Home made Heat Bank

The point is that it in effect it is accepting all of the heat that the boiler can manage. When there is a coil, it is acting as a heat exchanger itself and the heat transfer depends on the flow rate, the surface area and the temperature difference between the water inside the cylinder and what's going through the coil. The fast recovery cylinders have coils with larger surface area - e.g. by using a bundle of smaller pipes.

Effectively it's a buffer between the stove and the boiler as heat sources, AIUI.

I would think that during manufacture, th coil goes in before the bottom of the cylinder is attached.

Whether one could feed it out of the immersion heater boss I am not sure, but it doesn't really matter if it can't be.

The idea was to use the bosses of the coil to connect the boiler and make a direct cylinder.

Again this was a way to add connections to the cylinder for the boiler.

Reply to
Andy Hall
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oomph is to pumps as whoosh is to sewer falls.

Owain

Reply to
Owain

If you are running a vented primary, you could also convert the cylinder to a direct one by the addition of a couple of Essex flanges, and just ignore the indirect coil.

Reply to
John Rumm

system more user friendly.

On a slightly different topic, what would you say is the best way to mitigate the conflicting requirements[1] for boiler flow temperature when using a modern boiler in a system like this? Just balance the CH for a larger temperature drop?

[1] i.e. for optimal boiler efficiency the lowest adequate flow return temperature is desirable to promote condensing, but the heat bank will benefit from a higher storage temp.
Reply to
John Rumm

You mean if there isn't enough then a big smell ensues?

Reply to
Andy Hall

I've had this debate on numerous occasions with Drivel.

The discussion is somewhat circular, though.

Assuming that the boiler comes on when either the cylinder is cold or virtually so...

In that scenario, the cold water from the cylinder will flow to the boiler and it will heavily condense, with water flowing out to the cylinder determined by the flow rate, temperature rise, efficiency of the boiler heat exchanger (high) and the burner output (max).

As the water warms, the temperatures will rise and the boiler become less efficient.

This is also true if the cylinder is indirect because now the heat transfer rate from the coil to the water in the cylinder is influenced by the transfer through the coil. If anything the effect will happen earlier

If the target temperature for the cylinder is only 60 degrees (i.e. use of HW from there), then the period of time when the boiler is running at higher temperatures is less than it is if the target temperature is 80 degrees.

There is a red herring here which says that if the store temperature is maintained at 74 degrees, and the boiler heat exchanger can operate with a 20 degree temperature change, then the boiler is always condensing. The argument is bogus because all that happens when condensing is going on is an increase in the *rate* of efficiency with falling temperature. There isn't a Holy Grail.

Turning down the store thermostat will obviously improve boiler efficiency when running. Where there is an indirect coil it will reduce cycling at the top end - because as the cylinder temperature approaches the boiler max temperature, the boiler will cycle if it's a simple one or modulate otherwise increasing the time for full recovery. Of course the latter may not matter because the point of the store is that it is storing energy at a temperature above that required for use.

Most condensing boilers can't tell the difference between being presented with a heating load that is a set of cold radiators as opposed to a cold cylinder - i.e. they are not given any signal that tells them what they are supposed to be doing. For CH, one wants the boiler to modulate down to match the house heat loss at the outside temperature. For HW (thermal store or DHW from cylinder) one wants the boiler to go flat bollock out until the target temperature is reached.

Reply to
Andy Hall

Sorry, I think we are talking at cross purposes slightly... (and dribble was usually advocating sticking a store between boiler and rads as well thereby knackering its ability to load match)

I was not particularly worried about efficiency of heating the store - as you say, for all but the last few degrees you will be close to optimal anyway. I was thinking more about the CH role. Since many/most boilers don't have a way of externally influencing the flow temperature, you end up running your rads on a higher flow temperature that you otherwise might in order to be able to achieve the desired store temperature. (Hence associated touch risk from the rads, and also the potential that you lose some efficiency from the boiler while running the heating steady state, modulated down. You also would potentially get slightly shorter cycling due to the higher rad output at the 80 degree flow temp).

Hence my question really. Obviously it partly depends on how the boiler does its load matching - but one presumes that return temp, and selected flow temp will be the major inputs into the control loop. It would be nice to have a way of automatically selecting a lower flow temp when running the rads. I can see that balancing the system to drop more per pass will aid the efficiency of the boiler (while compromising the maximum available output a little), but does not reduce the maximum touch temperature that much. Alternatively you could balance for less CH circuit drop, and rely on the load matching to reduce boiler power more...

Reply to
John Rumm

Now I see what you're saying, John.

Assuming a modulating boiler with no input signals other than one demand on/off, flow temp control and temperature sensors on return and flow and controlling only burner rate (not pump speed), the options are more limited than where there is more instrumentation and control.

However, are we sure that the thermostat setting is what the boiler will always strive to do?

For large output requirements and cold water coming in, (i.e. startup) the burner can be going flat out but the flow will still only be 20-25 degrees above return temp.

However, I am wondering what it will do where there are TRVs involved. As the house warms up, the TRVs begin to close and flow reduces. Some will turn off. Less heat is being dissipated by the radiators and if the return temperature is being maintained as low as possible, the boiler will presumably throttle back to prevent the flow temp rising too high. The question is whether the boiler will attempt to maintain the flow temperature at (say) 80 degrees or will allow it to drop since there is enough heat being produced.

