"IMM" wrote in message
...
"Andy Hall" wrote in message
...
To a certain extent that's my feeling but I just couldn't resist
thinking
about how the pool could be used to make the whole thing work
better and perhaps get to the 20% extra promised by the boiler.
I still sort of think that there should be some way of making use of
a low temperature sink but I'm beginning to see that its
more difficult than I first thought.
mikej
Even if you connect the heat exchanger in parallel to the radiators
(as they should be) there will be a downward effect on the return
temperature to the boiler.
Think about the flows of water and the heat loads. Let's say for
simplicity that you arranged equal flows through the radiators and
through the pool heat exchanger, but that the pool heat exchanger
represents twice the load of the radiators. I am assuming that the
pool heat exchanger is not limited by its dT - which actually with
good sized stainless steel plate jobs will be the case - they can be
capable of transferring 200kW in a very small size. Since everything
else is equal, you will get a twice larger temperature drop on the
return from the heat exchanger than from the radiators. When the
water is mixed to return to the boiler, it will be lower than with
either load connected alone.
I have an arrangement like this for providing heat to my garage
workshop. It runs with a separate circuit and via a heat exchanger,
for a number of reasons. When there is a heat requirement to the
garage (which is generally equivalent to a couple of large radiators),
the boiler return temperature does drop and the burner is wound up a
bit as is the flow.
The one questionmark that I raised with this is that with the pool,
the heat load is going to be vastly greater than that for the
radiators. Therefore to get a reasonable distribution of heat,
some balancing would be needed to effectively throttle back the heat
supply to the pool as IMM suggested.
They do have rather different characteristics as well, which is why
there is generally a switchover arrangement for heating cylinders.
This is why having two separate boilers may have proven to be an
effective solution, simply because the load characteristics are so
different.
1.
Assume a Keston Celsius with load compensation control. When the flow and
return temps become closer together the system assumes the house is
nearing,
or up to, temp. It then modulates the burner down. When the return and
flow
temps become wiser the assumption is that the house is becoming colder and
winds the flow temp up.
CH is only on the house warms up and the boiler drops the flow temp.
Fine.
The pool heater is switched in. The return temp from the pool is very low
(17C) and this blends with the return of the house heating (say 30c) to
say
give a combined return temp of 21C. Great for boiler efficiencies. Less
gas is being used. The control system thinks the house is cooling and
ramps up the flow temp to max. Fine, as the pool heater needs this heat.
This means the flow to the house is too high. This is fine as TRV rad
valves will compensate for any heat gain; assuming they are fitted in the
rooms.
2.
The Keston boilers quoted are small commercial jobs with no modulation.
With these boilers it is best to fit a Danfoss Randall BES 5000 outside
weather compensator (£166 from discountheating) . This will drop the
return
temp of the boiler to the outside weather promoting efficiency. The room
TRVs will compensate for any highs or lows in the room trimming off
locally.
The pool heater kicks in. No matter what the combined heating and pool
return temp is, there is enough heat flowing to the pool to heat it. The
burner will stay full on as the return will have a good flow and a
constant
low return temp. The boiler will take a long time to make an impact on
that
volume of cold water. The pool temp is only around 21-23C. That will
work.
This goes for a couple of basic Ravenheat condensers too.
I see B&Q still have the Ravenheat 25 kW CSI primary at £399. This is a
basic condensing boiler with on-off control, no pump, no pressure vessel and
can be open vented or pressurised at over 90% sedbick efficency. Two of
these is the same price as the Ideal cast-iron clunker and a boiler for
backup too. Well worth considering and implememting as i suggested above
in No. 2.
With either of the above have the DHW heating on a priority, cutting out
the
pool heater to heat the cylinder.
Better still have one boiler do the house and DHW and one do the pool
(assuming a 25kW boiler will heat the house). With a few valves to
alternate boilers if say the house boilers drops out, always having a
backup for essential services. Or if there is a constant large demand
for
DHW, it may be best to have the pool and DHW on the one boiler with a
priority system for DHW.
Now it depends on what deals you can get with boilers. The two boiler
option in a commercial concern is by far the best.
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