When we moved in, the underfloor heating wasn't working. If you're going to have no heating, then it's a pretty good house to be in. The next morning it was -8 outside and 14 at the coldest place inside.
The heating engineers are on a direct contract, so they are not under the skilled and careful control of the builder, which may be why they hadn't got this ready, and it is probably a very busy time for that trade. There seemed to be some problem with the eco cute boiler and a new part needed sending. The architect had brought the heating engineers in a couple of years ago when we were still trying to work out some basic approaches. After this problem, he said he'd get them to throw in the air conditioner for free, telling us that the list price was 300,000 yen on it. For starters we didn't ask for a 300,000 yen air conditioner, and it seems to be massively over spec. It was certainly useful, though, to have the air conditioner to add some heat to the house, but working underfloor heating would have been much better.
We moved in on 23rd, and the heating engineers came back three days later on Boxing Day to fix it. In the evening I went to play with the controls, which are still dangling from wires sticking out of a hole in the wall under the stairs. We went through lengthy discussions about the system with them last year, before the concrete slab went down and the pipes went into the screed on top of it. The original plan was to put pipes in the bottom layer of the slab too, so that it could be used as a heat store when solar heat was available but we didn't want to use it until later. In the event we just put pipes below the floor. Another item on the "wish I'd" list.
Basically there is a figure of eight circuit, with the boiler upstairs, the underfloor piping downstairs and a mixing valve in the middle. One pump will circulate the water around the pipes under the floor, and a pump in the boiler will send water through the tank to get heat down to it. The mixing valve will turn the hot water from the boiler to a lower temperature. Each pump has a switch with a timer.
The system seemed to have been working fine when I got back in the evening on Boxing Day. The floor was a lot warmer, up from 14 degrees in the morning to around 17 degrees. But the hot water tank was empty. In view of the heat capacity of the concrete, a good part of a metre thick and scores of square metres wide, this is not surprising.
Economically, I've been hoping to use cheap night-time electricity for heating the water, at 9 yen per kilowatt hour, and selling as much of the solar-generated stuff in the daytime as possible for 48 yen per kWh. So I set the boiler to start making hot water, and set the timer to start pumping heat around the floor.
In the morning, the error was back again, and the pump from the boiler wasn't working. I called the heating engineer, who had not been planning to come again until January 13th, and he said they'd get here in the evening. The sun was up and a fair bit of heat gets in during the daytime, and we would certainly survive until then, and were in no danger of freezing to death anyway.
As it happened the electricians were in that day, and part of their work involved switching the boiler on and off again, which cleared the error. I called the heating engineers, but they were coming anyway, and I did want to talk to them about what was happening. When they came they showed me how to reset the error, by pressing two buttons on the Eco cute control panel at once.
At that point, I hadn't realised there was a timer on the switch for the underfloor pump, which we're going to want to use during the daytime to get some of the heat from the sunny South side to cooler North. It just looked like a normal light switch, but they showed me how you can open it to reveal an LCD and some buttons.
The problem behind the error, it seems, is that the water coming out of the boiler is too hot. There's a thermyster on the pipe, and if it stays above a certain temperature for over 20 minutes, alarm bells start ringing and the boiler switches it off. The problem is a combination of the level of insulation we have making the water coming back from pipes into the boiler too warm, and the slow speed of the pump allowing it to heat up too much. This then gets to the mixing valve, and the hotter it gets the less goes into the floor circuit, because the mixing valve works to a certain temperature, so the slower it gets and the more it will heat up in the boiler upstairs.
Later they will fix this by moving the thermyster to somewhere less critical. Already we've had to trick the boiler to let us use heat under 60 degrees. In the mean time we have to juggle the timing so that we get heat under the floor but leave enough in the boiler for baths and cleaning.
When we moved in the slab was around 10 degrees, thanks to building work and doors left open over the beginning of winter, rather than keeping doors shut and using the ventilation system, which seemed like a good idea to me.
It was never intended to get that cold. When we pump heat into the floor, a lot of it will be moving down to the bottom of the foundation to heat it up. Once it's up and running, the system should only have to add a little heat when it gets really cold.
It's like we built a bicycle for climbing mountains and we're using it to start a hike from the beach. We will get there eventually though.