Apart from the possible safety issue of radiators running at high temperatures (although that can be looked on as a reason to make the radiator covers that you've always wanted); if the boiler could work in this way, it ought to be able to figure out the difference in characteristics of the HW load vs the CH load. In ideal circumstances, where the water in the store is not being slooshed around (technical term) by the draw off through the heat exchanger, the cold water from the bottom should be presented to the boiler until quite late in the reheating cycle.

On my boiler, there are a lot of inputs and controlled outputs. Apart from those mentioned above, there are:

- Sensor on cylinder providing boiler with means to calculate actual temperature rather than whether or not heat is needed

- Room thermostat which provides boiler with actual room temperature data

- Outside temperature sensor

- Continuous control of pump speed

- Control of DHW and CH zone valves separately.

- Readout for all temperature sensors, pump %age and fan speed (proportional to burn rate)

I can also hook up a PC and see exactly what is going on

With all of that configured, the DHW temperature is set on the boiler and there are also max settings for flow temperature in DHW mode and CH mode (I have them set to 85 and 70 degrees respectively.

In CH mode, it's clear that the boiler attempts to minimise the return temperature. It certainly doesn't attempt to maintain a high flow temperature explicitly AFAICS. During mild weather, it can be running at 40 degrees flow and not much more than 20 return, with about

4kW output and the pump at about 30% of max.

When DHW reheat is required, the zone valves are switched by the boiler, to connect to the cylinder and both burner and pump wind up to full tilt. I can watch the cylinder temperature and boiler return temperature on a graph on the PC. As the cylinder temperature gets to about 57 degrees, the boiler starts to modulate back and then power drops gradually. There is no overshoot of cylinder temperature.

As an experiment, I tried replacing the room thermostat (which is a special one) with a standard one and the cylinder thermostat likewise. I configured the outside sensor off as well. In effect this is giving it almost the same control sensors that a typical boiler would have apart from knowing the difference between CH and DHW load and being able to control the pump output.

There is some observed difference in behaviour.

In CH mode, there is still modulation down such that the boiler still does not attempt to maintain the set flow temperature. However, the control is not as accurate and there is more of a tendency to cycling at lower outputs. Also, the early morning startup and behaviour in the winter when outside temperatures drop rapidly (e.g. sky clears and it gets dark) is not as good. Nonetheless, it is fairly clear from comparing the data on the PC that the boiler still makes a reasonable attempt to maintain low operating temperatures even if based only on the flow and return sensors.

In DHW mode, the behaviour is more obviously different. The boiler does still begin to modulate back, but nothing like as much as with cylinder temperature sensing and much closer to the end of the reheat cycle. The cylinder temperature will overshoot by about 5 degrees because of the detection by the cylinder thermostat (bimetallic in the test).

So the conclusion is, I think, that it is *possible* that a more basic boiler *could* do some reasonably intelligent things based on flow and return temperature sensing. Whether it does, is another thing.

Reply to
Andy Hall

The reality is that one would need to carry our reasonably detailed analysis on a case by case basis, and then design suitable external "influences" to modify the behaviour of the control system. So probably not much chance of that happening. I must admit I can't think of many generic solutions to this one, other than specifying a boiler like yours that can differentiate between CH and HW roles.

Reply to
John Rumm

One simple solution that would do at least something, would be to have some sort of arrangement to control the boiler thermostat to be different for each role. Something with resistors, a pot and a relay might do it.

At least one could achieve the goal of buckets of heat for the cylinder and less for the CH.

Reply to
Andy Hall

Yup, could work, chances are the temperature knob is just a pot...

perhaps there is a market opportunity here to make a module that "breaks in" to the wiring loom to insert an external control element into the HW temp pot. Would obviously be boiler specific, but be something with male and female connectors that simply is inserted between PCB and the front panel leads.

Yup.

If I had a thermal store I might even have given it a try!

(although the sexier solution would be using an ePot on on an I^2C control bus, hooked up to a microcontroller. Then you could really go to town)

Reply to
John Rumm

But prolly mounted directly on the PCB which is mounted behind the front panel. (At least it is in some boilers.)

Reply to
John Stumbles

IIRC the (now discontinued) Worcester Greenstar HE series had a built-in diverter valve which allowed it to respond differently when providing DHW than when doing CH. Possibly something like the current Greenstart CDi conventional might do the same trick.

Reply to
John Stumbles

The front panel is a separate module to the PCB on my Isar..

Reply to
John Rumm

The above is tripe as dedicated heat banks operate at 74C comfortably and return low temperatures for most of the re-heat time.

That can be designed in, external to the boiler controls.

Reply to
Doctor Drivel

You haven't a clue about thermal storage that is clear.

You use a weather compensator that dictates the CH water section of the store, keeping the water at the optimum temperature and boiler cycling eliminated as the water mass ensures this.

An outside weather compensator does that for you. They pretty cheap these days.

Reply to
Doctor Drivel

The Potterton Promax can have modulation turned off and the bolier just runs to the flow setpoint, as ina simple boiler. Great for integrated CH/DHW thermal storage, where the store is centre of the system and controls all.

Reply to
Doctor Drivel

Have a "diverter" valve and DHW priority system. The boiler on full temp. A pipe stat on the CH flow after the valve in the stat circuit, set to a lower temp for the rads. When on CH, the rads temp are lower. When on DHW full temp.

Better still install a weather compensator and a DHW priority system.

Reply to
Doctor Drivel

You don't have a clue about the behaviour of control systems.

That is only useful if it is integrated with the boiler. The separate ones operate by cycling the boiler which defeats the object.

Reply to
Andy Hall

Pointless unless inetgrated with the control electronics of the boiler and able to modulate its burn rate.

Reply to
Andy Hall

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