We are experiencing an issue with CH short cycling on a Worcester Bosch Greenstar 25i ErP boiler.
From a cold start the boiler fires for only ~25 seconds before exceeding the set CH flow temperature and entering it's 3 minute short cycle programme. As soon as the short cycle programme kicks in the flow temperature drops rapidly. This cycle repeats indefinitely.
The CH flow temperature is set to 45 deg C. The Maximum CH output is set to 7Kw (the minimum the boiler will modulate down to).
The boiler in plumbed to an underfloor heating manifold with 2 no. ~80m long 16mm dia loops. There is no switching or blending on the manifold. There are no other CH circuits plumbed to the boiler.
Adjusting the pump speed to maximum and fully opening the flow valves on the manifold (giving a significantly higher flow than required, ~6 L/min total) does not have an effect.
Adjusting the CH flow temperature to above 75 deg C results in the boiler not short cycling and running continuously. This flow temperature is of course not suitable for underfloor heating. This was WB's only suggested solution when they sent an engineer out to look at the boiler under warranty.
Whilst we are aware that the 7kw minimum CH power that this boiler can modulate to is larger than required by the heating circuit, but we would have expected it to be able to run a little longer than 25 seconds from a cold start before short cycling?
Is there an issue with the boiler or is it simply that our expectations are incorrect, and the boiler is not able to modulate down low enough for a heating system of this size?
(A quick note to explain the logic behind this boiler and underfloor heating combination - the boiler was existing to the property when we purchased it and we are undertaking a staged renovation & extension. We have removed all the existing radiator based heating system and renovated 2 no. rooms including the installation of underfloor heating loops. The remaining rooms and extension to the property will also be heated underfloor and work will commence on these in the spring. At this stage I am simply looking to get the boiler functioning as efficiently as possible this winter with the currently small underfloor set up)
From a cold start the boiler fires for only ~25 seconds before exceeding the set CH flow temperature and entering it's 3 minute short cycle programme. As soon as the short cycle programme kicks in the flow temperature drops rapidly. This cycle repeats indefinitely.
The CH flow temperature is set to 45 deg C. The Maximum CH output is set to 7Kw (the minimum the boiler will modulate down to).
The boiler in plumbed to an underfloor heating manifold with 2 no. ~80m long 16mm dia loops. There is no switching or blending on the manifold. There are no other CH circuits plumbed to the boiler.
Adjusting the pump speed to maximum and fully opening the flow valves on the manifold (giving a significantly higher flow than required, ~6 L/min total) does not have an effect.
Adjusting the CH flow temperature to above 75 deg C results in the boiler not short cycling and running continuously. This flow temperature is of course not suitable for underfloor heating. This was WB's only suggested solution when they sent an engineer out to look at the boiler under warranty.
Whilst we are aware that the 7kw minimum CH power that this boiler can modulate to is larger than required by the heating circuit, but we would have expected it to be able to run a little longer than 25 seconds from a cold start before short cycling?
Is there an issue with the boiler or is it simply that our expectations are incorrect, and the boiler is not able to modulate down low enough for a heating system of this size?
(A quick note to explain the logic behind this boiler and underfloor heating combination - the boiler was existing to the property when we purchased it and we are undertaking a staged renovation & extension. We have removed all the existing radiator based heating system and renovated 2 no. rooms including the installation of underfloor heating loops. The remaining rooms and extension to the property will also be heated underfloor and work will commence on these in the spring. At this stage I am simply looking to get the boiler functioning as efficiently as possible this winter with the currently small underfloor set up)
