water flow rates in old radiators 1

[joecontrol]

I am going to replace a 40 yr old boiler with a new mod-con boiler. I will be heating with old cast iron radiators. I am going to zone the first and second floors and have a primary zone for the indirect water heater.
I am going to have pumps for each zone.
I am just not sure of the flow rates needed to get the most heat out of the old rads.I want to make sure all the rads in the zone get close to the same amount of hot water.

Thanks,joe

I am not sure if this is the best forum for this question??

 

[BIPVguy]

Do basic calculations .... heat losses, select appropriate number of radiators to cover heating load, hydraulic calculations considering the noise levels water velocities and pressure drop and then balance a system by valves.
For further information, take a peek in ASHRAE Handbook.

Thanks

 

[BronYrAur]

I think you are asking what output you can expect from the radiators using water in lieu of steam. I found a basic explanation of this at the following web site:

http://www.colonialsupply.com/resources/radiator.htm

 

[willard3]

Were the old radiators used for steam?

If they were, pressure wash the inside before you re-use them.

 

[BronYrAur]

I never thought of pressure washing. I had a job once where we re-used some radiation from an old steam system and converted it to hot water. As a matter of fact we used some of the steam piping as well. It was in a church with concealed piping, etc. and we did not want to change the appearance. We installed a sidestream bypass bag filter and pulled out garbage for months. It served as our chemical feeder too so it wasn't much of an extra cost to the job.

 

[walkes]

You may run into an issue where you are not maximizing the use of the condensing boiler. The old rads were likely sized with 200°F water with a 20°F temperature drop. If you want your boiler to condense your return temperature should be below 140°F (approximately), your average water temperature may be between 150°F and 160°F depending on temperature drop. The flow rate and average temperature will then change and your radiation elements may not have the capacity to provide sufficient heating. If the flow rate is too low there may not be enough turbulence to provide good heat transfer.

On the other hand you may be able to set up a cascading system such that the radiation gets high temperature water, then the return goes to heat up incoming air to further drop the water temperature back to the boiler.

Also, the existing heating system may have been oversized and there may be sufficient area on the radiation to provide enough heating with a lower supply temperature.

 

[ChrisConley]

Totally agree with walkes, I would also add that a good outdoor air reset will also give your boiler system a greater efficiency at lower than peak outdoor temperatures.

Might have to 'tweak' the reset schedule a little, but it will help bring you're return water temperature down.

Preheat coil on outdoor air is a good 'delta T' point in the system. Another strategy is single pipe layout (twin tees) which can be sized for a more aggressive delta T than a typical system.

Another thought would be to check the capacity of the radiators with a high delta T (180-120) the mean water temperature is 150F, which could be high enough for the capacities you need.

One of the mod-con manufacturer's (Weil Mclein) did an interesting case study where existing radiation (finned tube) was used with a high delta T (180-100) they found that the radiation was sufficiently oversized to allow for the lower mean water temp.

 

[joecontrol]

Thanks everyone for your input.
Does anyone make a preheater for incoming air??
I expect you are talking of the intake air piping.
I could get a fin tube and run it inside the intake air pipe.Would this be good??

I would be happier to buy one from a supplier that has been making them for years.

Thanks,joe

 

[joecontrol]

THe rads are from a hot water system.
It looks like they were running with about 14 gals/mi. That is the flow rate of the grundfoss 15-42 pump that was on the old system.

I guess that would be a good starting point.
Taco makes a 3 speed pump I might go with to give me some flexablity.

Joe

 

[walkes]

The preheat coil is in the air handling equipment for the building. The coil will heat outside air up to 55°F or so. Then the heating coil will temper the air as needed for space temperature control.