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Boiler Cycling Too Frequently

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Posted by John Merritt on August 22, 2005 at 11:23:25:

I have looked into the messages for this year, and therefore I hope this isn't a frequently asked issue.
I have a 400,000 output BTU boiler that provides in-floor heating and DHW via a 40,000btu heat exchanger tank. It is a 2-stage boiler controlled by a Tekmar control unit.
I currently have two issues, which may well be related:
When responding to DHW demand the boiler, being so powerful, kicks in at around 150F but quickly reaches the cut-out temp of 165F. I am presuming it uses the more powerful Stage 2 burning for DHW demand? The boiler burns only for about 15 secs (every 3 minutes). I am told it would be a lot better for its longevity if it burned for longer periods less frequently.
2. I currently have a higher temp on my return floor heating pipes than on the out. Some plumbers have told me that this is impossible, but trust me it is true, and I believe it is being caused by air locks causing the flow to be in reverse - at least past the temp gauge. I have bled these lines but the same problem seems to have emerged. What can cause this?
Are these issues related, or can I do something to minimise the cycle frequency by changing the mix temp/pipe volume/DHW temp?
Thanks for reading this far.

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