diff --git a/supervisor/session/rtu.lua b/supervisor/session/rtu.lua
index eed5cdc..bb86b05 100644
--- a/supervisor/session/rtu.lua
+++ b/supervisor/session/rtu.lua
@@ -3,6 +3,9 @@ local log = require("scada-common.log")
 local mqueue = require("scada-common.mqueue")
 local util = require("scada-common.util")
 
+-- supervisor rtu sessions (svrs)
+local svrs_boiler = require("supervisor.session.rtu.boiler")
+
 local rtu = {}
 
 local PROTOCOLS = comms.PROTOCOLS
diff --git a/supervisor/session/rtu/boiler.lua b/supervisor/session/rtu/boiler.lua
new file mode 100644
index 0000000..83ecd8b
--- /dev/null
+++ b/supervisor/session/rtu/boiler.lua
@@ -0,0 +1,210 @@
+local comms = require("scada-common.comms")
+local log = require("scada-common.log")
+local types = require("scada-common.types")
+local util = require("scada-common.util")
+
+local txnctrl = require("supervisor.session.rtu.txnctrl")
+
+local boiler = {}
+
+local PROTOCOLS = comms.PROTOCOLS
+local MODBUS_FCODE = types.MODBUS_FCODE
+
+local rtu_t = types.rtu_t
+
+local TXN_TYPES = {
+    BUILD = 0,
+    STATE = 1,
+    TANKS = 2
+}
+
+local PERIODICS = {
+    BUILD = 1000,
+    STATE = 500,
+    TANKS = 1000
+}
+
+-- create a new boiler rtu session runner
+---@param advert rtu_advertisement
+---@param out_queue mqueue
+boiler.new = function (advert, out_queue)
+    -- type check
+    if advert.type ~= rtu_t.boiler then
+        log.error("attempt to instantiate boiler RTU for non boiler type '" .. advert.type .. "'. this is a bug.")
+        return nil
+    end
+
+    local log_tag = "session.rtu.boiler(" .. advert.index .. "): "
+
+    local self = {
+        uid = advert.index,
+        reactor = advert.reactor,
+        out_q = out_queue,
+        transaction_controller = txnctrl.new(),
+        has_build = false,
+        periodics = {
+            next_build_req = 0,
+            next_state_req = 0,
+            next_tanks_req = 0,
+        },
+        ---@class boiler_session_db
+        db = {
+            build = {
+                boil_cap = 0.0,
+                steam_cap = 0,
+                water_cap = 0,
+                hcoolant_cap = 0,
+                ccoolant_cap = 0,
+                superheaters = 0,
+                max_boil_rate = 0.0
+            },
+            state = {
+                temperature = 0.0,
+                boil_rate = 0.0
+            },
+            tanks = {
+                steam = 0,
+                steam_need = 0,
+                steam_fill = 0.0,
+                water = 0,
+                water_need = 0,
+                water_fill = 0.0,
+                hcool = 0,
+                hcool_need = 0,
+                hcool_fill = 0.0,
+                ccool = 0,
+                ccool_need = 0,
+                ccool_fill = 0.0
+            }
+        }
+    }
+
+    ---@class rtu_session__boiler
+    local public = {}
+
+    -- PRIVATE FUNCTIONS --
+
+    -- query the build of the device
+    local _request_build = function ()
+        local m_pkt = comms.modbus_packet()
+        local txn_id = self.transaction_controller.create(TXN_TYPES.BUILD)
+
+        -- read input registers 1 through 7 (start = 1, count = 7)
+        m_pkt.make(txn_id, self.uid, MODBUS_FCODE.READ_INPUT_REGS, { 1, 7 })
+
+        self.out_q.push_packet(m_pkt)
+    end
+
+    -- query the state of the device
+    local _request_state = function ()
+        local m_pkt = comms.modbus_packet()
+        local txn_id = self.transaction_controller.create(TXN_TYPES.STATE)
+
+        -- read input registers 8 through 9 (start = 8, count = 2)
+        m_pkt.make(txn_id, self.uid, MODBUS_FCODE.READ_INPUT_REGS, { 8, 2 })
+
+        self.out_q.push_packet(m_pkt)
+    end
+
+    -- query the tanks of the device
+    local _request_tanks = function ()
+        local m_pkt = comms.modbus_packet()
+        local txn_id = self.transaction_controller.create(TXN_TYPES.TANKS)
+
+        -- read input registers 10 through 21 (start = 10, count = 12)
+        m_pkt.make(txn_id, self.uid, MODBUS_FCODE.READ_INPUT_REGS, { 10, 12 })
+
+        self.out_q.push_packet(m_pkt)
+    end
+
+    -- PUBLIC FUNCTIONS --
+
+    -- handle a packet
+    ---@param m_pkt modbus_frame
+    public.handle_packet = function (m_pkt)
+        local success = false
+
+        if m_pkt.scada_frame.protocol() == PROTOCOLS.MODBUS_TCP then
+            if m_pkt.unit_id == self.uid then
+                local txn_type = self.transaction_controller.resolve(m_pkt.txn_id)
+                if txn_type == TXN_TYPES.BUILD then
+                    -- build response
+                    if m_pkt.length == 7 then
+                        self.db.build.boil_cap = m_pkt.data[1]
+                        self.db.build.steam_cap = m_pkt.data[2]
+                        self.db.build.water_cap = m_pkt.data[3]
+                        self.db.build.hcoolant_cap = m_pkt.data[4]
+                        self.db.build.ccoolant_cap = m_pkt.data[5]
+                        self.db.build.superheaters = m_pkt.data[6]
+                        self.db.build.max_boil_rate = m_pkt.data[7]
+                    else
+                        log.debug(log_tag .. "MODBUS transaction reply length mismatch (boiler.build)")
+                    end
+                elseif txn_type == TXN_TYPES.STATE then
