Beauty and the Beast

A number of months ago we were approached by a church and given the opportunity to develop a hydronic heating solution for their main sanctuary and meeting area.  The existing system that served the church was a 1.5 MILLION BTU cast iron boiler.  To put that in perspective it's about 15 - 20 times larger than a furnace found in most residential homes.  That is also a measure of how much natural gas it is designed to burn, not how much heat it actually delivers to the space.  Even if we assume 70% efficiency (which might still be generous) it's over 1 million BTUs of heat being generated by this beast.  I think it really earns that name too, I mean, just look at the thing:

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To give you a sense of scale, that's a 6 foot ladder you can barely see behind it and I think that's a 12 or 14 inch exhaust pipe you're looking at.  It even has an angry face from the other angle:

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We're pretty sure this was original to the building, a suspicion that was confirmed when removing it we discovered the concrete floor was actually poured AROUND it!  So, after it was removed and the hole patched appropriately, it was time to replace the Beast with something a bit more modern and efficient, but what to use?

We began this project as we do any other, by conducting a survey of the building and calculating the heat loss using the Manual J calculation.  As I said before we were serving the sanctuary, narthex, organ loft, and basement as well as replacing another small boiler that served a new entryway on the back of the building.  I'm sure at some point in the past someone much have thought 1.5 million BTUs seemed about right for a building of this size, but I wasnt' sure.  The very conservative heat load (no one really seemed to know how much, if any, insulation was in the walls between the plaster and the brick, so I assumed there wasn't any) revealed that the total heat load for the entire building was 400,000 BTU/hr.  Remember that include the new entryway with lots of windows that added over 75,000 BTU or heat itself and that 400k is on the coldest day of the year.  Needless to say, "The Beast" was extremely oversized from day one and had been consuming far more than his fair share of gas the entire time.

Gas wasn't the only resource being consumed more rapidly than it needed to be, though.  The old system used modulating valves to control the amount of heat and the rate it was delivered to the spaces.  I was actually impressed by this arrangement and in theory it should provide very even temperatures.  The valves were opened and closed by thermostats that sent an electrical signal that varied depending on how far away the set point was from the actual temperature.  I'm not sure exactly how it was configured, but if you can imagine the sanctuary was 65 degrees and  you wanted it to be 66, the valve would open a little to heat it up a little, but if it was 60 degrees and you wanted it to be 65, it would open a lot (or all the way, probably) to warm up as quickly as possible until it tailed off the closer it was to the setpoint.  Not bad as far as comfort control is concerned for a 50 year old system.  

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These aren't the pumps, but they're similar

There was a problem, though.  The pumps for the system ran continuously.  They were actually on lightswitches and would run from roughly October to March every year.  24/7.  That doesn't seem like it would be a huge deal until you start to do the math.  These were larger commercial ciculators - I'm pretty sure some were larger than others, but we'll be kind and assume they were all of the 1/6 HP variety with an amp draw of 2 amps.  I'll spare you the math (See the bottom of this article if you want to see it worked out) but it works out to - near as makes no difference - $120 per year.  Per pump.  and there were 4 of them.  So, $480/year was being spent just to run the 4 circulator pumps continuously for  6 months.  

Needless to say, there was plenty of opportunity to improve on the efficiency side of the equation for the church, but we wanted to make sure we were putting together a system that was reliable and effective as well.  The initial plan involved three smaller modulating-condensing boilers similar to what we had put in our own shop when we built it.  The idea was to apply the KISS principle and two of the boilers would simply supply the two largest areas of the church - one for the sanctuary that had radiant floor heat and would use lower temperature water, and another for the basement that would use high temperature water for the fin-tube wall radiators.  The third would be the "complicated" one and serve 3 lower temperature zones and the indirect domestic water heater.  

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Like this, but x 3

This is actually the concept we were planning on using until our vendor suggested we consider doing a cascade of two larger boilers rather than 3 independent systems.  The cascade system would simplify some aspects of the install while complicating others, but brought with it some very tangible benefits.  I should first explain what I mean by a cascade system.  A cascade of boilers is simply multiple boilers that are connected to the same piping system and controlled in such a way that the capacity of both are able to supply heat to the space as well as provide redundancy for the boilers.  The cascade controller handles both; bringing on enough boiler capacity to meet the heating demand as well as making sure both boilers are used an approximately equal amount of time as well as allowing a fail over if one should stop working.

