On 03/12/2021 04:28, RJH wrote:
> On 2 Dec 2021 at 18:41:25 GMT, "John Rumm" <see.my.s...@nowhere.null>
>> On 02/12/2021 18:06, Tim+ wrote:
>>> NY <m...@privacy.invalid> wrote:
>>>> Having had various types of CH and HW systems in different houses I would
>>>> rate them (least preferable first, most preferable last)
>>>> - cylinder/tank-fed
>>>> - cylinder/mains-fed
>>>> - combi/mains-fed
>> My own experience is that the unvented cylinder trounces the combi in
>> every way... Massively faster delivery of water, at higher temperatures,
>> and people can have long showers without turning off the house heating.
> I'd have thought it costs more to run the HW side of things with the heat
> losses from the cylinder.
The losses from the cylinder itself are pretty small (couple of hundred
watts perhaps) - it is well lagged with the lagging contained in an
outer skin. Keep in mind that in many cases you want some of the heat
loss from the cylinder, since it can then heat an airing cupboard. (I
deliberately chose not lag some of the pipework so as to lose some extra
heat into the airing cupboard)
Combi heatloss is in theory lower - but you do end up running off more
partially heated water waiting for the output to get to temperature. Not
all combis can run in condensing mode when heating the DHW. Also combis
typically need to be near an outside wall, so pipe runs can be longer. A
cylinder is often best near the middle of a property.
> But in other respects I tend to agree that HW using a combi is a mixed
> experience. Takes a good 20s for hot water to come through, although when it
> does it's a decent amount and plenty hot enough. I don't use the preheat
> option - it's pretty quick if I do. Multiple showers etc. are not going to
> happen, which would be more of a problem for larger households. As it is, on
> balance, a combi suits me better overall.
I installed a combi in my last place when I converted the loft. Went for
a 35kW model. I found that it could do one really good shower or two
"adequate" ones at once - at least in summer. (the 1/2" mains feed in
that place could only muster about 17 lpm at best). All in all it was
ok, and did better than many combis I had experienced in the past. (and
the vented system it replaced was particularly poorly done, so I hardly
lost any bath filling performance).
In this place there is a decent MDPE mains feed that can do 30 lpm or
better, so the cylinder will do two reasonably good showers at once.
From the DHW point of view I was designing it for performance, and gave
no consideration to cost of running TBH. However I did focus on cost and
efficency for the system as a whole - going from a single heating zone
off the back of an old Ideal Mexico floor stander that was probably less
than 70% efficent, to a modern condensing system boiler with 95%+
efficiency, split heating zones, external weather compensation, and
split temperature operation (so the CH can run at 45 degrees when its
mild out, but the DHW still gets heated to full temperature)