Boiler Sizing Question

Robert O'Connor_7

If the heatloss and the net IBR are dead on, I'd go with the next size up (I may get some fallout from this). But if the net and heatloss are not close (meaning the closest you can get to the net without being below is say 20,000 above your calc.)I'd go for the net based on the buildings net. If you were to do this and it became a concern you could always add a priority relay for the domestic...Not very scientific but its worked for me. Getting the right answer from the supplier will always favor the larger unit for obvious rea$on...Robert O'Connor/NJ

Ken_22

Boiler Sizing Question

Let's say I do a heat loss calculation and come up with 80,000 BTU for my house with oil hot water heat (radiator).

But, if I'm going to add an indirect hot water heater, how much bigger a boiler do I need? There are 4 people in the house, no hot tubs or any special water needs.

What would be the right size boiler in this case? Does the 80,000 account for the domestic hot water too?

Dale

sizing

Go with the 80k net. If your house figured 80K loss the IBR rating figures a 1.15 pickup factor already so the input will be over 80K. For your geographic area the already conservative 80k is for your design day or whatever is the coldest day of the year, so for almost all the time 80k is too much and the domestic hot water load for normal use would not be a problem. As the other post suggested you'll probably want a "polite" control that gives the DHW priority.

Joe@buderus

Boiler size

The boiler used depends on a couple of things. Are you looking at the output of the boiler (DOE rating) or the IBR rating (IBR deducts 15% for boiler and piping loss) and are you using a control that has a priority zone for the domestic load.
Buderus boilers are normally sized based on the DOE rating unless it is a large mass system, has long runs of 1 1/2" or 2" black iron mains, unheated/non-insulated basement. If not using a priority type of control, domestic hot water recovery could be extended.

Mitch_4

Go with

DHW priority control. I like the ARGO. It has a timed priority, 30 minutes max and lets the heating system catch up.

Then your Boiler can do both without up sizing the boiler. 30 minutes is plenty of recovery, especially if the indirect has large enough storage. and the temp in the house wil lnot drop significantly in 30 minutes.

Heat loss already as a fudge factor of around 20%, so you have plenty of capacity. Using the IBR rating gives even more cushion, and if you are using radiant floor heat, the thermal mass inthe floor alone is good for a few DAYS.

Good luck.

Mitch