Upgrading the House on the Hill

It is worth reprising the options for heating the House on the Hill after we remodel it.

If we assume that the heating model made previously still holds (albeit that the oil price has fluctuated greatly since then), then we can see that the energy use is somewhere from 0.6kW/degree to 1.3kW/degree. The upgrade suggested in that study is quite achievable - the additional thermal resistance from the optimised depth is significantly higher than the baseline used.

We have also seen that an 80mm is optimum for heating with oil.

A quick experiment with assumptions:

• 28000 degree hours of demand
• thermal conductivity 0.035
• base thermal resistance 0.8
• cost of fuel €0.10 per kWHr
• cost of insulation and fixings €230/m
• 15 year time horizon

This suggests 53mm of insulation, and just about pays for the insulation installation.

From a convenience perspective, we would like to avoid loading a wood boiler - or the hopper of a modest pellet boiler - more than once a day.

If the day has an average temperature difference from target of 15C and we have sufficiently thermal mass to average it over the day. Then, the total energy requirement from the loading and firing is 183kWHr to 397kWHr. So, an 8kW to 17kW boiler running continuously can keep up.

For comparison, the hot water usage for a person over a year is around 600kWHr: the efficiency of space heating is much more important than the efficiency of hot water production.

However, a more telling statistic is that the low demand kevel is about 39Kg of pellets, and the high demand level is 83Kg of pellets. The yield from wood is slightly lower although it is slightly cheaper for a given heat output.

From a practical level:

• how much space do we need to give over to wood or pellet storage?
• how often would we have to take delivery, and move the biomass to storage?
• how much hard work is it to load and clean the boiler?
• how big would the accumulator have to be for a wood burner that delivers the whole burn in 8 hours? (Some Orligno gasification boilers seem to be able to burn for 12 hours, but I suspect that most are not as effectively controlled)

If we can get an accumulator to 90C and can run radiators until the accumulator is at 60C - and we want this to happen over 16 hours (ie when the boiler is off) - then we need to supply 122kWHr to 264kWHr in that period. With 30C deltaT and shc 4.2kJ/lK, that implies that each litre can give up 0.035 kWHr per litre. As a result, we would need accumulators of size 3500l to 7600l!

It is tempting to think that these capacity issues can be solved easily by adding more insulation but we already start to push the boundaries in terms of what is feasible without a complete rebuild that addresses air exchange rates and the lack of insulation to the floor.

Some improvements may be possible but cost effective reduction below 450W/K is likely to be costly or reduce the effective thermal mass - and the house behaves quite well in summer even as it is.

This limited burn period is thus highly impractical not least because accumulators with such capacity and good heat loss characteristics are quite expensive. It would seem to rule out using a simple wood burner.

It may still be feasible to use a more sophisticated modulating gasification boiler that can deliver its energy more slowly and adapt to demand, or a pellet boiler that can also modulate and stop/start as necessary. The hopper of many pellet boilers is normally sized to give 8 hours of so of independant operation, so it may be necessary to buy an oversized unit or accept that an external hopper with additional feed complexity is a necessity.

In practice the issues suggest to me an altogether different solution:

• insulate to the levels previously proposed or slightly higher
• use a reversible air-source heat pump, 12kW or more
• decouple space heating and hot water
• use multiple hot water cylinders with integrated heat pumps and legionella programs: they seem more cost effective compared to forced circulation solar with drainback, which needs a backup source
• use fan-coil radiators that can potentially cool as well as heat, if we can deal with condensate
• invest in solar PV and look for a microgeneration connection - or a battery-backed autoconsumo as a hybrid system

The downside would seem to be complete reliance on the electricity supply.

It may be that we do not need to consider cooling through the fan-coild radiators, or that if we cool then we run through a subset of them. We will need to rewire and replumb anyway, but running condensate drains from some of the existing radiator locations may be difficult and we need to consider how much benefit there would be. It does seem that the house is quite good at staying cool as it is - we get through drafts with evening wind in Tomar.

Note that if we go this way then a microgeneration solar system is likely to be more attractive than the self-consumption systems.

The bottom line is that relatively straightforward upgrades as part of the remodelling can bring us into a heat demand level that is practical to provide with a heat pump system, and that the daily average heat demand (assuming high thermal mass) is less than 10kW.