+                    -- state response
+                    if m_pkt.length == 2 then
+                        self.db.state.temperature = m_pkt.data[1]
+                        self.db.state.boil_rate = m_pkt.data[2]
+                    else
+                        log.debug(log_tag .. "MODBUS transaction reply length mismatch (boiler.state)")
+                    end
+                elseif txn_type == TXN_TYPES.TANKS then
+                    -- tanks response
+                    if m_pkt.length == 12 then
+                        self.db.tanks.steam = m_pkt.data[1]
+                        self.db.tanks.steam_need = m_pkt.data[2]
+                        self.db.tanks.steam_fill = m_pkt.data[3]
+                        self.db.tanks.water = m_pkt.data[4]
+                        self.db.tanks.water_need = m_pkt.data[5]
+                        self.db.tanks.water_fill = m_pkt.data[6]
+                        self.db.tanks.hcool = m_pkt.data[7]
+                        self.db.tanks.hcool_need = m_pkt.data[8]
+                        self.db.tanks.hcool_fill = m_pkt.data[9]
+                        self.db.tanks.ccool = m_pkt.data[10]
+                        self.db.tanks.ccool_need = m_pkt.data[11]
+                        self.db.tanks.ccool_fill = m_pkt.data[12]
+                    else
+                        log.debug(log_tag .. "MODBUS transaction reply length mismatch (boiler.tanks)")
+                    end
+                elseif txn_type == nil then
+                    log.error(log_tag .. "unknown transaction reply")
+                else
+                    log.error(log_tag .. "unknown transaction type " .. txn_type)
+                end
+            else
+                log.error(log_tag .. "wrong unit ID: " .. m_pkt.unit_id, true)
+            end
+        else
+            log.error(log_tag .. "illegal packet type " .. m_pkt.scada_frame.protocol(), true)
+        end
+
+        return success
+    end
+
+    public.get_uid = function () return self.uid end
+    public.get_reactor = function () return self.reactor end
+    public.get_db = function () return self.db end
+
+    -- update this runner
+    ---@param time_now integer milliseconds
+    public.update = function (time_now)
+        if not self.has_build and self.next_build_req <= time_now then
+            _request_build()
+            self.next_build_req = time_now + PERIODICS.BUILD
+        end
+
+        if self.next_state_req <= time_now then
+            _request_state()
+            self.next_state_req = time_now + PERIODICS.STATE
+        end
+
+        if self.next_tanks_req <= time_now then
+            _request_tanks()
+            self.next_tanks_req = time_now + PERIODICS.TANKS
+        end
+    end
+
+    return public
+end
+
+return boiler
diff --git a/supervisor/session/rtu/txnctrl.lua b/supervisor/session/rtu/txnctrl.lua
new file mode 100644
index 0000000..3d74484
--- /dev/null
+++ b/supervisor/session/rtu/txnctrl.lua
@@ -0,0 +1,95 @@
+--
+-- MODBUS Transaction Controller
+--
+
+local util = require("scada-common.util")
+
+local txnctrl = {}
+
+local TIMEOUT = 3000 -- 3000ms max wait
+
+-- create a new transaction controller
+txnctrl.new = function ()
+    local self = {
+        list = {},
+        next_id = 0
+    }
+
+    ---@class transaction_controller
+    local public = {}
+
+    local insert = table.insert
+
+    -- get the length of the transaction list
+    public.length = function ()
+        return #self.list
+    end
+
+    -- check if there are no active transactions
+    public.empty = function ()
+        return #self.list == 0
+    end
+
+    -- create a new transaction of the given type
+    ---@param txn_type integer
+    ---@return integer txn_id
+    public.create = function (txn_type)
+        local txn_id = self.next_id
+
+        insert(self.list, {
+            txn_id = txn_id,
+            txn_type = txn_type,
+            expiry = util.time() + TIMEOUT
+        })
+
+        self.next_id = self.next_id + 1
+
+        return txn_id
+    end
+
+    -- mark a transaction as resolved to get its transaction type
+    ---@param txn_id integer
+    ---@return integer txn_type
+    public.resolve = function (txn_id)
+        local txn_type = nil
+
+        for i = 1, public.length() do
+            if self.list[i].txn_id == txn_id then
+                txn_type = self.list[i].txn_type
+                self.list[i] = nil
+            end
+        end
+
+        return txn_type
+    end
+
+    -- close timed-out transactions
+    public.cleanup = function ()
+        local now = util.time()
+
+        local move_to = 1
+        for i = 1, public.length() do
+            local txn = self.list[i]
+            if txn ~= nil then
+                if txn.expiry <= now then
+                    self.list[i] = nil
+                else
+                    if self.list[move_to] == nil then
+                        self.list[move_to] = txn
+                        self.list[i] = nil
+                    end
+                    move_to = move_to + 1
+                end
+            end
+        end
+    end
+
+    -- clear the transaction list
+    public.clear = function ()
+        self.list = {}
+    end
+
+    return public
+end
+
+return txnctrl