Controlling DHW temp. with AGA.

[chrishutt]

I'm looking at a system where the DHW is heated via gravity circulation from a gas fired AGA, although there is a conventional gas boiler supplying CH system. The problem I'm anticipating is that because AGAs are running continuously, there doesn't seem to be a straightforward way of controlling the DHW temperature since the cylinder acts as a kind of heat sink.

This is a potential problem because if hot water temperatures get excessive, the design parameters of various components (pumps, shower valves, taps, etc.) will be exceeded and damage (reduced life) may result. Furthermore scale formation increases greatly above 60C, causing further problems. I'm not that familiar with AGAs, so I would appreciate advice on the following points.

Firstly is there any way of limiting DHW temp., say with a motorised valve, without having to provide an alternative heat sink (say radiator)? Alternatively, is it feasible to drain the AGA circuit and run the AGA "dry", so relegating the AGA to being just a cooker and run HW off the boiler?

 

[Agile]

After reading the other posting about the unfortunate affair with working out of scope on a Range, I hope that you have taken that into account!

These are fairly primitive devices and seem to rely on a daily usage to soak up the heat output. You cannot limit the flow otherwise they will boil the water in the jacket. Fitting a radiator may be fine during the winter but a pain in the summer.

All I can suggest is asking Aga if they can be run dry but I doubt it.

Alternatively it may be possible to reduce the heat output to just below the daily requirement and toping up HW temperature with electricity or the CH ?

Tony

 

[Gasman1015]

If you want to run it dry it would entail removing the boiler and replacing with insulation, the burner would probably require adjustment as well.

 

[chrishutt]

Thanks for responses, Tony and Gasman. I also did a bit of research on AGA site which confirms what Gasman says about needing to remove boiler section and installing insulation to run it dry.

The alternative approach is to fit a thermostatic mixing valve on the cylinder outlet supply (after the vent connection branches off, of course), so limiting the temperature of hot water to taps.

Although this deals with the temperature issue, does it also solve the scale problem? I guess scale would still be deposited in the cylinder and on the hot inlet of the mixing valve, but not downstream. Is this right?