Limitations / advantages of ‘batch heaters’ like Versa Hydro (@Dana)
I have just recently grasped that with the Versa Hydro you can keep the tank temperature at 140 for bacterial safety but then the variable speed DT circ will modulate down and sip the heat out of the tank to achieve the 85, 90, 100 degree supply temperatures desired. I understand that the circulator modulation represents the Versa’s thermal efficiency. However, doing that, the temperature at the combustion chamber will be at the higher temperature,–out of condensing range–which is what drives the combustion efficiency, which is where fuel savings comes in. Such is the issue with all–in-one units like these.
So even if I’m not truly condensing–as I would be with a regular mod con and indirect, but I’m getting the advantages of the thermal efficiency and built-in 55 gallon buffer tank, is the whole package still substantially superior in overall efficiency to an 82-85% atmospheric (ECM circ, zone valves, etc.) with either a standard indirect or turbomax reverse indirect? (Reverse indirect also presents the issue of constant maintenance of high water temperature in the tank.)
Of course I have to factor in additional installation and maintenance costs for the Versa.
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Was there a question in there?
Anything over 120F will keep legionella from growing, but it takes 140F to outright kill it. It'll hit the advertised 95% thermal efficiency range if stored at 120F. Even at 140F storage temps it'll hit in the 88% range.
It's also possible to "sterilize" the tank with high storage temps for a few weeks, then lower the temp for higher efficiency for the rest of the season. I forget what the max temp is on the Versa, but 160F would kill most pathogens in a few hours.
Thanks. The question was: "is the whole Versa package substantially superior in overall efficiency to an 82-85% atmospheric (ECM circ, zone valves, etc.) with either a standard indirect or turbomax reverse indirect?"
Given the Versa is a boiler-buffer-indirect in one, and tank temps have to be kept above condensing range --unlike with a regular modcon with separate indirect--are the extra few percentage points of efficiency at 140º, whether thermal or combustion, worth the extra costs and maintenance above that of an atmospheric boiler? A tech has told me that he can get more efficiency from a regular modcon since tank temps stay in the condensing range. That makes sense to me, but I have no way of calculating how the Versa's mix of thermal efficiency and buffer function may offset its higher than condensing tank temperature. (assuming as you say 88% thermal efficiency and _____% combustion efficiency.
Tank temps do not have to be kept above the condensing zone for "...bacterial safety...".
Even at non-condensing temps it's thermal efficiency is better than 85%. The fact that it's all self-contained and pre-engineered minimizes distribution losses, and any inefficiencies related to your separate component design.
>"A tech has told me that he can get more efficiency from a regular modcon since tank temps stay in the condensing range."
Calling BS on that one. A standard indirect tank would have to be kept at the same temp as the Versa for the same level of pathogen safety, ergo no condensing advantage there. It does NOT "...stay in the condensing range..." if the storage temp is above the condensing range.
The standby losses of the Versa are comparable to those of a reverse-indirect like the TurboMax. But if you're keeping the TurboMax at 140F there is no condensing advantage either. The return water temp entering the mod-con would be the tank temp, most of the time, just like the Versa.
If the heating system can run at a substantially lower temp than a 140F water heater there is a condensing advantage to treating the indirect as a separate zone, only taking the efficiency hit when serving hot water. That advantage is pretty slim if the storage temp is 120F though.
My own heating & hot water system is centered around a 48 gallon Ergomax at storage temp of 130F in order to be able to hit the condensing range at least at the beginning of a burn. I need 125F AWT on the radiation to heat the place adequately, and the entering water temp at the boiler is below 120F during heavy space heating or long DHW calls, but only 120-122F at the beginning of a burn when just the micro-zones are sipping, finishing up at 130F (above condensing) at the end of a burn.
Combustion efficiency and thermal efficiency are essentially identical in a space heating application.
So is the Versa substantially superior? Depends on the competence & diligence of the designer. Designing a system from separate components isn't "free", and how you set it up makes a difference.