In message , John
Rumm writes
On 03/11/2020 18:32, Tim Lamb wrote:

It is that time of year...
I was a bit taken by surprise when someone recently suggested that
condensing boilers work best at 65 deg. C.

Its the return temperature that matters most, since that dictates how
much it can cool the flue gases. The more water you can condense from
the flue gas, the more latent heat you recovered. The ideal temp is
usually suggested to be around 54 since you get a step improvement
about that temperature. (it will still condense above that - just not
as effectively).

If you balance the system for a total drop of around 20 degrees, then
that allows a flow temp of ~75.

(old system designs were often balanced for a total drop of only ~11
degrees - partly to *prevent* condensation in the main HX which would
cause accelerated corrosion on systems not expecting to have water
sloshing about the outside of the HX)

My thought was that it does not leave much room for DHW at 60deg. to
avoid bacteria.

Another trick you can play on some systems is split temperature
operation, when it uses different flow temps for heating the CH and DHW.

My flow and return piping is insulated so there should not be much
loss tank coil to boiler.

Is it a fast recovery cylinder? And what heating "plan" are you using?

Old cylinders had relatively short coils, and could probably only shift
5kW at best. So they were well suited to Y plan where the system could
run both at once, to provide adequate load to avoid short cycling.

Modern setups with a fast recovery cylinder may be able to swallow the
full output of the boiler, and so are well suited to W, S, S+ plan
setups and can still keep the boiler condensing for most of the DHW
recovery.

The other question I have related to weather compensation. My system
boiler has provision (not fitted) for this but no other arrangement
for reducing the set temperature when running the underfloor heating.
The manifolds have local thermal control valves but it seems daft
running the boiler at 65 when 55 would be ample.

Having split temp operation then gives you freedom use weather
compensation safe in the knowledge that it can decide to run the
primary flow into the CH at 40 degrees if it likes but you can still
get a proper DHW reheat.

With weather comp the flow temperature is set in sympathy with the
outside temperature, and also possibly the delta between current inside
temp and the inside set point temp and taking into account the heat
lost characteristics of the house.

That will lead to long gentle burns at "just enough" heat input to keep
the place comfortable.

However you normally still want blending valves on the UFH, since you
can't assume that the primary flow temp will always be cool enough to
run that directly - it might just be -5 outside and the weather comp
decides its time to run the CH at 75.

Could I fit a suitable fixed resistor in place of the Weather
compensation detector (assumed to be a thermistor of some sort)?

Yup it would be a way of tricking it into changing flow temperature in
response to some control event.

(and yes they are usually NTC thermistors)

This weekend is *move the by-pass valve* event!

Well it passes for entertainment during lockdown I suppose :-)

Fast recovery cylinder and W plan. Caused some raised eyebrows with the
plumbers!

There are blending valves on each of the manifolds. 9 underfloor
circuits in total.

While typing this I checked the price of the sensor and decided to buy
one:-) £57.84 del! Seems a lot for a thermistor in a plastic box. Oh
well.





--
Tim Lamb