw and free electricity. I don't need much wood at all, There's getting on for three years worth in this tree.
It is an utter rip off! Not only the boiler cost but being locked into one supplier. Oil would probably give cheaper running costs but obviously yhe cost of an oil tank would need to be factored into the calculations.
I wish I knew how to use a pipe bender. Knowing where to put the bend and how much of a bend to make seems like magic to me!
I had one for microbore but took it back for a refund! If I knew how to use one, one for 15 and 22mm would be useful but they are expensive at that size. Do you have to get a branded one or are the no-names just as good?
Yes, I found that out when I phoned around last year and they told me they had already quoted for me! Curiously they owned three of the brands I called and although two were the same, I got a different price from the third one; perhaps I was put through to a nearer office and the transport costs were lower?
There was a post recently suggesting that 3 phase supplies were not as competitive as domestic supplies. If that's the case, could you have a
3 phase supply for the heat pump and whatever big toys you have in your workshop and then a "normal" domestic supply for the rest of your new build, or isn't life that simple?
I'm sure I'm about to embarrass myself asking a daft question but is there a reason domestic supplies are 100A? I'm guessing it's because you would need bigger cables and is it to do with losses (I^2xR)?
I run our local syndicate, and a year ago, all GB companies gave the same price, except for Total Butler, who were much cheaper than anybody. But now, they have been tamed, and quote almost the same as all other GB companies. But I still managed to get a price 6ppl lower than the norm at the time, from a different supplier. Probably ordering >40,000 litres helps.
Just buy a pipe bending spring and bend it over your knee. These fancy machine benders are a new thing, no-one had them years ago. The trick is to bend the pipe a little "too far" & then open it a little. You always have to have a bit extra pipe to have the leverage to bend it. But the bits come in useful elsewhere.
Three phase supplies are only of advantage if you are using electric motors over over say 2Kw or have large loads of any sort. The main advantage is to the supplier as it results in a more balanced load (ie the same load on all phases)
Obviously there are four wires instead of two so it costs more. And three phase control gear is more expensive too.
Three phase motors are more compact, efficient, cheaper, reliable and with better starting torque than the same size single phase.
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