Tuesday, 3 April 2012

Luke warm water

It was a beautiful sunny Sunday in February, so we went skiing. There's nothing nicer, after a hard day of skiing, than a nice hot bath. Usually I'd go to an onsen, a natural hot spring, and soak in abundant mineral rich waters, not worrying that they are probably heated with dirty oil because they lose so much heat to the outside air.

There was school the next day, and anyway, we have this fantastic new bath which can supply hot water, and it's only a short crawl to bed from there, rather than a drive. 

I just went with the kids, so I called as we were setting off asking the wife to put the bath on. We didn't get on that you could telephone.

When we got home, looking forward to a piping hot bath to sooth are muscles, the bath was not really all that hot. The first sign of something wrong was that the control panel didn't have the red light on to show that it was keeping the bath water warm. It's set to keep it warm for a couple of hours.

The previous day, we'd used too much hot water in the underfloor heating in the morning, and so there was no hot water in the evening and we didn't run the bath. That meant that it was starting from cold. 

I pressed the button again, and it announced that it was going to start running the bath, and the red light came on, and the little bath icon with lines floating up. A few minutes later it just stopped. No warning, no error message and no bleeping. It just gave up, put the lights out and the icon vanished. There were three bars when we'd got home, and these were starting to go down to two and then to one. 

When I checked the bath, it was not getting much hotter, but did seem to be getting fuller. 

The problem is the strategy for filling a bath. 

If the bath is empty, it's easy. It just needs to fill the bath with how ever many litres we've told it, at whatever temperature it's set for. Simple calculation based on the temperature of water coming out of the tank. 

Usually the bath is not empty though. As we get clean outside the tub, we don't need to change the bath water every day. I guess it measures the amount of water in the tub by the water pressure at the fancy jet thingy where it sends water in and out. If there is less than the required amount of water in the bath, it can get the water to the right temperature by adding hot water. Again this is a fairly simple calculation.  100 litres at 30 degress needs 50 litres at 60 degrees to make 150 litres at 40 degrees.

If the water in the tank is not hot enough to do this, then it can go to plan B, which is to circulate the water from the bath through the boiler, and re-heat the water in the bath. This is known as Oidaki.

The problem with this strategy is if the water in the boiler isn't hot enough, it will just take away heat from the boiler, and still leave the bath luke warm. Remember the second law of thermo dynamics. 

So I suspect what's happening is that the Eco cute tries to add hot water to the bath, realises that it can't get it hot enough with hot water, then tries Oidaki, and then gives up.

We just have to make sure that there is enough hot water, and in this case there wasn't for about four reasons.

First of all, the night before had been very cold, minus 11 in the morning, and not ideal conditions for an atmospheric heat pump. Exactly how unideal is an ongoing concern.

Second, we'd left the underfloor heating on for rather too long in the morning and run off a lot of the heat from the boiler. This left it in a situation of abundant water at low temperature.

Third, we had a full bath of cold water that had not been heated the previous day. Part of the reason why it  was full is that the washing machine is not working properly. It's supposed to be able to reuse the bath water in part of its cycle, but doesn't seem to be working. The bath water, controlled by a special tap and with a separate hose into the washing machine, keeps running into the washing machine when the tap is on, while the washing machine should actually be switching it on or off itself. That's another story though.

Fourth, and this may have been fairly critical, the temperature of the boiler was set to low. I'm not sure why, but I remember showing the wife how the temperature setting worked a couple of days ago, and maybe I didn't set it back again. If the boiler had been set to a higher temperature, in fact, everything would have probably been OK. It is now.
at least influencing