On Sun, 5 Sep 2004 10:51:52 +0100, "IMM" wrote:
"Andy Hall" wrote in message
.. .
On Sat, 4 Sep 2004 21:53:07 +0100, "IMM" wrote:
It does if it is going to be involved
in an efficient control system
where the heat output to the radiators
and the heating space are
measured
They are measured by the weather
compensator, with any room temp influence
to drop the slope if need be.
It should be the other way round of course.
The outside temperature is monitored
to provide a correction to the slope.
failed again. I'm fed up marking you as it's always a poor score.
That's good. You weren't invited to in the first place.
The
outside weather gives the slope and the internal temp sensor (if one fitted.
Many internal temp sensors can be fitted and the temps averaged on large
buildings) raise or lowers it to suit.
We are talking about a simple domestic system here, not a commercial
one
For you:
1. With heat bank and a weather compensator, a mass of water is heated at
the bottom of a heat bank to the temperature what the building requires (the
rads). The temp can be anything from 25C to 80C depending on the outside
conditions. The temp of the lower mass of water is dictated by the weather
compensator, which is near enough the correct temperature that the rads will
require.
So the boiler will be put into a situation where it will cycle
frequently when the heatbank temperature is low.
You still haven't suggested a suitable controller.
2. A load compensating modulating boiler generally goes down to about
5-7kW, any lower than that it cycles on-off. Some super expensive boilers
go down to 3kW.
As most of the time buildings are up to temp (part load on
the heating system) this will be significant cycling in most modulating
boilers.
This is based on a bunch of assumptions which in practice are not the
case.
Also, any cycling that does happen will be at the bottom of the
modulating range and not full power as in your example.
This makes a substantial difference.
These boilers tend to be complex and expensive and have more
expensive parts and servicing to go wrong on them.
This is untrue. The extra complexity is in the firmware of the
controlling microprocessor. There are no additional mechanical
components
- With a heat bank the rad circuit(s) can draw-off only 1 kW of heat from
the heat bank and no cycling from the boiler. Only when the whole mass of
water cools does the boiler come in and re-heat the mass of water in one
long efficient no boiler cycling burn.
Of course there will be cycling.
You still haven't said whether you are going to maintain the heatbank
at the setpoint or allow the temperature to drop to practically room
temperature.
- The temp of the mass of water is not totally fixed as the temp will drop
slightly as the mass of water is cooling.
How much is "slightly"?
This is insignificant and TRVs
trim off local temps.
Only if the heat being provided by the water is within the range
required and controllable by the TRV. Since you haven't indicated
what that will be, this suggestion is guesswork.
Weather compensating slopes are never fully accurate
and may out by +-5%. The slope is a best fit at the best of times.
5% accuracy on a system for domestic use is pretty good/
It may well be like this or worse on an external box controller such
as you describe because the controlled device is an unknown
If you have an external sensor connected to a boiler designed with
weather compensation and for use with a nominated sensor, then all the
pieces can be matched together as a system and will produce a much
better result than can be achieved by throwing bits and pieces
together - especially cheap ones as you are suggesting.
By the way, what about that controller?
internal temp sensor (if one fitted), raise or lowers the slope it to suit.
Many internal temp sensors can be fitted and the temps averaged on large
buildings for greater accuracy..
We are talking about a house, not a commercial building.
- A simpler, and cheaper, condensing boiler can be used, similar to any
regular heating boiler, that can have the optimum flow through the boiler
at all times for greater efficiency and boiler longevity.
Where is this cheaper boiler? Where is the £400 model that you are
dying to tell us about?
How are you going to control the flow to optimise it?
- A variable speed Alpha pump can be used on the rad circuits reducing noise
electricity consumption.
A modulating pump is always a good idea to match heat source and load.
Better yet if incorporated into the boiler and controlled by its
microprocessor.
- If having a heat bank for DHW, then increasing the size to provide:
* a cheaper, simpler and more reliable boiler,
Which one? I'm still waiting.
How big is this heatstore going to be?
At least 100 litres is needed for the DHW, and 200litres for the space
heating. What about the space for these cylinders?
Normally you are squeaking about space saved by combis. Now you are
proposing a half baked, unreliable system with huge space
requirements.
* a neutral point for all circuits,
Only a benefit if there are multiple heat sources.
* weather compensation
Except that your suggestion does it poorly in comparison with a full
integrated modulating boiler.
* elimination of boiler cycling when on part load, which gives far greater
boiler longevity on all components.
It does not eliminate boiler cycling because the damned thing is being
switched on and off.
* oversized rads to cope with a condensing boiler giving lower return
temperatures.
If possible one should always do that with condensing boilers if
possible.
How is your system going to cope with heat output required for
different radiator design schemes? It is not monitoring the return
temperature.
* Variable speed pumps(s) on rad circuits.
Even better if one is integrated with the boiler and controlled by it.
* instant heat available first things in the mornings. Rads are fully
heated with afew minutes.
The same happens with a modulating boiler.
It is better if th control system can monitor the room temperature and
back off the heat supply at the right time.
If you dump heat from a store directly into radiators when it has been
a cold night and presumably the heatbank temperature is high, you are
going to get huge overshoot and waste of heat.
Now understand this if you can. It is not difficult. If there are parts
you are confused about, just accept it as right.
I am not and have never been confused.
Perhaps you would like to answer the questions that you have been
avoiding.
..andy
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