I was a bit taken by surprise when someone recently suggested that condensing boilers work best at 65 deg. C. My thought was that it does not leave much room for DHW at 60deg. to avoid bacteria.
My flow and return piping is insulated so there should not be much loss tank coil to boiler.
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.
Could I fit a suitable fixed resistor in place of the Weather compensation detector (assumed to be a thermistor of some sort)?
Modern boiler have separate temperatures for CH to HW. 65C is the maximum for efficient condensing, ideally it needs to be less than that
- I have mine set for Max 60C for CH, Max 80C for HW production.
OK
Weather compensation and a clever control system, means the boiler output can go up to the maximum set, but it certainly does not have to hit the max temperature. It would only heat the water to what ever temperature it decides is necessary to achieve your desired temperature.
My system throttles the boiler back, as it near the set temperature, it predicts the input power needed, so it does not over shoot.
What would be the point of that? The boiler needs the outdoor ambient temperature detector, to improve it output Kw setting, to better match the houses heat loss.
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)
Another trick you can play on some systems is split temperature operation, when it uses different flow temps for heating the CH and DHW.
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.
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.
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)
Well it passes for entertainment during lockdown I suppose :-)
In message <rns9re$org$ snipped-for-privacy@dont-email.me, Harry Bloomfield <?.?@harrym1byt.plus.com.invalid> writes
snip.
The Veissmann has no provision for two different settings other than using the weather compensation. Elderly, retired couple with underfloor heating, 24 hour occupation and heating kept on.
Heating water temperature is limited to 50 deg. Is there any point in circulating water at 65 with the boiler spending a lot of time on the lowest burn?
Heat pumps often have a flow temp much below 60C. The trick some of them use is to run an immersion once a week to boost the tank up to 60C - that's sufficient to kill the bacteria.
Since the heat pump tries to keep the (well insulated) tank about 40C anyway, it's only the weekly 20C boost that you're paying electric heating prices for.
My Vaillant has an anti legionella cycle that you can enable. That allows it to heat to the normal DHW set temp (60 in my case) for six days a week, and then have one day where it heats to 70.
Yup, although most I have looked at are reactive rather than anticipatory... not quite sure how you would do an optimising weather compensator.
On my system, its by choosing a response curve:
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I believe the Viseman system boilers also use a similar approach.
On the Vaillant setup it also factors in the set point internal temp shifts the whole curve up to force higher flow temperatures if the temperature is set higher :
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So in this case it actually works the way many people think a thermostat does - turn it up higher and it *does* heat up quicker! :-)
My weather compensated Viessmann system boiler - quite old now - seems happy enough to heat the water above 60c at all times, but modulates down when driving the heating to the most economical temperature once the house is up to the set temperature.
I have heating to get the house to the right temperature as quickly as possible, rather than to save money.
Would likely be different with underfloor heating left on 24/7.
In message snipped-for-privacy@davenoise.co.uk>, "Dave Plowman (News)" snipped-for-privacy@davenoise.co.uk> writes
I'll fit the transducer and see how it goes.
Indeed. Ground floor rooms in the original part of the house (where there is much less insulation) get most heating at the moment. I have been meaning to ask if exterior: ground level to foundation, insulation would pay back and, if so, what would be a suitable material.
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