There is a horizontal 22mm copper pipe containing hot water at 55C; local ambient is around 25C. What rate of heat loss, per metre, should be expected?
In theory, I should be able to work it our from first principles; but perhaps someone knows the answer.
Graph
Paul
Sounds about right. An indicative figure for convection from a vertical plate in air is 1 - 10W per square metre per deg C.
Thanks. That agrees with my feeling that it hurts when touched, but not as much as a 60W incandescent lamp bulb did.
I'm trying to decide how the lack of pipe insulation near someone else's 170L hot water cylinder relates to her complaint that, on Economy 7, the hot water is not hot enough later in the day. Neither resident displays any technical sense whatsoever or any relevant numeracy.
I myself have traditional tankage, open cold tank over hot tank, with an "overflow" from the top of the hot tank up over into the cold tank. That pipe is rather warm, and should be insulated in summer (in winter, it provides a modicum of useful day-time warmth using night-rate power) - so I'll buy some insulation for that too.
/ P.S. message from newshound now appeared; it will give another estimate of loss. :-) /
Is that person's hot water cylinder actually insulated? Is the thermostat on the immersion heater set too low?
I wonder if that could be due to loss of stratification. Let's say tank of water is at 60C, inlet water temp is 10C. Resident takes half of it for a bath in the morning. The hot water is replaced with water at 10C.
The water immediately after the bath is hot, because it's at the top of the tank. If there's stratification, the top stays hot. If there's gradual mixing the whole tank ends up the average temp of 35C.
Is the tank in question well insulated?
Theo
There will be conduction and radiation of heat in the tank, and some agitation when water is entering at the bottom. But if that happens there, it should happen in all hot tanks including yours and mine - I do not notice such an effect.
I did; and I fitted it. I estimate that the cost of the metre of foam will be saved before 2023. The down-feed pipe below it is, when not recently used, much cooler, as expected.
It was made on Friday 2021-11-05; the makers say "It is highly insulated with environmentally friendly foam".
Another thought - is the immersion heater a dual element type, i.e. one long element, one short element, and has the long one gone open circuit, meaning that only the top half of the tank gets hot?
I wouldn't expect it to have much effect. More likely not enough heat being put into in the tank for the amount used. Most people leave the shower running wasting a lot. On most HW cylinders one can increase the set point.
Having been professionally involved in the precision measurement of energy and power, I was interested in putting numbers to the amount and cost of the heat loss from the piping. The temperature loss proved to be unimportant; less than a degree. The cost was a few pence a day, most of which can be saved by a metre of foam costing under a pound. Foam was purchased 360m away. The payback period of fitting insulation, using free labour, is less than a month. All much as expected.
BUT I was also interested in assessing the competence of the cylinder fitters, if any. The installation instructions clearly indicate that the insulation must be fitted and signed for - and that is the most obvious breach of the instructions. There are others.
A thought about the UK.d-i-y Wiki - perhaps there should be a section on when not to d-i-y - examples of when "Competent Persons" are required, the existence of the Building Regulations and their Approved Documents and of Building Control and of
SURPRISE - the Senior Resident had told me that the temperature of their hot water varies too much during the day. I assumed that the water was less hot later in the day. But when I quizzed her about measuring this drop, she explained that it was not so; the water gets hotter during the day. I have yet to gain access to investigate the matter. Due to past follies, although the Electricity Meter does charge for energy at day rate and night rate, its night output is not connected. Instead, there is an external timer connected to the lower immersion heater, and it SHOULD be set to apply power only after night rate has started. Perhaps its clock is far from correct now.
There was Economy 7 in my previous bungalow. When the meter changed from standard rate to E7, a) the whole house went on to the cheaper rate, allowing washing machines, dish washers, immersion heaters etc take advantage, via individual time switches on the ring-main sockets. We had a few night-store heaters which were on their own circuit, which were also switched on at about midnight by the meter; they couldn't be switched on at any other time, but could be switched off if not wanted.
But a neighbour of long ago also had an afternoon boost to their night-store heaters although we didn't. I assume this switched on the night-store circuit, as otherwise there'd be no way of directly activating them, e.g to the house ring main for example. IIRC there was a separate consumer unit for that PM boost. Not sure what rate they paid for the boost - possibly the standard rate.
It sounds as though your 'Senior Resident' has their lower immersion heater connected to the afternoon boost, resulting in the water getting hot later in the day as well as overnight.
Some circuit diagrams here
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