"Andy Hall" wrote in message
...
On Thu, 17 Feb 2005 10:48:22 -0000, "Doctor Evil"
wrote:
"Andy Hall" wrote in message
.. .
All of this takes you to the point that it is most efficient to run
this type of boiler continuously at lower temperature than to force it
into a situation of reheating a cylinder in bursts.
Which is exactly what it will do when
heat demand is les than the minimum
output of the boiler. Most of them it is
around 8 kW, some go down to 5 kW.
You will get cycling.
There are a number of models now going down to around 4kW.
With your approach the boiler will *always* be forced to do high power
burns into the store so there will never be an opportunity for it to
operate in its most efficient range.
Not so. Wghenthe temperature of the store is ciontrolled by an outside
weather compensator the return temp will always be cool. A boiler raises
the temperature of water a certain degree rate. If an outside weather
compensator operating on the lowest part of a store It may be setting ity to
say 45C. when the store gets below this the compensator tells the boiler to
re-heat. As the return temp will be below this the operation is very
efficient because the return temp is low. The store of water prevents
boioer cycling. Best have the water volume about the same as the heating
circuit, so when the heating demands heat all the rads are up to temp
immediately, the the boiler comes in to re-heat in one long efficfient burn.
If this type of boiler is connected directly
to the radiators and you have TRVs, as
the room temperature reaches the set point, you have
effectively balanced the building heat loss with
the radiator outputs.
THe TRVs will begin to close. If the boiler is directly connected to
them, it is able to sense the reduced heat
demand and modulate down accordingly.
But not enough and inefficient boiler cycling occurs.
This of course is untrue.
'fraid it is true.
If you put a store in the way, you
will be emptying it at a low
continuous rate of (e.g.) 8kW,
but then when most of the energy has
been used, replenishing it in bursts of
perhaps 25kW - all controlled
by a thermostat on the cylinder.
Putting a thermal store between the
boiler and the rads with the temperature
controlled by an outside weather compensator
will eliminate boiler cycling
and operate at very low return temperatures
promoting efficiency.
This is nonsense
'fraid it is not nonsense at all.
When a house only requires 1kW of heat,
it can just take this from the thermal
store at the exact temperature dictated
by an outside weather compensator.
The boiler stays off until the stored water
drops below what the outside
weather compensator dictates. Then
it comes in to reheat to the required
temperature in one long low temperature
efficient burn.
This is an attempt at obfuscation.
It appears you don't undertsand. It is very clear.
THe two exceptions that I made were a) if it's a conventional or
non-modulating boiler - there you would make an improvement by having
it recharge the cylinder once as opposed to driving the radiators
directly and cycling to match effective power output to load;
Most modulating boiler modulate to maintain a flow temperature. These are
perfectly suitable for a thermal store. Even those modulating on load
compensation operate quite well too. Best to buy:
a) A condensing non-modulating boiler
b) A condensing modulating to maintain a flow temperature.
These generally are cheaper and less sophisticated and less to go wrong.
Keep it simple.
This is an attempt to alter the agenda.
I'm afraid you are confused.
Best system:
1. Heat bank with a dedicated flow and return from the boiler for the top
half DHW set to 75C. Have two cyl stats in this section precvent boiler
cycling.
Bottom heating section with a dedicated flow and return from the boiler.
Temperature of botom section controlled by an outside weather compensator
for heating. This could be set to a very low temperaure by the compensator.
This gives two separate temp zones in one cylinder. The DHW takes priority.
If DHW calls, all the boilers heat is directed to the top half for quick DHW
recovery. When the DHW is satisfied the boiler is controled by the outside
weather compensator and only keep the bottom section to what the compensator
dictates.
2. A cheaper simpler, more reliable, non-modulating condensing boiler.
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