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My Opinion about Post Purge Circulating Pumps on Hot Water Systems

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 loose it's punch or fuel savings in a bad application. A not so great product can shine in the proper application. The same applies on ppp (post purge pumps). 

We have to look at two things when considering ppp ( post purge pump). The first would be fuel used and the second would be electric used. When we burn fuel to create heat due to a call on the thermostat or aquastat for hot water the fuel is already burned when the unit shuts off. Now, what to do with the heated water already produced during the run cycle. Do we let the hot water in the boiler or pump it into an indirect or heating system?

Let's first look at the operation of ppp. The demand for heat or Indirect water heater demand ends, the pump continues to run for a certain period time. Maybe a few minutes to 15 - 20 minutes. Who decides what is the best time period. 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) the ppp makes a lot of sense provided we control it properly. Without ppp the heat that does not end up in the system will end up going up the chimney due to constant drafting. Getting the heat out of the boiler and into the indirect water heater or the 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 mass and water volume we have more stored energy to benefit from. The smaller the mass the lower the benefit of using ppp. 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 ppp.

The system plays a part in ppp. If the system is micro-zoned, high temperature copper tube baseboard makes less sense dependent of control strategy. On cast iron system radiation system's, radiant in-floor 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 ppp time is set too long the transfer of heat from the boiler to the system is nill. 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 ppp unless the boiler is chimney vented. If the boiler is heated to 130ºf on a given outside temperature the water will cool too quickly. The lower the water temperature the lower the delta-T in the system. If delta-T reduced to a point the cost of electricity is greater than the heat transfer.

Some controls dump into an indirect water heater. 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 ppp 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 ppp is by temperature. If the supply and return temperatures get too close together the benefit of ppp 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 ppp the idea is to drop boiler water temperature. So now we would slow down the water during ppp 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 ppp.

Do not confuse this with the idea of constant run pumps. This is another concept although not a new idea if also a fuel saver.

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