Logging On

Those who know me will understand that I'm a bit OCD about data and measuring things. This obsession is not limited purely to my solar power generation, nor my electricity consumption.

At the end of November I thought I'd have a go at measuring the consumption of my log burning stove, merely as a comparison with what I'm saving by not having the underfloor heating on. If I'm not making a saving, then what's the point?

Anyway, I started from some basic assumptions:

1. A load of logs, comprising 1/3rd of a tipper truck, costs us £150, delivered (it's cheaper by the full tipper truck, as the delivery cost is apportioned over 3 'loads').
2. Having consulted numerous websites, as well as Google Bard, I reached the conclusion that a 'load' of kiln dried logs weigh roughly 500kg.
3. My log basket, which I use to replenish the log pile by the fire, weighs 0.67kg.

On the above basis, and using the hand-held scales I use for weighing our motorhome gas bottles, I calculated, over the space of a month, that I use 23.11kg of logs a day - and that heats the whole house, upstairs and downstairs, at a rate of 12.3kW when fully loaded. The log burner goes out at night.

Some days, such as when we were away for the day, results in just a couple of kg being used. We haven't yet had any weekends away, so the amount will reduce a bit over time. However, we haven't really had any bitterly cold weather yet, so consumption would likely increase.

Anyway, the results so far mean a load of logs lasts me about 22 days, at a cost of a fraction under £7 a day. Assuming a winter of 5 months, which is an overestimate, I need just over 2 full tipper trucks of logs so see me through the winter, for a price of around £1,000. Let's say 2 full tipper trucks, or 6 loads.

Handy to know when we're ordering our stock of logs, as we want to buy them in summer, when the price is low, rather than having to top them up when prices are high due to high demand.

However, when I add the average of £3.60 a day I spend on electricity anyway for the lights, cooker and domestic hot water, it's debatable whether I'm actually saving anything at all by using the log burner, as the daily average with the Air Source Heat Pump and underfloor heating on at full tilt is under £10 a day; between £6.18 (current warm spell) and £10.31 (during the very cold snap).

The thing that buggers up my electricity usage is having to heat the spare bedroom with the underfloor heating when No.2 Son is home from university, as it's at the end of the pipeline and it never gets above 19 degrees, no matter the temperature of the water. The pipes to the spare bedroom take a circuitous route round the living room, rather than the most direct route, resulting in a lot of the heat being extracted within the living room before it even reaches the spare bedroom - a case of bad design.

This winter I haven't heated it at all, having installed a couple of 800W infrared wall heaters for the few days No.2 Son inhabits the room. The IR heaters have the advantage that they can be switched off when there's no-one in there and that they heat surfaces, rather than air, meaning you feel the heat immediately and don't have to wait till the air, which is a poor conductor of heat, warms up.

The Sunday before last, taking advantage of the sunny weather (free electricity) and the fact I only get charged half price for any I use between 11am and 4pm, I cranked up the Air Source Heat Pump and the underfloor heating, taking the water temperature up to 47 degrees (my usual winter setting) and the floor to 25 degrees within 6 hours. This cost me £10 in total (spread over 24 hours). That was reduced by about £3 because of the half price deal between 11-4, and I made £5.10 courtesy of the sun. Overall, it cost me £2.90 - a deal.

Kicking the underfloor heating into operation from cold takes far more electricity than keeping it running at a steady temperature once warmed up. 

Monitoring my electricity usage over the subsequent weekdays showed, conclusively, that I'm not actually saving anything by using the log burner, even at the current inflated price of electricity. So, the log burner is off for the remainder of the winter and relegated to its original purpose of being a backup.

Lighting the log burner every morning is a pain on the bum anyway, plus a vague hint of woodsmoke continually permeates the house (not unpleasant, but unwelcome on clothing). Also, while logs are sustainable, the soot generated is not conducive to clean air. You could make the same argument about the electricity, but an ASHP is between 300% and 400% efficient and there is a chance that the power is generated, at least in part, from renewables.