Intermittent high pressure in hot water heating system system

We have a hot water heating system where this winter every morning in cold weather the pressure rises to around 30 psi and water gets dumped on the floor from the pressure release valve. It was originally an old gravity system with old-fashioned expansion tank - about 10 years ago we got a new boiler and they replaced the expansion tank with one of the smaller compression-type tanks and added a circulation pump. But we still have big oversized pipes and radiators.
The mysterious thing is that the pressure during the day stays in the high teens and 20's and there is no leak, even when the water temp is fairly high - say 170 deg. The overpressure only happens in the morning when the system has been running hard bringing up the temp from the 55 deg we set it for the night.
The plumber first replaced the pressure release valve. That had no effect. He then replaced the expansion tank - problem continued. Then replaced the expansion tank with a bigger one, as he felt the small one was undersized for the house (couldn't understand how it had worked for 10 years without incident, though - thought maybe that there had been some air in the radiators acting as an additional expansion tank). Still not fixed.
I'm wondering if there could be an intermittent leak in the water refill valve when the system has been running for a while - can't be a continuous leak as the pressure does go down to around 15 when the system is cold. Plumber is quite skeptical - claims never to have seen such a thing.
Any ideas?
Peter
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wrote i

I am so sorry! the answer is no, it is magic house where common since supersedes stupidity and arrogance
Respectfully; Dido
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wrote:

Yeah, but if he gets one, she might have a papoose. I've heard that you can stick stuff in a lady's papoose. Might as well put some beer cans in there...
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expanded metal is full of holes, making it much lighter. It is used to make lath. Expanded water is used to take a bath. sounds very similar.
Expanded water, therefore is LIGHTER because of the holes, that's why it forms a layer on top of the non-expanded water. Hah?!
Expansion is a very good thing, except when our wives expand to a size 20, when you met her when she was a size 2.
My head is expanded from giddiness now. It is too big for my hat. Makes me look like George Lopez.
Bob

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With a head like Ike on southpark, who'd marry him?
wrote:

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Ya, come on Steve! It's not dangerous unless you're anticipating the pipes busting! All you have to do is check the schedule to see when the pipes are supposed to burst, and then you'll be fine!
Now, go ahead Dildo, call me some names....you are one stupid fucking idiot! Even someone who's not in the trade has more common sense than you when it comes to boilers, A/C, furnaces, Carbon Monoxide!!!....you name it. Every time you post, you show just how little you know about HVAC!
--
Respectfully, Bob

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