Power Vented Water Tank

Did an inspection today with a power vented John-Wood Superflue. This was a single power vent system. IPEX run of approximately 16 feet and 4 elbows. 2" line.
As this was an owner inspection(11 month warranty inspection), I was informed that the tank would not deliver hot water at all times. Mostly on windy days. Heat was not constant.
I have had this happen on other 2 vent systems and the span was too long but when researched the span, it appears this has a proper span. I did notice however, that there was no combustion air line for the system and the furnace was a high efficiency furnace.
Anyone else had the issue of heat loss in these superflow tanks or similar? Kind of stuck in this one or maybe somethings staring me in the face I can’t see the problem for looking.
Any assistance would be appreciated.

I think max distance is 40 feet , Each elbow is = to 5 feet times 4 =20 feet
So if as you say 16 feet plus 20 feet comes in under the 40 foot rule .
Sounds like air starvation . Is there an HRV and is it set up correctly .
Could be a simple as adding a two inch air supply with a fish hook at bottom to add air if needed but this is not our call…