odd hot shower problem

We recently renovated a house, bringing it from about 900 sq ft to about 1800. We kept the original boiler as we were told it was adequate. It is a Weil Mclean Gold series 3 (P-SGO-3). In the original house it was steam heat. Now it is converted to FHW baseboard. There is tankless hot water. We used the house for a summer prior to the renovation and the shower was always hot and did not run out.
OK, so now we have a second floor and a new bathroom and shower with a Moen fixture. The mix on this is as far as it can go. So, I get in the shower and it is plenty hot, I don't even have to turn it up all the way. I take a long shower sometimes as I shave in the shower (the whole head). After about 5 min or so the water will go luke warm, just on the edge of tolerable. After another 5 to 7 min or so, it will go back to hot and stay there. The weird part is sometimes it will stay hot the whole time, but this is becoming the exception more then the rule. I have a few theories, which may be totally off base:
1. The heating system is calling for heat, thereby diverting away for whatever heats the water. 2. The shower is not restrictive enough and is overcoming the tankless system. I was going to try one of those little washer type restrictors to test this theory.
I have tried turning the mixing valve up, but all that does in increase the initial hot water temp.
Any ideas?
-Jim
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When the shower water turns lukewarm, have you tried shutting it off and immediately checking the temp at the bathroom sink, letting it run long enough to be sure the supply pipe is completely filled with the hottest water available? If you've tried this, was the sink water also lukewarm?
I'm asking because if the sink water was hot, it might point to a problem with the shower faucet. I seem to recall that many shower faucets have some sort of anti-scald thing built in to protect people who don't realize that hot water is hot. If that's the case with yours, perhaps it's malfunctioning.
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And if it's not a bad mixing valve as joesparebedroom suggests, it may just be that other zones calling for heat are lowering the boiler temperature to a point that the coil can not produce sufficient hot water. If this is the case, you may want to install an indirect tank

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If this has happened multiple times, it seems unlikely that the boiler would get a call for heat during a shower more than once. But if it is this, it seems like your system would be giving priority to the heating zones vs. the HW. Should be able to change it so HW is priority. I have no idea how to do it on your system--I've worked a bit with Munchkins and know that it is fairly simple to change the priority. A pretty simple test of this hypothesis would be to turn your thermostat(s) down during your shower and see if it happens again.
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He may have several heating zones in the house which can be calling for circulation. If his boiler is undersized, it may not be able to keep up during times of peak demand. He currently has a domestic coil in the tank. There is no way to give it priority. If he had an indirect, he can set it up on a priority relay, though

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