Hi,
I've done a fair bit of googling and reading posts in this forum regarding my current setup. Probably stuck with what I have, but would like to be 100% sure. So any extra insights would be very welcome. Thanks in advance.
I have a large 5 bed detached house, 16 radiators in all, with a Vaillant Ecotac 637 (37 kW) boiler and a 210 l Vaillant unistor hot water cylinder. In S plan with 2 motorised valves, all radiators TRV (except bathroom (3) and hall (1)) and thermostat (on/off type) in hall.
The radiators can heat the house (some rooms faster than others, but all get there) so are near the heat requirements, as far as I can understand. Total maximum heat output of the radiators is around 16 kW (Some bigger ones at 1700 W, some smaller ones at 500 W).
This boiler is oversized I guess (for some strange reason - I guess the previous owner thought bigger is better);
This is even more so as I don't use a few of the rooms, so their TRV's are set lower, say setting 2, and close down quickly once the chill has been taken off the rooms.
So on normal, I estimate only to use a sustained 6 to 9 kW once all is warmed up.
The boiler can modulate down to 6 kW so that should be fine (and it does, no cycling), and I've maxed/downrated the boiler to 18 kW (setting d.00) in the hope it helps with efficiency.
BUT
With house reasonably warm, the TRV's closing up just enough to keep everything warm, the radiators only outputting about, say, 8 kW that means only around 700 litres of water (at around 10 degree drop, I've balanced as well I could) flows around the radiator. Even though the boiler can modulate down to 6 kW; the pump doesn't. So it pumps around circa 1300 litres of water per hour minimum (I guess to keep the heat exchanger safe) and so the internal bypass is open most (of not all) of the time, mixing hot water straight back in to the return:
Boiler puts 6 kW into heating 1300 litres of water by about 4-5 degrees. Even though the return is 10 degrees cooler, this is mixed with half of the flow resulting in a return of only 4 to 5 degrees below flow.
I've read somewhere the bypass opening means the efficiency of the boilers drops (like a rock, I think was the exact frase used). Sounds believable to me.
SO
I'm surprised they (Vaillant) even sell a boiler that can modulate down to as low as 6 kW when that results in bypasses opening up and reducing efficiency?!
Is that correct?
Are there ways to adjust the setup to increase efficiency, i.e.
Can one have the pump replaced (downsized), effectively changing a 637 into a 618 for at reasonable cost?
Or would weather compensation help (still having the bypass open all the time but at least the return temperature would be in a more efficient range most of the time).
Any other suggestions?
Thanks in advance,
Waldo
I've done a fair bit of googling and reading posts in this forum regarding my current setup. Probably stuck with what I have, but would like to be 100% sure. So any extra insights would be very welcome. Thanks in advance.
I have a large 5 bed detached house, 16 radiators in all, with a Vaillant Ecotac 637 (37 kW) boiler and a 210 l Vaillant unistor hot water cylinder. In S plan with 2 motorised valves, all radiators TRV (except bathroom (3) and hall (1)) and thermostat (on/off type) in hall.
The radiators can heat the house (some rooms faster than others, but all get there) so are near the heat requirements, as far as I can understand. Total maximum heat output of the radiators is around 16 kW (Some bigger ones at 1700 W, some smaller ones at 500 W).
This boiler is oversized I guess (for some strange reason - I guess the previous owner thought bigger is better);
This is even more so as I don't use a few of the rooms, so their TRV's are set lower, say setting 2, and close down quickly once the chill has been taken off the rooms.
So on normal, I estimate only to use a sustained 6 to 9 kW once all is warmed up.
The boiler can modulate down to 6 kW so that should be fine (and it does, no cycling), and I've maxed/downrated the boiler to 18 kW (setting d.00) in the hope it helps with efficiency.
BUT
With house reasonably warm, the TRV's closing up just enough to keep everything warm, the radiators only outputting about, say, 8 kW that means only around 700 litres of water (at around 10 degree drop, I've balanced as well I could) flows around the radiator. Even though the boiler can modulate down to 6 kW; the pump doesn't. So it pumps around circa 1300 litres of water per hour minimum (I guess to keep the heat exchanger safe) and so the internal bypass is open most (of not all) of the time, mixing hot water straight back in to the return:
Boiler puts 6 kW into heating 1300 litres of water by about 4-5 degrees. Even though the return is 10 degrees cooler, this is mixed with half of the flow resulting in a return of only 4 to 5 degrees below flow.
I've read somewhere the bypass opening means the efficiency of the boilers drops (like a rock, I think was the exact frase used). Sounds believable to me.
SO
I'm surprised they (Vaillant) even sell a boiler that can modulate down to as low as 6 kW when that results in bypasses opening up and reducing efficiency?!
Is that correct?
Are there ways to adjust the setup to increase efficiency, i.e.
Can one have the pump replaced (downsized), effectively changing a 637 into a 618 for at reasonable cost?
Or would weather compensation help (still having the bypass open all the time but at least the return temperature would be in a more efficient range most of the time).
Any other suggestions?
Thanks in advance,
Waldo