Heating with wood pellets: the question to ask before choosing this method of heating?
Domestic hot water problem with pellet or wood boilers
The preparation of domestic hot water (which we will call DHW hereinafter) is a problem completely (or almost) independent of the problem of dimensioning that of the heating.
Indeed: a boiler must run for heating 4 to 8 months (depending on the region and the behavior of the inhabitants) in the year while it must permanently provide DHW. We cannot therefore “in theory” shut off the boiler during the summer months (just like with fuel oil), it will simply operate at reduced load. However, we have seen that this can pose regulatory issues on a wood boiler (Acceptable on a pellet boiler but quasiement incompatible with a wood boiler).
Over the year, the energy needs in DHW are generally for an average family and house about 20% of the heating needs.
Also, generally, the pellet boilers have no integrated water heater. We must therefore prepare the ECS apart.
Some boilers have a "summer" function. The boiler then only starts up the DHW circulation pump and regulates the power on the minimum. If this makes it possible not to consume at all another energy source for DHW, the efficiency of the boiler is greatly degraded, which does not seem very wise to us.
So what are the possible solutions to have DHW all year round with a pellet boiler without overconsuming and clogging your boiler in summer? Here are the 3 possible solutions: electric, wood-electric and, the best, solar-wood-electric.
A completely independent electric DHW
Using electricity for heating (even DHW) is not very econological overall (it is economical and "ecological" in France because of the cheap nuclear kWh and which emits little CO2 but it is everything) but if the rest of your house is heated in a green way, this can be a rather interesting ecological compromise.
The solution is to make a completely independent DHW circuit of the heating circuit.
The energy consumption due to the DHW of an average household is between 2000 and 4000 kWh / year which corresponds, with an electric kWh at 0.1 euros (subscription included) to an invoice between 200 and 400 € per year, if you have the dual schedule it's even better.
For information, a shower uses about 2 kWh.
Practical example: if you consume 4000 kWh per year for the DHW, financially (bill of around € 400) corresponds to 2T of pellet but energetically this only corresponds to around 0.8 T (4000/5 = 800 kg). Electric heating is therefore 2.5 (2 / 0.8) times more expensive than pellet heating.
This electricity bill is therefore significant for an average house which would consume around 4 tonnes of pellets. Economically to do without electric DHW heating, this would make it possible to have 1,2 tons (2-0.8 = 1,2) of “free” pellets per year. In other words: to heat free of charge one year out of 5.
But electric DHW is also the solution that costs the least to invest: an electric tank costs 100 to 300 € while a dual-energy tank costs about 5 times more.
A dual-energy water heater
It is a compromise solution allowing not to use electricity when the boiler is running.
A coil pass inside the flask with water from the boiler. The ball is a radiator again, but with a separate circuit other radiators and a specific management (already provided in most boiler control). The ball also has an electric heater for summer periods or it is not advisable to operate the boiler because of the low power required for the preparation of the ECS (see sizing problem).
This solution is strongly criticized by professionals who say that it is better to burn pellet than electricity, given the low efficiency in summer, this is not necessarily true.
practical example: if your boiler is shut down 6 months a year (compromise), this saves around 50% on your DHW electricity bill. If we go back to the reasoning and the figures above, not using an electrical supplement would “just” save 1 year of heating every 10 years.
This solution therefore seems to us to be the best compromise between investment and price, but environmentally friendly, the best solution is to use neither pellets nor electricity in summer when the sun is shining! We are obviously thinking of the use of solar energy!
The Best of a storage tank tri-energy: wood-solar-electricity
The wood-solar compromise is, for us, the best solution as much in terms of gross energy as in terms of annual heating budget, but it is the one which obviously costs the most to invest.
(personally it is this solution that we have as you can see it on this page: Solar wood pattern for floor heating and DHW.)
The term tri-energy is a bit abusive because if your installation is well dimensioned electricity should be useless. Indeed, the solar coverage can go up to 80% of your DHW needs. The remaining 20% being supplied by wood in winter. Electricity should therefore be used for less than 10% of needs.
For example on our installation (see Solar wood pattern for floor heating and DHW) we do have an electrical resistance but it is not wired and we obviously never intend to wire it (it serves as a "plug" see the photos).
Technologically: it is like a normal boiler tank with the difference that an additional coil has been added to the solar.
In winter: most of the ECS produced by the boiler.
In summer most of the ECS produced by the sun and the complement (low) by electric.
This solution is therefore the best compromise in terms of polluting discharge and heating budget but the investment is heavy (count 30 to 35 € for 000m² of panels and the wood boiler excluding premiums or tax credit) but ultimately this constitutes a much better econological compromise than a heat pump whose prices can be much higher and whose maintenance, cost to use and lifespan are more unfavorable!
However, it is the heat pump that is on the rise, in particular because of the advertising campaigns carried out by EDF. We believe that the disillusions related to this energy (falsely called renewable geothermal) will soon be very large among users (maintenance costs, electricity consumption and lifespan) ... Furthermore, using a heat pump means developing and renewing nuclear power: it is therefore only ecological on the paper of the salespeople.
General conclusion of the file: wood pellets, the ecological compromise to fossil fuels!
Wood pellets is a compromise between the econological heating wood logs and heating fossil fuels: the pellets provide comfort of use that logs will probably never be able to provide.
Obviously this at a cost: the investment but also the use.
Indeed, a kWh of pellet delivered in bulk costs around 200 € / 5000kWh = 0.04 € while a kWh of wood in logs costs approximately (at 60 € per cubic meter of dry beech): 60 € / 2000kWh = 0.03 €. Bulk pellets are therefore 33% more expensive than logs (and € 60 per cubic meter is already a high price), however this difference is compensated by the yield of pellet appliances generally higher than that of log appliances.
Wood pellets are an economical and ecological means of heating (no binder is used, the pellets hold by frying, that is to say by internal pressure).
This heating method has a bright future on the condition that the supply of pellets is reliable (limited resource all the same) and does not (too) align its prices with oil. See : What future for the price of wood pellets?
The pellets are not a panacea but a very good compromise for those who dare to invest for the future.
Indeed, the field of wood waste would not, far from it, to heat all the French with granulated.
The energy future is made up of diversified solutions and especially energy savings!
1 comment on “Domestic hot water DHW and wood or pellet boiler”
Hello, is it possible to heat a greenhouse for seedlings (tomato / zucchini / cucumber etc ....) between January and April to have 20) C constant all day and evening with a simple pellet stove or pair with a water tank hot that could send hot water in a piping circuit distributed in the greenhouse please? If so, what could be the price for a volume of 80m3 please?
Good day to you.