Wednesday, March 28, 2012

Optimizing Boiler Controls for Energy Efficiency

Well, my attempt to increase the frequency of my posting in March fell by the wayside.  First one.  Almost 1 month since I last posted.  I'm hopeful that I will give a better effort as we move into Spring.  My bad.

Today, boiler controls.  More specifically, touching on two specific Honeywell products I come across: the L91B proportioning (modulating) pressuretrol, and the L8124A aquastat relay, and working to optimize their set points for improved energy efficiency.

L91B: Modulating Pressuretrol
This is a really neat product I typically see on steam boilers.  Bellows within the base of the control swell and recede based on the pressure within the system.  This function works to modulate the current flow based on load demand: when load is close to being satisfied and pressure in the boiler builds, the bellows swell, and a potentiometer reacts by reducing the flow of current.  This drives the burner toward low fire in an effort to meet the reduced load demand.  Conversely, when boiler pressure drops, the bellows recede and the potentiometer reacts by increasing the flow of current and driving the burner into a higher firing rate to meet the load.  

The beauty of this control is that it will modulate as needed to keep a steady firing rate at a desired system pressure.  This is good for both energy efficiency and wear and tear on the system.  The problem is that the dang things are almost never set properly.  

The differential on the L91B is additive; the operating range of modulation is equal to the Main set point plus the differential.  This must be configured carefully with regard to the Operating Pressuretrol set points.  Too often, the settings are incorrect.  The entire range of modulation for the L91B should occur within the range of the Operating Pressuretrol (which, to confuse the situation, has a subtractive differential).  Basically, you don't want the burner to cut-out on high limit while still the L91B is still modulating the firing rate.  The L91B Main+Diff should be slightly less than the cut-out setting for the Operating Pressuretrol.  Also, the Diff on the L91B directs the rate of modulation; a small Diff means a more aggressive rate of modulation, while a larger Diff means a more subtle rate of modulation.  This can be important to fuel consumption and other system operating functions.

Retro-commissioning the L91B so that it is appropriately configured will provide even and efficient combustion across the entire firing range.

L8124A: Aquastat Relay
I often see this Honeywell control on small home hydronic systems, specifically where there is a single boiler that provides for both space heating and domestic hot water needs.  This relay control has a high limit adjustable setting with a fixed Diff of 10F, and a low limit adjustable setting with an adjustable Diff range of 10F-25F. 

The low-limit is going to control the DHW, and the high limit is going to control the space heating.  I last saw this over the past weekend at a buddy's house.  They are getting killed by their oil consumption, exacerbated by $3.90/gallon fuel oil prices.  This is a tiny house with fine-tube convector baseboards and a Weil-McLain Gold series oil boiler.  The L8124 was set to 185F at the hi-limit for the baseboards (10F fixed Diff) and 160F with a Diff of 25 at the low-limit for the DHW.  With this setting, the burner will fire at low-limit between 135F-160F, and at high-limit between 175F-185F.  I feel that both of these settings are inappropriate.

This house is tiny and there is ample radiation.  I turned the hi-limit control down to 175F so that the burner fires to maintain a range of 165F-175F at a call for heat, which should be more than enough to heat this home.  I changed the setting of the low-limit to 150F with a Diff of 10F, so that the burner fires between 140F-150F, which should be more appropriate.  Reducing the low-limit target temp will bring down the excessively high DHW temp of 160F, which needs to be mixed down at the tap to prevent scalding.  Reducing the Diff will reduce total burner firing time, which should improve fuel efficiency.  This will promote more frequent cycling (short cycling), but that shouldn't be an issue with a low mass boiler housed within conditioned space.

Unfortunately, this scenario is really trying to do a little bit better with a sub-optimal configuration.  A larger differential on the low-limit will allow the burner to remain off for longer periods of time, but will then require the burner to have longer on-cycles to achieve the set point.  In this case, there is just no reason to be making DHW at 160F; that is too hot and potentially dangers.  The new setpoint of 150F is still too hot, but less too hot and less potentially dangerous.  I would love to be able to leave it at 150F and crank the Diff up to 25F so the burner fires only between 125F-150F.  Unfortunately, the boiler has minimum hot water return temperatures to ensure condensation does not occur in the flue, so we can't go below 140F.  For this reason, a standard, open combustion boiler with a tankless coil is not an ideal method of producing DHW; sometimes you just gotta work with what you got.

The moral of the story is to scrutinize boiler controls, consult manufacturer's specs, and make some changes to the protocol where appropriate.  You can often tweak a few percentage points of heating fuel savings by making adjustments to optimize settings.

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