I'll first state that I'm not a professional. With that being said I will spew a few things anyway. Your Logstor 31mm (just over 1" pex) can handle between 38,000BTU and 75,000BTU with a flow velocity of 2ft/sec and 4ft/sec velocity without pipe wear/noise and with a 20deg delta-T. I think the key might be your delta-T and or your blower cfm. If your design allows for a 20deg delta-T then your stated heat loss of 48,000BTU is considerably lower, heck, even your rated furnace output is lower at 68,000BTU than what your pex 'allows' at 75,000BTU. If however your design is say a 10deg delta-T then you would need 12gal/min and no...your pex is too small for that. I'll assume your design is 20deg delta-T since that is the industry standard in North America. Now the question is what is your flow rate of your pump? Doing some quick calculations for your head loss (using some equivalent length numbers) you should be able to achieve a nice flow rate with a Taco 005 or 007 or equivalent and meet your 6.8gal/min flow target assuming again a delta-T of 20deg. Using a 20deg delta-T 1GPM=10,000BTU's. I'm using the higher furnace load for safety although I would want an accurate heat loss of the space your intending to heat. Even at that, your 1" Logstor can handle 75,000BTU's and be within the 4ft/sec parameter.
Probably your most important factor is finding the output ratings of your particular water-air exchanger. I did a quick search and found this on the web to use as an example.
http://www.ctwoodfurnace.com/PDF/V101.2 VAL Air2Water_logo_pg2.pdf Your particular exchanger might vary so I would definately find out about your own particular models specs. I think you might find that the cfm of your furnace blower is not giving you enough velocity to achieve your requirements using 130deg entrance water into your exchanger. Your particular exchanger is nicely oversized to take advantage of lower temp water so that leads me to believe your not blowing enough cfm across it or your flow rate is too low through it but that seems impossible with the dinky size pumps you need. If if were me there is no way I would change out your 1" pex unless your heat loss was over 75,000BTU's your design is for a smaller delta-T and or your headloss was so high your wire to water was so inefficient that you couldn't find a pump that could pump your required flow. In your case it seems like that at least is not the case, assuming again a design of 20deg delta-T. I know I know delta-T again but it is very important in this case.
If however you need more than 7.5GPM flowing through your exchanger than yeah, you would probably want larger pex. But you could even flow 9GPM through that pipe (90,000BTU's) for short periods of time say when your water temp is lower. Or blow more cfm across your heat exchanger. That I would definately find out about.
I know I'm spewing and again I'm not a professional but I would seriously look at all the design parameters before I changed out my pex. Because in your case everything looks good (assuming a 20deg delta-T
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).
Good luck, and maybe a professional will chime in and really actually help you...I'm just spewing you see.