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The upper left is the cascade controller, the upper right is the outdoor reset, and the lower green control is the pump controller

The largest complication of switching to a cascade system vs the independent boilers was the different temperature zones we would need to have.  The majority of the zones are low temperature radiant floor heating, but the lower level has radiators and the indirect water heater that need hotter water to work correctly.  That problem was solved through the use of mixing valves that take the high temperature water produced by the boiler for the radiator zone and mixing it with water returning from the zone to make the water the lower temperature needed the radiant floor zones.  We used 4 individual mixing valves so we could adjust the zones independently to balance heating capacity with comfort. 

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This valve allows mixing hot and warm water to make hottish/warmish water for the radiant zones

In addition to the mixing valves and cascade control, the system had two other controls that help maximize efficiency and flexibility of the system while remaining very simple to control.  The first is the outdoor temperature reset control.  In a nutshell, it changes the temperature of the water produced by the boilers based on the outdoor temperature and by association the heat load of the building.  Lower water temperatures means the boilers don't have to get as hot and allows for maximum condensation of the flue gasses, maximizing boiler efficiency.  That works great for allowing the boilers to make 140 degree water to send to the radiators when it's warm outside, but that doesn't work very well for making 120 degree water in the indirect water heater.  That problem is solved by the cascade controller and the next control, the Enhanced Switching Relay, or more simply, the pump control.

The pump control takes the input of the thermostats from around the building and brings on the appropriate pumps and sends a signal to the reset control that it would like heat.  The reset control takes that signal, determines what temperature water it would like from the boilers and sends it on to the cascade control that determines which boiler to bring on and at what firing rate to satisfy the temperature requirement from the reset control.  Let's simplify that a little:

Thermostat > Pump Control > Reset Control > Casade Controller > Boilers

As more zones come on, but boilers adjust their firing rate and even bring on both boilers to try to get to the temperature requested by the reset control.  This all happens automatically, and at any given time there is exactly as much heat being produced as needed and it is doing so as efficiently as possible.  But what about that water heater I mentioned earlier?  Doesn't it throw everything off?  After all, you don't want to try to heat 50 degree incoming water to 120 degrees using 140 degree water - it would get there eventually, but it would take forever.  Something I forgot to mention, the reset control won't even let the boiler come on if it's more than 70 degrees outside.  

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So does that mean no hot water in the summer?  No, we've got a little trick up our sleeves.  You see, the pump control has a special priority zone that we can hook the water heater up to.  When the aquastat on the water heater says it needs heat a few things happen.  First, the pump to the water heater turns on and the other zones turn off - that is the priority part - it jumps to the front of the line.  The pump control also has a priority output that connects directly to the cascade controller - bypassing the outdoor reset control and brining the boilers on at high fire - producing 180 degree water for as long as the water heater needs it.  So to recap, it looks like this:

Aquastat > Priority zone control > Cascade controller > Boilers

This arrangement allows us to make hot water very efficiently and in very large quantities. 

Some of the other efficiency and performance features of the system are the thermostats and the pumps.  The thermostats are internet connected and allow for system monitoring, adjustment, and scheduling remotely.  They will also send an alert to email addresses if there is a power loss, high or low temperature problem or any number of other conditions that can be set in the system.  The pumps are also special - they aren't simply fixed speed pumps (with the exception of the indirect water heater) but rather vary their speed based on the temperature difference between the supply and return water and get as close to 20 degrees as possible.  That means that the pumps are able to speed up or slow down throughout the heating call to get the zone up to temperature as quickly as possible (by flowing more water initially) and then slowing down as the pipes heat up to minimize energy use and maximize comfort.  Where the old pumps used over 240 watts to run, these pumps, that only run when there is a demand, only use about 40 watts at the most and run even lower as conditions allow.  The pumps alone should save over $400 a year in electricity.  

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That's 40 watts and 12.1 GPM showing on these two pumps

 So I titled this entry Beauty and the Beast - we've already met the Beast - and while I think the new system and its installation is a thing of beauty, the real Beauty is the savings that will be achieved by this system.  By using available technology, properly sizing the system, and designing intelligently to maximize efficiency, effectiveness and reliability the church should see tremendous gas and electrical savings vs the previous system.  Maybe some day I'll be able to post a follow up to this, but I wouldn't be surprised if the savings approach 50% and that, my friends, is a thing of beauty.

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The Math:

2 amp x 120v = 240 watt per pump

240W x 24 hr/day /1000 = 5.76 kWhr/day/pump

5.76 kWhr/day x 180 days/year = 1036.8 kWhr/year/pump

1036.8 kWhr/year/pump * $0.115/kWhr = $119.23/year/pump

4 pumps x $119.23 = $476.92 ~ $480/year pumping cost.