There is a lot of conversation going around about the need for post purge circulator pumps on hot water boilers. I have my own opinions on this idea. I feel the need for post purge pumps is a great idea or a mediocre idea to fuel savings. In real estate they say everything is "Location, Location, Location". I say something similar; everything is "Application, Application, Application". The best product or idea can lose its punch or fuel savings in a bad application. A not so great product can shine in the proper application. The same applies to post purge pumps. 

We have to look at two things when considering post purge pumps. The first would be fuel consumption and the second would be the ratio of electric consumed to radiation btu output. When we burn fuel to create heat due to a call from the thermostat or aquastat for domestic hot water, the fuel is already used when the unit shuts off the excess energy, hot water left in the boiler, just sits there and is lost in standby losses. Now, what to do with the heated water already produced during the run cycle sitting in the boiler? Do we let the hot water in the boiler or pump it into an indirect water tank or heating system?

Let's first look at the operation of pump post purge. The idea of post purge is get the hot water left in the boiler out to where we can take advantage of the Btu’s.  When the demand for heat or Indirect water heater ends, the pump may continue to run for a certain period of time, maybe an only few minutes to 15 - 20 minutes. Who decides, what the best time period is. Some say the longest and some say the shortest and everywhere in between.

Application is the answer. If the boiler is chimney vented (waste of fuel) and cast iron the post purge makes a lot of sense provided we control it properly. Without post purge the heat that does not end up in the system will be lost up the chimney due to constant drafting considered as stand by loss. Getting the heat out of the boiler and into the indirect water heater or the heating system makes sense. If the boiler is sidewall vented, we lose some of the need to get the heat out of the boiler. This application will have much less standby losses then chimney vented products.  The larger the mass and larger water volume the more stored energy to benefit from. The smaller the mass the lower the benefit of using post purge. If we look at a cast iron boiler there is a mass of iron and water volume to cool down where the newer low water volume wall hung boilers is less advantageous. The hotter the water and more mass the more benefit of using post purge. When we consider post purge we must be able to move water. If we utilize zone valves or multi-zone relays it is more difficult to use post purge.

The system plays a large part in choosing post purge. If the system is micro-zoned or high temperature copper tube baseboard makes less sense dependent on control strategy. We must be concerned with overheating the zone. Cast iron radiation systems, radiant in-floor systems, in other words slower response systems, and indirect water heaters may make more sense dependent on control applications. Each system will operate differently. We just do not want to circulate water in the system that is not transferring temperature from the boiler water to the system. In other words if the post purge time is set too long the transfer of heat from the boiler to the system is very low. Therefore a waste of electric. This can happen quicker in a copper tube baseboard system or radiant heat in slab with ODR (outdoor reset). ODR will reduce the need for post purge unless the boiler is chimney vented. If the boiler is heated to 130f on a given outside temperature the water will cool too quickly. The lower the water temperature the lower the system delta-T. If delta-T is reduced to a point where the cost of electricity is greater than the heat transfer you don’t gain anything.

Some controls can dump into an indirect water heater. This is a good strategy if the indirect is well insulated, has a hot water mixing valve for safety and has a heat trap piped in. With the use of ODR today the boiler temperature may be less than the tank temperature. Now the water entering the indirect water heater is lower than the tank temperature the boiler water will absorb temperature from the tank and heat the boiler up even more.

First I would like to say pump pre-purge should not be controlled on a time period, it should be controlled on temperature. When we take very little heat out of the water due to the type of heat emitters and water temperature, there comes a point where you end up just chasing water around the system using more electric then the benefit of heat.  

I feel the best way to control post purge is by temperature. If the supply and return temperatures get too close together the benefit of post purge becomes less of a fuel savings. The use of a variable speed pump acting on delta-T makes more sense to me. I talk on other pages of this sight about speeding water flow up increases heat output. When using post purge the idea is to drop boiler water temperature. So now we would slow down the water during pump pre-purge to drop the water temperature and still be a benefit to the system. With variable speed delta-T pumps you set the delta-T you want in the system and the pump will slow down as the water cools off trying to maintain the preset delta-T during post purge.

Do not confuse this with the idea of constant run pumps or even pre-purge pumps. These are other concepts although not new ideas are also fuel savers. Again more options with Hydronics.