knocking in radiant floor

Anyone tell me why my wirsbo tubes (oxygen barrier tubes) chug and knock? They seem to chug where they go in, and come out of the cement floor, and knock at the Heat Link manifold. The cement floor in question is a one and a half inch pour suspended on the second floor. The manifold where the knocking occurs is located below, on the main floor. To my way of thinking, the first likable suspect is air in the tubes above, since air rises, and there is no way for air to escape if it travels to the second floor, since the air snifter valve is directly installed on the manifold below. But, stangely, I do continue to get circulation, and warm floors. I poured the floor, and included rigid insulation around the outside edges for a thermal break from the outside walls. But I did pour the cement directly up against the bottom plates of the interior walls. The plumber that finished hooking up the electric boiler, manifolds, etc. told me the knocking was from expansion of the cement floor. He explained, since it was poured against hard edges, it has no place to expand outwards, and therefore knocking results. He suggested either chipping away at those edges of the cement floor where they come in contact with the bottom plates, or gouging away a little at the bottom plates themselves to make a small gap to allow for expansion when the floor heats up. But, before I take on this task, I still wonder if there's air somehow still trapped in the tubes above on that floor. Maybe he's right, and I'm wrong. What do you think? Thanks in advance, John
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I kind of doubt what you described is air. Wirsbo expands like 1 foot for every 100 feet for every 100 degree of temp rise. If you have not allowed for that you will hear it. I use plastic clips that the PEX can easily slide through to help stop this. It should NEVER be wedged agianst wood or it will make noise. Use the 90 degree support bracket if you need to but don't let it be wedged agianst stuff.
Why electric boiler?? Are you on Solar?

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Thanks for the reply. You obviously know more than I do. The problem has temporarily disappeared, I adjusted the return valve to slow the flow of water to this floor, and upon further reflection, the problem may also stem from the fact, that the length of one loop is now much shorter as the plumber hooked it up differently from the rest. If this is the case, then it can easily be fixed, by coupling it back onto one of the existing other loops which is a little short now, but in the meantime I will contiue to ensure none of the tubing is directly in contact with wood, or wedged where it cannot expand. One thing I did when originally installing the tubes, I simply hammered j-clips (maybe those same plastic clips you mentioned), directly onto the plywood subfloor to hold the tubing in place before pouring the concrete. If this is a problem, there's not much I can do about it now, without tearing up the whole floor, and redoing it. I think I'll experiment by letting the warm water to go into the flow at a slower rate to slow down the rate of possible expansion. The electric boiler is a low temperature system. The water is only a 100-110 degrees by the time it enters the floor. I've been heating my house exclusively with a couple of wood stoves until now, and figure if the cost of electricity becomes too high to maintain, then I will think about the possibility of incorporating another heat source. I do like the idea of solar too. What do you think? Thanks again, John

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Don't worry too much about the flow rate, in fact turning it down could be a bad thing. The different lenth loops can be dealt with at the manifold. The flow rate is important if you have a mixing valve especially.
Electric is very expensive if it comes from power company. Wood boilers are nice, I work on a number of them. I think a system with a wood boiler and an oil boiler combo is great. The boiler you mention is only for use in a slab if the temp is only 100-110. I have been impressed with the solar pannels and acc. that have come out. A friend of mine has just made a house that is off the grid and you would never know it unless he told you. You have normal outlets in the house and everything is the same. You connect the solar to your electric pannel. If you can have a wind powered generator to help the pannels recharge the batterys then you will never run out of power. We connected a gas powered generator for chargeing the batterys back up in a jam.

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