Boiler output not on a par with demand after short run period?

[apple1]

Folks,

In the last week or 10 days, I've noticed that when my UFH is calling for heat for any moderate amount of time (e.g 30 mins+), the output temp from the boiler appears to drop significantly. Typically the output is circa 80 deg C but I noticed at the weekend that I after a run time as short as 30mins, I can literally grasp and hold the exit pipe from the boiler and it is only moderately warm. When the boiler returns to pause mode, it again reads at circa 80 deg, so the problem is only when there is demand on the system. Any ideas as to what my problem may possibly be? As an fyi, the fuel source is wood pellet but given the "resting" water temp is 80, it leads me to believe the problem is not necessarily with the boiler but elsewhere in the system. Thanks in advance.

 

[ted]

Hi apple1. When you say you can "grasp and hold the exit pipe from the boiler and it is only moderately warm" are you handling the flow pipe beside the boiler or beside the manifold?

At this time of year I run my UFH for about 2-3 hours a day downstairs and 1 hour upstairs. I've set the boiler cutout to 60 degrees C (i.e. when the return water reaches 60 c the boiler will stop) - I've set the UFH controls for max. heating and I find that I can run the heating for up to 3 hours without the return water ever exceeding 50C. I put it down to the fact that there is so much water in all the pipework and the flow rate of the incoming hot water being so fast that the boiler temp in the Flow pipe rarely exceeds 50C. This means my boiler will run non sop for the 3 hours or so of heating.

In your case my guess is that your mixing valve is open full and that your settings are somewhat similar in mine.

Remember an UFH system contains many multiples of the amount of water than a radiator system.

 

[apple1]

Hi Ted,

Yes, its the flow pipe that exits the boiler at the top. When the initial demand is placed on the boiler, (i.e. when the stats begin calling for water), the pipe is very hot to the touch (flow temp around the 70 deg C mark) but quickly (within 30-45 mins), the flow temp drops significantly. I've the mixing valve set to send water through the UFH circuitry @ circa 50 deg but I doubt if it is even entering the mixing valve @ 50 after about 30 mins running which is having an obvious negative impact on space heating. I also have my room stats set at max temfor 3 hours daily so the boiler runs non-stop for this period but the problem appears to be that its not sending out hot water for this duration. The return water is entering about 25-30 deg C. Out of curiosity, what is your mixing valve reducing the flow temp through the manifold to? Thanks ted, apple1

 

[ted]

I have the water entering the manifold set to max (i.e. lttle if no mixing) - the water from the boiler will come in at about 20C and rise to 60C after 2 - 3 hours in a 24 hr period at this time of year. I have a poorly insulated pipe run underground from the boiler approx 10 m and I'd say I have losses there but it would make sense to me that if there is a large volume of water to be heated in UFH circuits and if the pumps are sending the water throughthe boiler at max flow rate then the water in the boiler will take quite a while to heat up. I supose it would depend on the boiler size and heat demand of the house. Mine (a Firebird 120) I would imagine is prob undersized for the house size but our costs are okay with us. If the water from the boiler is cold and the boiler is still running then there must be some setting that's incorrect or a fault imo.