setting up 2 Keston Celsius 25 boilers

[ajrobb]

I have been playing with the bypass (low loss header?) on my dual boiler installation. The manual states a temperature differential across the boiler between 8°C and 20°C. The manual quotes efficiencies only for 20°C differential.

I think I've cracked it (the set up that is, not the heat exchanger). I want to check that I haven't got things wrong:

First, I balanced the radiators for a 20°C drop with the radiator pump (Dunfoss 15/60) adjusted down to middle speed.

While the system is warming up on full power, I set the bypass valve so the temperature rise across the boiler is nearly 20°C - hopefully suggesting the bypass is adequate but not excessive. I take it that it is vital the temperature rise across either boiler does not exceed 20°C.

With the system up to temperature and the boilers and their pumps modulated down close to minimum, both the return temperature from the radiators and to the boilers match and are nearly 20°C below the matching feed temperature from the boilers and to the radiators - suggesting the flow through the radiators and boilers are similar.

The feed temperatures are 70°C (so hot water reaches 60°C) and the return temperatures are about 50°C (so there is reasonable condensation).

 

[htgeng]

I have been playing with the bypass (low loss header?) on my dual boiler installation.

Congratulations but are you insane TWO working Keston C25s!

 

[ajrobb]

Congratulations but are you insane TWO working Keston C25s!

They weren't my choice! Fortunately the double glazing and loft insulation have rendered them oversized (they tend to run down close to minimum i.e. 7 kW each), so I have some redundancy. I daren't turn either off...

 

[Biggles..]

Sounds like the installer actually kmew how to get the best out of them.

Keating get a bad reputation because their fitted purely for the flue option by people that have no real under standing of how the wet side should be done.

 

[ajrobb]

Sounds like the installer actually kmew how to get the best out of them.

Keating get a bad reputation because their fitted purely for the flue option by people that have no real under standing of how the wet side should be done.

I beg to differ; 8 years, two heat exchangers and countless circuit boards down the line, I'd replace them if I could afford to. They're fitted on an outside wall, so the 60m flue option offers no advantage.

 

[Boilerman2]

Another completely satisifed KESTON owner..
...Skip anyone?

 

[ajrobb]

I think I was fooling myself. Yes, the boilers have nearly 20°C rise at full power, but the temperature rise at part load varies (within the 8°C-20°C range of the boiler requirement).

Also, I have a 300 litre cylinder with 28 mm heating coil connections. When the water is cold, this can absorb a lot of the 50 kW put out by the boilers. When this comes on, the pressure from the auxiliary pump practically disappears. I think it's connected as an 'S' plan but would be better as a 'Y' plan, to take the entire flow from the auxiliary pump but for a shorter time.

The auxiliary pump is a Grundfos UPS 15/60 and there is no ABV. Given that I have a low loss header that separately ensures adequate flow through the boiler, could I have an ABV across the pump rather than across the radiators? I don't want an ABV to raise the return temperature. Might a better alternative be to replace the 15/60 with an Alpha, which regulates itself?

The boiler manual quotes efficiencies at two temperatures:
88% at 60°-80°C temperature rise
96% at 30°-50°C temperature rise

It claims that condensing starts when the inlet temperature drops below 55°C. I take it that efficiency would rise from 88% with an inlet temperature of 55°C up to 96% at 30°C. With the boilers cycling on the room 'stat, the flow barely reaches 70°C (return about 52°C) before the 'stat turns off. So at least I get the efficiencies of low return temperature while the radiators are warming up.

 

[ajrobb]

now I'm getting cross. We paid £10,000 to upgrade the heating back in 2002. For that we got 2 Keston Celsius 25 boilers , 300 litre unvented cylinder with bronze pump secondary circulation and 7 new/replacement radiators.

The reason I'm cross is that the boilers have never been stretched. I have just totted up the 18 radiators and got 30 kW (I missed a couple of towel rails), a far cry from 50 kW from two boilers. The boiler size calculator suggests 44-48 kW.

To cap it all, I've just balanced the radiators for a 20°C temperature drop with a flow temperature of 70°C. This has de-rated the radiators to about 75% (22.5kW).

Second hand Keston anyone?