Does anybody know of a good quality hot water cylinder which has a facility to replace the cylinder thermostat with a temperature sensor for better control?
The hot water system will be unvented and I'll be using a condensing boiler.
Does anybody know of a good quality hot water cylinder which has a facility to replace the cylinder thermostat with a temperature sensor for better control?
The hot water system will be unvented and I'll be using a condensing boiler.
Use a quick recovery cylinder with a condensing boiler. Also use two cylinder stats to prevent boiler cycling. One top one bottom. The stats can be strap on. High accuracy stats are not required for a cylinder. Only when the cylinder drops many degrees, differnce set by the two stats, will the boiler kick in and re-heat with one long efficient burn.
Albion, Range do suitable cylidners. Travis Perkins do the Telford Typhoon which is well priced.
If it's an unvented storage system you can't install it as a DIY job in the UK. However, since your address suggests Belgium, maybe it is allowed there.
To answer your question, though, there are electronic thermostats that will fit a standard cylinder if you remove the insulation to give a good contact. There is a separate sensor and controller. A good example of that is the Danfoss Randall TP75 family (direct wired and RF versions).
Alternatively there are boilers which will allow connection of a bullet type temperature sensor (e.g. MAN Micromat). For this type, the cylinder needs to have a pocket fitted by the manufacturer. This is basically a closed tube into the cylinder and the sensor is inserted into that, thus sensing the temperature in the core of the cylinder rather than the surface of the copper.
Could I ask why you want better control than that provided by an ordinary stat?
As the hot water near its setpoint temperature, the boiler can slow the primary heat rather than overshoot.
See my post on this.
So this wouldn't be a direct replacement for a cylinder stat? But integrated into some form of microprocessor control?
Yes, sorry if this was unclear. Some boilers have a facility to continuously vary their output..
continuously
No need in cylinder re-heat with a quick recovery coil. You want the boiler to dump all its heat ASAP into the cylinder and then shut off when up to temperature.
Most modern boilers have some sort of burner modulation.
continuously
Indeed, but they don't really need a continuously variable input from the hot water cylinder. The additional benefit of knowing this information to a control system is marginal at best.
Due to stratification, the tank doesn't so much heat up uniformly, as divide into a hot and a cold zone, the boundary between of which moves down as heating progresses. A single thermometer will show a very rapid move from cold to hot as this boundary passes its installed location. This is why it is difficult to provide good hysterysis from a single cylinder thermostat. You actually need to sense the temperature at different heights to get a feel for the amount of hot water available, not know the temperature at one height.
Room thermostats are a very different matter and much useful information could be gleaned from an analogue output sensor.
Christian.
Actually there is some value.
In the past I've used bimetal type thermostats, more recently thermostats with temperature sensor and electronic control and currently an arrangement with temperature sensor inside the cylinder in a pocket.
The bimetal type I found to have quite poor response and taking a while before a rapid temperature drop is detected and hence firing up the boiler. Moreover, the hysteresis was around +/- 5 degrees - 10 in tota - for slow changes in temperature.. So, with a nominal set point of 60 degrees, when a shower was started or the bath run, with hysteresis plus poor response, the boiler wouldn't come on until the cylinder surface temperature was at just over 50, then on the way up, the boiler wouldn't go off until around 67 or 68 degrees. Accounting for all of these things the response window is close to 20 degrees.
An electronic type(e.g. TP75) does rather better because although it is a switch on/off type, the hysteresis was smaller (+/- 3 degrees, IIRC) and the probe gives a faster and more accurate response as well.
In my current system, the sensor is wired directly back to the boiler and the controller measures the temperature (and displays it). The HW temperature is set to a nominal 60 degrees as before. I've monitored the behaviour with a PC connected to the boiler and the response is quite intelligent.
If the water temperature has been drifting slowly down through small amounts being used, nothing happens until it drops below 55 degrees - i.e. 5 degrees below set point. At that stage, a hot water cycle is initiated with the boiler being wound up to full power. As the temperature approaches the 60 degree set point, the burner and pump are modulated down so that there is no overshoot of temperature. The controller appears to learn the characteristics of the cylinder load because the behaviour improves over a few cycles following a reset.
If I start a bath etc. and there is a more rapid drop in temperature, then the HW cycle is started earlier - typically as the temperature is falling through about 58.5 degrees.
The result is that even with some stratification, the water temperature is maintained within a narrower temperature range than with the simpler thermostat.
