Boiler flow temperature

Joined
9 Sep 2005
Messages
12,769
Reaction score
3,495
Location
Sussex
Country
United Kingdom
It's a dog of a boiler...there have been many many postings about similar problems with this model...flawed heat exchanger (it collects debris and is difficult to clean even with powerflushing) and software (it needs just the right flowrates and temperature gradient).

Do yourself a favour and start saving for a new boiler, at 10 years old with all the problems this model has it's not worth salvaging.

BTW I've inherited the same boiler...and it's going in the skip asap.
 
Sponsored Links
Joined
8 Apr 2008
Messages
10,731
Reaction score
2,712
Location
Essex
Country
United Kingdom
Your pump is too small for heat exchanger resistance plus rads, get bg to put either a bigger pump 25/80 or another 15/60 in series with the pump you got.
 
Joined
18 Oct 2007
Messages
10,587
Reaction score
1,353
Location
Kent
Country
United Kingdom
Your pump is too small for heat exchanger resistance plus rads ...
It's only too small if the system has been balanced incorrectly.

The boiler is designed to work with a flow/return differential of up to 20C. The chart below shows the resistance through the hex for a 21 kW output when the differential is 20C (green) and 11C (red). You will see that the resistance of the heat exchanger is proportional to flow rate squared, i.e doubling the flow rate increases the resistance four times.

38hxi.PNG


At the flow rate required for an 11C drop at 21kW output the pump is not even capable of providing the head to overcome the resistance of the heat exchanger, ignoring the rad circuit. The boiler with therefore modulate down until the output of the boiler matches that of the radiators, which will be less than 21 kW.

Increasing the differential, which reduces the required flow rate and the resistance of the hex, should leave sufficient pump head for the rad circuit.

With a flow of 55C and return of 42C (13C lower as per first post) the rads will be producing about 46% of their rated output. So the 19 kW of rads are only producing 8.75 kW. This means the flow rate is 580 litres/hr, so the hex resistance is 0.5m. At this flow rate the pump is delivering a head of approx 4.5m. But this does not mean that the resistance of the rad circuit is 4.5 - 0.5 = 4 metres. It's just that the working point on the selected pump curve (speed 3) gives a head of 4.5 m.

Set the pump to a lower speed (2 as a start) and check the flow return difference. Provided it is not over 20C, rebalance the rads. A good starting point is to set all lockshield valves to half a turn open. Then make small adjustments (1/12th turn) to correct for any rad which are hotter (close) or colder (open) than the average. Keep a watch on the temperature difference at the boiler.
 
Joined
8 Apr 2008
Messages
10,731
Reaction score
2,712
Location
Essex
Country
United Kingdom
Meanwhile in the real world with dirt in the system and ad hoc additions to the pipework who knows what the actual resistance of the whole system is, the op has said all the rads heat up evenly so balancing isint the issue, so bigger or extra pump.
 
Sponsored Links
Joined
18 Oct 2007
Messages
10,587
Reaction score
1,353
Location
Kent
Country
United Kingdom
the op has said all the rads heat up evenly so balancing isint the issue
It's not balancing as such which is the issue, but balancing for the wrong temperature differential. That is the real cause of the problem.
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Top