I'm in the process of reviewing my CH system, and have re-calculated all my heat losses and compared them with the radiators currently fitted. Many of the rads are over generous - partly because they were fitted a long time ago when the house was far less well insulated.
I currently have a conventional boiler, and run the system at a mean water temperature of about 76 degC (82 flow and 70 return), giving a typical Delta-T of 55 degC. At some point in the future I may replace the boiler with a condensing model (probably won't have a choice!) and would like to re-asses the adequacy of my rads for running at a lower Delta-T.
60 flow 40 return. Top quality (expensive) boilers can modulate down to 3 to 5kW, which is still high when the house is up to temp. 90% plus of run time boiler is on part-load so does not need 82C - 70C.
Assess your DHW, a heat bank may be the better option with TRVs on all rads and a Smart pump on the CH and also mains pressure DHW. Then a cheaper simpler boiler can be used to heat the heat bank.
Or a packaged solution: ACV Heat Master (integrated outside weather compensator), Gledhill Gulfstream or Atmos Multi. These makers will provide a Smart pump with the units if requested.
I did this exercise a few years back to achieve exactly this.
The first thing is that a condensing boiler can manage a much larger temperature drop - 20 or 25 degrees rather than nominal 12. This means that for a given flow rate, more heat can be transferred. Secondly, it means that radiators will output more heat because there will be a larger temperature drop.
But....... of course, there's a caveat. That is that the mean water to air temperature, that you had already alluded to is much lower because the conventional design is to go with 70 degree flow and 50 return.
If you plug that into your radiator data sheet, you will see that the derating factor falls from about 90% (82/70 case) to about 60% (70/50 case) which makes a huge difference to radiator size if you decide that you want to use these temperatures.
I did a complete design check for the house and found that over half the radiators were oversized enough that they would still be OK at
70/50. There was one room which had always been cool in the winter and it turned out that that had only about 70% of the needed capacity at 82/70. The remainder were under the requirement for 70/50.
So I changed four radiators (making them double panels with fins) and put one much larger one in the room that was too cool. I also added a separate circuit for the conservatory. All of this permitted 70/50 operation.
The boiler that I used is a MAN Micromat, which is a fairly sophisticated boiler with external weather compensation, continuous range pump control and temperature sensing. For example, both the room and cylinder thermostats provide the boiler with temperature figures as opposed to on/off signalling. There are settings to control the interaction of the weather compensator vs. the room monitor and their control effects. In addition, the boiler will modulate down to about 4kW in heating mode if required. Later in September, it will start coming on a little in the evenings as the weather cools and runs with a flow temperature of 40 degrees or so. There is a data logging output to which I can hook up a PC. In the coldest weather, the flow temperature has not yet been above 65 degrees. I can have a limit set on flow temperature of 70 degrees if I wanted to, although I have actually left it at the default 85 degrees. The HW is handled separately. When there is demand there, the boiler is wound up to
30kW or so to rapidly heat the cylinder.
It's fair to say that most boilers don't have this sophistication. the only other that I know that does is Viessmann, although there may be others. The majority have a thermostat and do some form of modulation. With these, it may be interesting to set the thermostat seasonally to prevent the operating temperature going too high, although generally the boiler ought to optimise for low temperature if it can.
Only if it is able to provide an analogue or equivalent signal to the boiler.
If it is handling the boiler by simply turning its burner on and off, it is not a good solution by comparison with a boiler that has full external temperture sensing and acts upon that.
The flow temperature will fluctuate up and down as the burner is turned on and off because the thermal intertia of the heat exchanger is small.
This is incorrect.
A boiler with built in weather compensation by virtue of measuring temperature directly will achieve a better controlled result than an external device that turns the burner on and off even if there is anti cycling.
You are suggesting breaking the control loop with on/off switching, and that, by definition, is inferior to full analogue measurement.
If, on the other hand, the boiler has no weather compensation at all, an external device *may* be interesting .
However, if anticycling also has to be brought into the equation in order to avoid the inefficiency introduced by turning the burner on and off, you will have introduced two discontiguous control elements into the system thus making the control even worse.
Matt, it is correct, mine has anti-cycling circuitry.
Matt, no one is arguing with you.
Matt, read back, a simple cheaper boiler can be enhanced by using stand alone weather compensator.
Matt. very interesting indeed.
Nope. the flow temperature remains low and the boiler does not continually cycle. Best of a buffer tank is used, or a heat bank, which the weather compensator controls the temperature of, then no to little cycling.
Except that by the time one has bought all that lot, one can have bought a boiler with the proper controls and sensing, so the exercise in adding an external compensator plus the bits to make it work is pointless.
If one already had the boiler and wished to add an external box of tricks, then it may be interesting. It certainly isn't if one is starting with a clean sheet of paper as in the case. The decision would be to go for a boiler with the functionality built in or not to bother with any means of weather compensation.
Which are separate to the point being discussed here and may or may not be relevant.
That argument doesn't really hold weight.
Consider the differences between the so called "cheap and reliable" boiler and the more expensive one such as the MAN or Viessmann.
The firmware of the more sophisticated boiler will be more complex. Does this lead to greater unreliability? I don't think so. More to the point would be boards used in cheap boilers engineered down to a cost. One thinks of certain Potterton models here which are notorious.
The burner and heat exchanger in the more sophisticated boiler are of better quality. That has been said many times, even by you.
Therefore what is it precisely in one of these "simpler boilers" that makes it more reliable?
How does adding a heat bank, with extra controls, valves, relays and thermostats result in a more reliable system overall?
Once the total cost of installation and ownership of something like these vs. your collection of bits and pieces are added up, there is likely to be very little in it.
What exactly is absent that makes for more reliability on these "simpler boilers"?
Which on a modulating boiler consists of a fan with variable speed control operated by the electronics. Are you now going to tell me that this alone worsens reliability, or would it more likely be the shitty bearings in the cheap boiler's fan?
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