We had a very strange problem last night. Got home to a very cold house and turned on our forced air furnace. We have 2 Tankless Water Heaters. We went to take a shower and one 1 of them, after about 30 secs of very hot water it turned completely COLD ... went to the other shower (different Tankless heater) and flipped on the HOT water ... NOTHING .... About 10 mins later, went back to the 2nd one and tried it again - we got HOT water for 1 complete shower. In the middle of the 2nd shower it went totaly Cold again.

So my question is : Is there a possible gas pressure issue when running the forced air furnace and trying to generate HOT water ??? I mean the fact that BOTH tankless heaters bhaved the same way leads us down that path of gas pressure ... Thoughts??

Thanks in advance