I've found the results to be remarkably consistent with the analogue sensing by the boiler. It is monitoring temperature accurately as well as rate of change so seems to do a good job of overall control
Indeed. The room controller that I have does that and also allows you to adjust the relative sensitivities of the terms for the indoor loop and the weather compensation.
I'm guessing but it looks like the boiler control system may well implement a PID (or even just PD) type controller.
Some of the control systems that are now being used in some boilers are a significant (even you might say a quantum) leap ahead than simple on/offs. Some of them are sophisticated enough to work out a good enough strategy simply from the on/off signal and the return flow return temperatures.
They do and that is designed for the heating function. DHW usually throws them, unless they know it is DHW by a signal from a stat.
Which is not that hard since something will be switching motorised valve(s) anyway.
I have mine set up to work the opposite way around to conventional Honeywell plans
The boiler is given outside temperature, cylinder temperature and room controller data as inputs. There are a choice of functional programs for different valve or external pump configurations using three relay driven outputs. I use one where the boiler has control of opening heating motorised valve(s) or DHW valve as required - in effect an S plan configuration but with the boiler acting as controller and no auxilliary contacts used.
When a hot water cycle is needed as monitored by the cylinder sensor, then CH valve is closed and HW opened together with full power. On completion, the CH operation is recommenced by closing the HW valve and reopening the CH.
You have a state of the art boiler, most cheaper versions: Ideal, Glow Worm etc, do not have a simple function to bypass the modulating control system and keep the burner full on. Or a function to tell the control system that what it heating is not the heating circuit, so any self adaptive memory must not be used. Ravenheat of all people, even have this.
Well, maybe some, but it isn't even slightly critical. A large amount of hysterysis is actually desirable, so that the HWC can be heated in one (relatively) long burn.
What your sensor isn't telling you is the relationship between the measured temperature and the amount of energy stored. I strongly suspect that there isn't a simple direct mapping between the two, as it depends on many factors, such as the recent history of water takeoff and heating, leading to different levels of stratification. A simple thermostat will turn the boiler on when the stored energy is below a reasonable value, so it perfectly adequete in terms of the function of the system. I doubt that an analogue control system with a single analogue sensor could substantially increase the seasonal efficiency of the system.
Have you done any calculations on your system to ascertain whether this is true or false, though?
But that sounds quite good. It waits a minute or two for the bottom of the cylinder to get properly cold before turning on and doesn't turn off until the HWC is well and truely warm. It will only take maybe 10% of capacity for the bottom to get cold enough to turn on the stat. Where this is useful is overnight or other times without any hot water drawoff, as it allows the HWC to cool down a bit, rather than trying to maintain a precise temperature, which results in excessive burner firing and unnecessary primary circuit losses.
Christian.
Clearly it can't directly. Multiple sensors at different depths would be needed to do that. However, it is certainly able to monitor rate of use indirectly which was the main point.
That would probably not be substantial. I felt that the behaviour for heating the water was more interesting in the sense that the temperature is maintained at a more constant level at the top of the cylinder.
I do log the boiler behaviour, but have not run the sums.
I found it to be poor on the old system because it was a long time before the boiler would come on, by which time a lot of water had been used. Then the temperature would overshoot by a long way and the water too hot during later use. Having more accurate control has helped that, and the issue of reheating not happening during the night also taken care of because of detection of rate of use,. I don't really care about primary circuit losses because thay are into the house and I run the heating sith setback anyway.
The simplest method to re-heat a cylinder. Have two cylinder stats, top and bottom of the cylinder either strap-on or immersed. Immersed is better, but not that crucial. Have the bottom stat set to 60C for Legionella problems, the top, about 1/4 of the way down and set to just below the desired DHW temp you want. Have a DHW blending valve on hot draw-off pipe, which are to become mandatory. Set this to your desired DHW temperature, say 50C. DHW is then user adjustable.
When the cylinder is up to temp at 60C the bottom stat cut out the boiler. When the temp get below say 48C, the top stat will bring the boiler in when the temp is below say 47-48C. The boiler heats the cylinder with one long burn and no inefficient boiler cycling.
If the cylinder is running out of hot water too quickly, then there are three options:
Doing the above effectively increases the size of your cylinder. Quite simple and easy to do. Many people when installing full lengthy body jets have a larger cylinder installed. This in many cases is unnecessary. The above could be done and money saved in boiler efficiency.
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