Special feature on Buderus Logamatic DHW priority

Joe@buderus_2

I think there is a little confusion on the R2107 and domestic hot water. The R2107 will go into DHW ERR if it sees no rise in tank temperature within twenty minutes. If the domestic hot water recovery is slow, but steadily rising, the R2107 will not error out after twenty minutes.

D107

If HW recovery is not complete in 20 min, HW production stops

until Logamatic is manually turned off and on.

EDIT: The previous statement is not fully accurate and was based on a slight misinterpretation. Look further down in the thread for an explanation from Buderus: "The R2107 will go into DHW ERR if it sees no rise in tank temperature within twenty minutes. If the domestic hot water recovery is slow, but steadily rising, the R2107 will not error out after twenty minutes." So the concerns listed in the following paragraphs are moot, even though one of my proposed features might be nice on a wish list. END OF EDIT.

While that's a nice feature to protect against space heating loss on a below-design day, it would seem to mandate a possibly larger boiler size than necessary for the heat loss--aside from the nuisance of having to manually reset the Logamatic. Suppose one sized the boiler to heat loss and sized storage to handle the HW load and that it was designed --during certain peak hw periods--to take 40+ minutes for full recovery. Seems it would be a far better feature for the 20 minute period to be variable--certainly upwards--and then to go back to HW priority after the space heating was satisfied. Note that this is for a Buderus G115/21 75DOE output, ST-200 indirect and 47K loss/100K gain house.

(side point: Note that Buderus says that if the FM241 module is installed in the R2107, then there is an option to turn the domestic hot water priority "off".)

Thanks,

David

Dan_15

Interesting. I have an R2107 without the FM module and I am pretty sure I can turn off DHW priority. Maybe I am mistaken.

[Deleted User]

If I understand this correctly, the indirect is not getting satisfied and after 20 min. the control shuts off the indirect circ and goes to space heating? If this is the case, instead of oversizing the boiler you could turn up the temp on the indirect and install a mixing valve. This would allow less draw off the indirect. Honeywell has a mixing valve specifically for indirects and hot wtr heaters. I hope I understood you correctly and that this idea might help.

D107

yes, you got it right

it's not that going to space heating is a problem, it's that it turns off the HW recovery after twenty minutes. Your solution is a good one of course. Don't think I'd want to go above 140 deg... Then you can mix down to 110 or so. Problem would be that in the case of a huge hw draw, recovery might still take more than 20 minutes and occupants would never know it until they noticed their hw getting lukewarm, then they'd have to reset the logamatic.

Of course, for one occupant, the argument could be made for a 40 gallon tank instead.

Thanks,

David

D107

Thanks Joe

Well I guess that makes the feature some kind of a safety, which sounds like a good thing.

Thanks,

David