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[wreckit87]

Just getting back from Texas where it was almost as cold there as here in Wisconsin before this weekend anyway.

Well wreckit87 if you want to put in a 40 plate heat exchanger that's rated anywhere from 250,000 to 400,000 BTUH at close to twice the price for a 40,000 BTUH water heater you could probably keep incoming and outgoing temps very close to the same. But installing it in the cold inlet to the w/h you still need to keep your water heater on to prevent tepid water during prolonged off cycles. Extreme overkill in my opinion.

E Yoder you hit the nail on the head with properly sizing equipment to loads.

A respectful disagreement ( 2 actually); I don't know where you buy your exchangers, but a 40 plate can be had for under $200 all day. Last time I bought a water heater which was 3 months ago, was right at $400 for the cheapest piece of crap Menards sells. Secondly, unless you're leaving the house for a week at a time, the WH can stay off. Mine hasn't run once since I installed the boiler 6 years ago, and I often leave for several days at a time with nobody else around to use any hot water. 40 gallons of 170 degree water isn't going to be "tepid" overnight unless you were born on the sun and need 150 degree showers. I'm mostly baffled at where you are finding $100 water heaters; I could save a lot of money with those.

If I read that wrong and you were saying the 40 is double the price of say, a 10 plate, then yes you are absolutely correct. For $60 though, I'd still rather have a tank full of hot water for when I want to do laundry and dishes and shower at the same time. Nothing worse than running out of hot water in the shower IMO. Especially if I'm in a hurry and the boiler isn't up to temp, there's nothing you can do about it if the plate is after the tank. Personal preference I suppose, it's just what I like

[E Yoder]

I would agree with wreckit that I haven't seen water heaters cool down much when preheated with a flat plate. Maybe 10 degrees after 24 hours at most. We've done quite a few that way with very happy customers, endless hot water, with fairly even temps. We use 3X8" 30 plates and aim for about 140℉ water in the water heater to avoid needing a mixing valve. A bigger flat plate (hotter water) and mixing valve would allow a longer idle time between drawing hot water as the mixing valve would even things out... Hmm... I learn things every day. I like these discussions.
Years ago we did sidearms and those were much harder to get right. I hope whoever invented the flat plate was treated real well, he made my life easier.

[schoppy]

Yes, I was comparing plate to plate cost. Of course if you have the temp high enough in a water heater it will maintain an adequate temp but 170 is crazy IMO. I can run hot water all day with my 10 plate and never run out. Obviously this is with the OWB running in operating temp range. Even when water isn't being used a flat plate will transmit some heat into the tank which can help hold temp as well.

To each their own, I like to keep it simple.