hot water boiler sizing

Brad White_153

Take note of how much radiation of each type you have and what the output is at your design water temperature.

As Tim said, size a HW boiler to a calculated heat loss. But knowing your radiation output will tell you at what temperature you can run your system on the coldest day. From here you can reset the temperature by outside air temperature to save money.

If by chance the radiation you have is close to the heat loss, this is a cross check opportunity for you. No point in installing more boiler than you have radiation output. But always start with a heat loss when sizing a HW boiler.

And, because you have two kinds of baseboard, low mass copper and high mass fin, each should have it's own zone and priority. If the CI zone governs, the fin-served areas will cool off rapidly. If the fin zone governs, the CI areas will overheat.

For the indirect water heater general practice is to not add capacity for this, especially if you are using a low mass condensing boiler. These heat rapidly, often putting 100% of their output toward domestic water production, then get back to heating the house before you know it.

Often if a high-mass CI boiler is used (if oil is your only option for example), the indirect tank manufacturers have different "time to hot tank" numbers taking into account the slower response times of CI from a cold start. The difference is in minutes, though.

Most any boiler will be incrementally over your calculated heat loss, (just so many sizes commercially available), so you will have some small margin, just don't go to the next size if you can avoid it. This is one reason ModCon boilers are recommended; they tailor their output better to the load at a given time, becoming the boiler you need.

Jim_109

hot water boiler sizing

How can I properly size a hot water boiler for my home?

I have cast iron radiators on the main floor and slant fin basebards on the second floor.

I am also considering an indirect water heater.

Timco

Top of the page...heatloss calcs...follow the directions, and fill in the fields. The heatloss is your boiler size. A heatloss calc is the only true way to size a HW boiler.

Tim

Jim_109

Most of my radiation is cast iron, each with it's own zone. Don't know about the priority part. How would I know?

How do I find out "what my output at design temperature" is?

My current boiler is about 50 years old. I am considering a 3 pass type boiler. Several I am considering have the same AFUE rating and similar BTU, however one holds about 4 gallons of water, while another as high as 11. All things being equal, which is more efficient, more or less water? THANKS!

Timco

Output at design temp is boiler supply temp at coldest day needed to keep spae at design temp...usually around 70-74. Usually output is 180* on coldest day of the year. Priority means your heating circs will shut off when the DHW calls for heat, thus if using priority, you would not add the WH to your heatloss calc / boiler sizing.

Tim

Brad White_9

But.... and there always is a but....

The 180 degrees supply water is traditional sizing for most hot water systems. Sometimes it is 200 but 180 we see 95% of the time.

That said, your balance point or actual design water temperature has to do with the amount of radiation you have relative to your heat loss.

If your house once had radiation sized for 180 degrees supply (160 return = 170 average) and your house was older and uninsulated, that is your baseline.

Since then say you insulated, caulked, added storm or replacement windows, etc. etc. Your heat loss is now 60% of what it was. You now have a 66% surplus of radiation which means that on a design cold day you could get by with a maximum hot water temperature of 150 degrees -and modulate down from there!