I should have also mentioned that my 2 in-wall electric heaters installed in the North Wall have built in fans. So, when those are on, some air is already getting pushed around. However, even with these on constantly (still cold outside!), there is a significant temp difference between the peak and ground. This is why I feel like I need some sort of vertical/sloped circulation to push/pull the heat from the peak down.

hi Soshh, Are you looking for tub with more than 20'' deep? So far, I found the following tubs from MTI with inline-heater options for our candidate. They are all 19-21'' deep. Since you can install 42'' wide tub, you will have more option than me! :) Here is their search page if you haven't looked yet: http://www.mtiwhirlpools.com/products/ >MTI-5 Whyndam 1 (59 3/4 x 35 3/4 x 19 3/4), rectangular, $1086 >MTI-22 Holiday (59 1/2x35 1/2 x 20), rectangular, $1138 >MTI-15 Charleston 1 (60x36x19 1/4), oval, $1101 >MTI-9 Victoria 2 (59 3/4 x 35 1/2 x 21 3/4), oval, $1195 As I searched more, it seems Aquatic doesn't drill hales for tub spout&valves. So I'm leaning toward MTI at this moment... -b

I am in the process of researching electric tankless water heaters for our remodel as I want the space our traditional tank heater requires. I stumbled across your post looking for people posting their research on which size electric they have. The tankless sites have spreadsheet style charts showing gpm flow plus temperature rise. You may have a third option of installing a new electric tankless that will produce a greater temperature rise at your greatest demand (gpm). However the electric heaters have huge energy demands when they turn on so you may not have enough capacity in your electrical panel to upgrade. the ones I am debating between for our two adult one bathroom home are either the 96 amp size or the 120 amp size. For the same situation you have-solar preheat most of the time but during our California winter storms we can be 7-10 days with no solar to speak of and then the water passing through the solar storage tank is not much warmer than it is coming out of the ground, maybe 50 degrees. I assume you are being reasonably careful about not doing two hot water demand things at once? It is easy for our household to not shower and do dishes/laundry, for example. But with a baby & visitors maybe coordinating running the dishwasher/washing machine when you also need hot water to bathe baby or yourselves gets too complicated? Another option that crosses my mind is to change the solar storage tank to one that could be turned on (gas or electric) when you absolutely need to have warmer water coming into the on demand heater. You could set it to the lowest possible temperature setting and then switch it off when the solar is working to preheat the water. Or possibly get one that can be set to preheat the water on a timer so it runs just once a day, just enough to preheat the water to the level that the tankless can keep up because it needs to raise the temperature only 30 degrees instead of 50-60 degrees. Sorting out what exactly is the issue may help you decide what to do. Do you not have enough temperature rise only when you are trying to do two hot water demand activities at once? If this issue arises only when you have guests you may be able to suffer through the inconvenience of not running the dishwasher/washing machine when someone is showering? I am curious as to what size heater you have and how long you have had it and have you had maintenance issues yet (do you have hard or soft water and do you pretreat the water if it is hard?) and anything else you would share with someone who hasn't taken the plunge!

"I read somewhere that to heat MUA unit that is balancing a 650 CFM hood you nneed a furnace big enough to cover a Bill Gates-type house." No idea where that came from but it's quite erroneous Hi opaone, yes I made a typo: it should have been 900 CFM not 600 CFM. I was just trying to make the point that it had been noted we need to have a lot of heat to warm the make up air, and I don't know how to get this done without having some massive framing work and pllumbing and electrical too. I believe I got that from a post up thread from kaseki about four years ago. He wrote: "For example, 900 CFM is 54,000 CF/hr being raised as much as 50 degrees (more in N. Minnesota). This would be equivalent to a house volume (assuming no leakage) of 270k cu. ft. being heated 10 degrees per hour. A 27,000 square foot house with 10 ft ceilings would be of this volume. Lacking a Bill Gates' house sized furnace, you may need supplemental heating." Now, your link to Electro's electric MUA heater is really interesting. I had seen Fantech not this. Both big and costly! The daily use cost is what I wonder about, and I don't see any reviews (at least not at supplyhouse.com). Of course, operating these varies based on one's power needs/costs, and where I live electricty it not cheap. I am incapable of calculating what my daily use cost might be, using the Electro model MD-10 as an example (designed for 600 CFM fans), the specs are beyond my skill level to assess a cost to run the thing. I do see that unit has a 10" duct, and puts out around 34K BTUs of heat, which is about what I was planning to use to heat the additon in its entirety. Heating a house in my area, using electricity, is not considered the best option as I looked at an electric combi boiler and was reading comments and this was pooh poohed.... You did a good job of explaining the best- to worst-case examples of brining in the MUA, and of course like an idiot I cut a nice big fat 6" hole in my rim joist so I could bring the air into the middle of the LR/kitchen via the ceiling, which was the worst option you very intelligently noted. I learned a little about air flow reading a paper from the dept of energy website where they talked about throw and how the air hugs the walls, etc. Balanced vs supply vs exhaust vs ERV/HRV and how all of that relates to this thread makes the whole topic in general complex for the layperson, and as mentioned previously: good affordable help these days is... hard to find. These types of threads and forums are all most of us have! Thank you for your valuable contribtions.