Floor heating when the sun don't shine

It can't only be Solar

As per my previous attempt at this 10 (or so) years ago there are times when you can't use Solar (not least because we have nigh as well as day) such as when its rainy or overcast. So in anticipation of this I wanted  to prepare a different solution to last time (which I used LP gas) that might be cheaper.

Ad mentioned in my previous post I thought I'd try this:

which is a 1000W aquarium heater (electronically themo-regulated).

So 1000W (if you think heaters) isn't likely to heat your house by itself (unless you have a small and very well insulated house) but it can bring something to the party. 

Yesterday I ran the system and recorded the following data

RED = interior GREEN = under floor BLUE = outside

So lets look at the points:

1. turned on the reverse cycle AC to heat and set that to 20C
2. the floor pumps turned on and started pumping (which meant that the heater started having to heat the water because cooler water was flowing into the junction tank from the now cooler floor)
3. I lit a fire because I wanted to spend time in the living room (and the floor pumps had just stopped as they are on a timer).
4. note the change in slope on the red (the interior) curve as it fell and eventually became the same slope as the floor. 

The final red line is when the fire went out. Now when I went to bed I set the AC back down to 16C  (and it had ceased providing heat after I lit the fire) and the house now started to cool relying only on its insulation and the AC to keep its heat.

I woke at about 5:30am (pretty usual) and noted that the AC was making some gentle stirrings as the house temp fell into the zone where it was kicking in. This means that basically the house kept its heat sufficiently just on thermal insulation alone. Not bad.

Lastly there is the slope on the red and green curves. Some things I anticipate at work here are: the higher the difference in temperature (between inside and outside) the faster the rate of loss. Then as the interior reaches about the same delta T as the floor then the thermal mass of the floor (recalling its on the inside of that layer of insulation) works to hold the whole system higher. Eventually they'll fall together at the same rate as the difference diminishes. I've seen this in other plots.

Some Further Notes 

Power consumption: The setting on the thermostat on the water tank heater is 20C but I've observed that when you use a quality thermometer to measure the water temperature in the tank its 18C when it thinks its 20C (who'd have though, its not a calibrated scientific instrument?). This is interesting because the floor temperature reading (from between the insulation and the floor boards) was almost that. So the floor reached an equilibrium temperature within 3 hours and stayed there (just as the room temperature did) for 8 hours. This despite a falling of outside temperature during that time (so yes during the day).

I have a watt meter on the tank heater and it showed that the heater only consumed 1.2kWh during the day and 1.5kWh 24 hour period (yes, including keeping the tank warm all night). 

This clearly shows the benefits of the insulation (at reducing losses) and the benefits of a more modern and efficient approach to heating (reverse cycle AC).

Other benefits: without heating the floor there are many problems with using the AC to heat the house. For instance it only heats by moving warm air and so does not directly heat the floor, far down the walls, under beds or other rooms. Sure some radiation from the ceiling eventually warms the floor, but by heating the floor directly you gain this benefit this heat coming through the actual floor boards and into the carpets and under beds ...

Basically this makes the floor boards into a thermal mass for storing some of that heat after it ends and because its on "our side" of the insulation barrier keeps our space warmer longer. Evidenced by that slow trail-off of temperature.

Where to next?

We are moving out of winter and into Spring now, but we are slated for some 0C temps next week. I'll be interested to post on the results for that then. 

I'm thinking about if a better area of Solar Collection will help, but frankly I'm not entirely sure it will. This winter has been significantly more wet and overcast than previous years, so it will remain to be seen how this impacts the generation of warmth inside the house. My current area of collector is about 0.4 square meters, this suggests that I can only capture at most 100W of energy from that. So it may be worth increasing that area. Knowing that answer will require an experiment to find out.

So, as always, further research is needed.