Pump head and water temp question

[gorsuchmill]

I'm trying to determine the ability to reduce my water temp to get the most out of stored water. The formula for calculating gpm includes a lot of variables that, for me, might be difficult to measure. Is there an easy rule of thumb for calculating the flow rate of a Taco 007 (0-23 gpm range) given the following variables:

The pump drives a hydronic coil in a closed loop system using 3/4" pex. The coil is located 25' or so vertically (40' of total pipe length) above the pump and incorporates a single 90* fitting on the supply run (after the pump). The return line is similarly piped.

Based on the coil manufacturer's chart and my heat/loss calc for the area served, I could get enough btu out of the coil at a flow rate of 5gpm or better and 120* water temp. If the flow rate drops to 3 gpm, my water would need to be 130* or higher.

Lastly, what is the lowest temp you would want for cast iron radiators? My system consists of 2 hydronic coils for second floor area and radiators for the first floor. If I could get the water temp down to 120* I could achieve a delta T of 50*, which would give me nearly 8 hours of storage at an average load. That said, if the radiators need at least 140* water then I'll have to accept a smaller delta.

Thanks.

 

[Eric Johnson]

120 works alright with cast iron radiators in milder weather, but when it gets cold (I forget how cold), they can't keep up.

 

[BrownianHeatingTech]

I'm trying to determine the ability to reduce my water temp to get the most out of stored water. The formula for calculating gpm includes a lot of variables that, for me, might be difficult to measure. Is there an easy rule of thumb for calculating the flow rate of a Taco 007 (0-23 gpm range) given the following variables:

The pump drives a hydronic coil in a closed loop system using 3/4" pex. The coil is located 25' or so vertically (40' of total pipe length) above the pump and incorporates a single 90* fitting on the supply run (after the pump). The return line is similarly piped.

The vertical height is not important, since the water coming down the return balances the water going up the supply. For a closed system, anyway.

Based on the coil manufacturer's chart and my heat/loss calc for the area served, I could get enough btu out of the coil at a flow rate of 5gpm or better and 120* water temp. If the flow rate drops to 3 gpm, my water would need to be 130* or higher.

What is the pressure drop on the water side of the coil at that flow rate?

Lastly, what is the lowest temp you would want for cast iron radiators? My system consists of 2 hydronic coils for second floor area and radiators for the first floor. If I could get the water temp down to 120* I could achieve a delta T of 50*, which would give me nearly 8 hours of storage at an average load. That said, if the radiators need at least 140* water then I'll have to accept a smaller delta.

As Eric notes, the radiators can function down to 120 degrees.

Whether they will function for your particular application at that temperature is dependent upon how much radiation you have relative to your heat loss. Just as your fan coil can run on a lower water temperature and put out enough heat for your needs, but might need a higher water temperature if the same coil was used to heat a bigger house, the temperature that your radiators need will depend on how much they are oversized (or not) for your application.

Joe

 

[gorsuchmill]

I'm not sure how to calculate the pressure drop on the water side.

Regarding the radiator temp, simply knowing 120* water could work would at least allow me to give the greater delta a try, recognizing the possibility I could add additional radiators if necessary. Further, would it be possible to set up some type of outdoor thermostat to allow different storage tank temps depending on the weather? If so, I could allow the tank to get cooler when the weather is mild and force a tighter temp band on colder days. I live in the Baltimore area, and our winter days usually fluctuate from the 20's to low 40's.

 

[BrownianHeatingTech]

I'm not sure how to calculate the pressure drop on the water side.

The specs for the coil should list it at each GPM flow rate. It should be in psi or feet of head.

Further, would it be possible to set up some type of outdoor thermostat to allow different storage tank temps depending on the weather? If so, I could allow the tank to get cooler when the weather is mild and force a tighter temp band on colder days. I live in the Baltimore area, and our winter days usually fluctuate from the 20's to low 40's.

The charging of the tank is pretty much manual, based upon when you light a fire in the wood boiler, so that's really just a matter of learning your system. If you notice that it's not quite keeping up, make a note of the outdoor conditions and the tank temp, then re-charge the tank with a new fire. If you keep a log of the conditions at which the tank temp is no longer able to heat your house, you should be able to estimate based on the weather.

In other words, one you know that the tank temp will (for a hypothetical) heat your house fine when the tank is 130 and the outdoor temp is 10 degrees, then any other day it is forecast to be 10 degrees, you know you will need to keep the tank above 130, so when you see your tank thermostat dropping towards that point, you burn some wood and recharge it.

Joe

 

[Bob Rohr]

Here is one way to accomplish what I think you are asking. This system has a solar tank to feed the radiant, (2- 1" ball valves ) but it could just as easily be a wood fired boiler.

It uses a pair of differential controls and an outdoor reset control. So if the solar tank is warm enough the 3 way zone valve allows flow from that source, via one of the delta t controls. The other differential control runs the solar. When the tank falls below a usable temperature the Laing back up electric pump/ heaters take over.

The three way mixing valve, in this case a Viessmann, although it could be tekmar, Taco, pick a favorite, is tied to an outdoor reset control. This alows that solar, or wood buffer to be pulled down to the absolute lowest temperature based on the outdoor temperature.

If the building can be heated with 120F supply then the mixer adjusts to that temperature, etc.

It really doesn't matter what the heat emtters are, but with low temperature emitters, like radiant slabs, the outdoor reset may allow that tank to be pulled down to within a few degrees of the slab temperature.

Particularly nice for solar heated systems as it presents a cold tank to the solar every morning to leverage delta t and keep the panels at a high efficiency. same for wood boilers, really. the cooler the return the higher the efficiency. Although return temperature protection is needed for non condensing boilers.

Thanks to Radiant Engineering for sharing this simple but effective control logic.

hr

 

[Eric Johnson]

I like the square conduit.

 

[gorsuchmill]

Impressive setup Master, though I must admit I prefer Joe's simpler approach better at this point. No need for me to complicate my initial install. Better to burn more when cold, and less when milder - seems straight forward. Once I've got that down I can move to full automation.

Thanks to all.