ᐅ Is the T8 Poroton brick only significantly better than the T12 in terms of heating costs?
Created on: 16 Dec 2015 16:38
K
kalle2000K
kalle200016 Dec 2015 16:38Hello everyone,
we are converting a barn into a residential house. The existing walls will remain, and essentially a new structure will be built inside. The wall composition is as follows:
30 cm solid clinker brick
4 cm air gap
36 cm T12 Poroton blocks
1.5 cm lime-cement plaster
This results in a U-value of 0.27 W/m²K.
For comparison:
30 cm solid clinker brick
4 cm air gap
36 cm T8 Poroton blocks
1.5 cm lime-cement plaster
This results in a U-value of 0.19 W/m²K.
Our energy consultant calculated heating costs of 930 € for the T12 version, based on a building area of 240 m² (2583 ft²) and heating with an air-source heat pump. For the T8 version, the estimated cost is 890 €. However, it should be noted that he incorrectly used an electricity cost of 7 cents per kWh. Currently, our rate is at least 18 cents per kWh.
Is it possible that the better-quality blocks only have a minimal effect on heating costs?
Regards, Tobias
we are converting a barn into a residential house. The existing walls will remain, and essentially a new structure will be built inside. The wall composition is as follows:
30 cm solid clinker brick
4 cm air gap
36 cm T12 Poroton blocks
1.5 cm lime-cement plaster
This results in a U-value of 0.27 W/m²K.
For comparison:
30 cm solid clinker brick
4 cm air gap
36 cm T8 Poroton blocks
1.5 cm lime-cement plaster
This results in a U-value of 0.19 W/m²K.
Our energy consultant calculated heating costs of 930 € for the T12 version, based on a building area of 240 m² (2583 ft²) and heating with an air-source heat pump. For the T8 version, the estimated cost is 890 €. However, it should be noted that he incorrectly used an electricity cost of 7 cents per kWh. Currently, our rate is at least 18 cents per kWh.
Is it possible that the better-quality blocks only have a minimal effect on heating costs?
Regards, Tobias
Hi Tobias,
The U-values seem to be roughly correct at first glance: a pure T8 wall with 36.5 cm (14.4 inches) thickness has a U-value of 0.21, and a pure T12 wall with 36.5 cm (14.4 inches) thickness has 0.30. I’m somewhat surprised that the 30 cm (12 inches) brick with an air gap only contributes so little, but considering that a 30 cm (12 inches) solid brick wall has a U-value of 1.85 (!!!!!), it might make sense.
However, I would question the final result:
A difference in wall insulation of about one third is supposed to cause only a 5% difference in heating costs? That would mean the house is either almost all windows, or you have a very poorly insulated roof, so that the walls hardly make any difference.
I also find the overall calculation strange:
About 900 euros heating cost at 7 cents/kWh (which is not very trustworthy either that he uses such a number) corresponds to roughly 13,000 kWh electricity consumption. With an annual performance factor (COP) of 3 for an air-to-water heat pump, that results in a heating demand of 39,000 kWh, which is 160 kWh/m² (15 kBTU/sqft) per year?! For a house with a U-value of 0.19?! No, that can’t be right!
Did he calculate with open windows?
For comparison: We currently live in a house with 135 m² (1450 sqft) of living space plus 60 m² (645 sqft) of heated basement and, with a wall construction also at U 0.19, we use about 9,000 to 10,000 kWh of heating energy per year.
He should explain the calculation to you carefully again. That would also mean you have to expect heating costs of about 2,600 euros per year.
That’s more than my mother pays in her 250 m² (2700 sqft) mansion from 1975!
It might be that the calculation is correct, for whatever reason, but at first glance, I would be very suspicious.
Best regards,
Andreas
The U-values seem to be roughly correct at first glance: a pure T8 wall with 36.5 cm (14.4 inches) thickness has a U-value of 0.21, and a pure T12 wall with 36.5 cm (14.4 inches) thickness has 0.30. I’m somewhat surprised that the 30 cm (12 inches) brick with an air gap only contributes so little, but considering that a 30 cm (12 inches) solid brick wall has a U-value of 1.85 (!!!!!), it might make sense.
However, I would question the final result:
A difference in wall insulation of about one third is supposed to cause only a 5% difference in heating costs? That would mean the house is either almost all windows, or you have a very poorly insulated roof, so that the walls hardly make any difference.
I also find the overall calculation strange:
About 900 euros heating cost at 7 cents/kWh (which is not very trustworthy either that he uses such a number) corresponds to roughly 13,000 kWh electricity consumption. With an annual performance factor (COP) of 3 for an air-to-water heat pump, that results in a heating demand of 39,000 kWh, which is 160 kWh/m² (15 kBTU/sqft) per year?! For a house with a U-value of 0.19?! No, that can’t be right!
Did he calculate with open windows?
For comparison: We currently live in a house with 135 m² (1450 sqft) of living space plus 60 m² (645 sqft) of heated basement and, with a wall construction also at U 0.19, we use about 9,000 to 10,000 kWh of heating energy per year.
He should explain the calculation to you carefully again. That would also mean you have to expect heating costs of about 2,600 euros per year.
That’s more than my mother pays in her 250 m² (2700 sqft) mansion from 1975!
It might be that the calculation is correct, for whatever reason, but at first glance, I would be very suspicious.
Best regards,
Andreas
Saruss schrieb:
I think the consultant calculates the heating kWh using 7°C (45°F), which makes sense, whether it’s gas or heat pump, at least approximately
I had considered something like that as well. However, that results in an annual heating energy consumption of only 13,000 kWh, which I find very optimistic for a 240 m² (2,583 sq ft) house with average insulation.
As I mentioned before, I think the original poster should ask for a detailed explanation of the calculation...
Best regards,
Andreas
Yes, that can be quite possible. Assume a heat loss of 20% through the masonry. This means you have 0.2 (20%) * 0.3 (30% worse insulation) = 0.06 → 6% energy savings. €930 * 0.94 → €874
7 cents per kWh for heating energy is fine – that corresponds to the gas price. An annual performance factor (COP) of 3 for the air-to-water heat pump means 0.238 / 3 = 8 cents per kWh of heat… so that fits as well.
Do you mean 240 m² (2583 ft²) of floor area or exterior wall area? If it’s 240 m² (2583 ft²) of floor area, 13 kWh per m² is ambitious... but not impossible either.
Please post your energy performance certificate or related documents; that would make things much clearer.
7 cents per kWh for heating energy is fine – that corresponds to the gas price. An annual performance factor (COP) of 3 for the air-to-water heat pump means 0.238 / 3 = 8 cents per kWh of heat… so that fits as well.
Do you mean 240 m² (2583 ft²) of floor area or exterior wall area? If it’s 240 m² (2583 ft²) of floor area, 13 kWh per m² is ambitious... but not impossible either.
Please post your energy performance certificate or related documents; that would make things much clearer.
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