Outdoor Reset Control Theory FULL versus PARTIAL reset

Transcription

1 Outdoor Reset Control Theory FULL versus PARTIAL reset Conventional boilers must be protected against sustained operation at low temperatures. If not, they will be damaged by sustained flue gas condensation. The blue line shows partial reset control for a conventional, gas-fired boiler. The supply temperature is not allowed to go below a target of 140 ºF, regardless of how warm it is outside. Supply temperature (ºF) target temperature heat source off heat source on this portion of! reset line not used Outdoor temperature (ºF) 190 ºF

2 Outdoor Reset Control Theory FULL and PARTIAL reset in same system combined boiler + mixing reset controller 3-way motorized mixing valve mixed supply temperature! sensor VENT boiler supply temperature! sensor 210 conventional boiler boiler inlet temperature! sensor Supply temperature (ºF) target temperature! (boiler outlet) target temperature! (mixing valve outlet) 190 ºF 105 ºF Outdoor temperature (ºF)

3 Outdoor Reset Control Theory Compensation for non-proportional heat loss/gain (parallel shifting) Water supply temperature (ºF) Outdoor temperature (ºF) starting point line parallel shift T balance = T indoor starting point (moves along starting point line) Q gain UA building ( ) ( ) T start = T inside + R T balance 1+ R outdoor reset controllers with indoor temperature sensors will automatically parallel shift the reset line based on internal gains or losses.

4

5 Instantaneous Domestic Water Heating From Thermal Storage Tanks

6 Starting points: Nearly all thermally-based renewable heat sources require significant heat storage. Solar thermal system Geothermal heat pump systems Wood- and pellet-fired boilers Most of these systems use for thermal storage. It almost always makes sense to use these heat sources to provide domestic hot, as well as space heating. Even low storage tank temperatures are useful for preheating domestic hot. Keeping all portions of the DHW system outside the thermal storage tank has several benefits. Hydronic based instantaneous domestic heating has been used in thousands of European installations. Modulating electric tankless heaters have some distinct advantages in dealing with preheated. Brazed plate stainless steel heat exchangers are readily available and have very fast response times.

7 One way to make a solar combisystem... P3 solar! circulation! station solar collector array CW DHW! anti-scald! tempering! valve P&T air vent! w/ shut off valve relay A check! valve B mod/con boiler AB 3-way! diverter! valve (D1) closely spaced tees (P1) heat source! circulator 3-way! motorized! mixing! valve! (C2) Drawbacks: Top of thermal storage has to be maintained at suitably high temperature for domestic hot delivery - reduces collector efficiency. Space heating from storage may be significantly limited by capacity of upper internal coil in storage. outdoor! temperature! sensor relay (R1) pressureregulated circulator (P2) make-up end switch leads from actuators space-heating circuits! (highly zoned) manifold valve actuator end switch leads thermostat! (1 per zone) solar storage tank! (dual coil)

8 Another way to make a solar combisystem... Pressurized drainback system w/ internal coil for domestic heating. lift head solar! collector! array 3-way mixing valve air return! tube All solar circuit piping sloped minimum 1/4" per ft. PRV cold hot air return tube outside temperature sensor auxiliary boiler Passive domestic preheating Boiler maintains temperature at top of storage. Thermal mass of tank buffers space heating load. Single tank simplicity static level sight! glass air space heat source! circulator! w/ check A AB B 3-way! diverter! valve 3-way mixing valve space heating! distribution! system collector! circulator DHW coil closely spaced tees Sourcing the tank may be an issue Internal coil may be difficult or impossible to service differential! temperature! controller storage / drainback tank

9 Wood gasification boiler with heat storage. air vent temperature sensor well bypass wood-fired boiler! loading! unit pressurized thermal storage tank Large thermal storage is often used with wood gasification boilers.

10 VENT Geothermal heat pump space heating with heat storage. low temperature distribution system to / from other heating zones outdoor! reset! controller Comp. temperature! sensor variable-speed! pressure-regulated! circulator compressor evaporator condenser TXV -to-! heat pump (heating only) buffer tank geothermal manifolds purging! valves earth loop circuits

11 Air-to- heat pump with heat storage. OUTSIDE INSIDE outdoor! temperature! sensor outdoor! reset controller pressure regulated! variable speed circulator manifold valve actuators antifreeze! protected! circuit zoned radiant ceiling panels make up air to heat pump flow switch buffer tank

12 Gas-fired tankless heaters and preheated Many gas-fired tankless heaters have limited modulation ability. Most require an inlet temperature at least 20ºF lower than their setpoint as a condition to fire their combustion system. heat exchanger Thermal load on heater: Minimum heat transfer rate of heater: outlet! temperature! sensor inlet! temperature! sensor flow sensor Where: Qthermal = current thermal load on heater (Btu/hr) f = flow rate through heater (gpm) Tseptoint = desired outlet temperature leaving heater (ºF) Tin = inlet temperature to heater (ºF) QHTmin = minimum rate of heat transfer to (Btu/hr) Qgas = minimum gas input rate to burner (Btu/hr) n = combustion efficiency of burner / heat exchanger (decimal %) IF Qthermal QHTmin THEN burner doesnʼt fire hot cold controller isolation &! flushing valves premix gas burner gas or propane supply ASSE 1070 mixing valve

13 Assume the minimum gas input to a tankless heater is 10,000 Btu/hr, and the combustion efficiency is 90%. Solar preheated enters the unit at 110 ºF and 0.8 gpm. The setpoint of the heater is 120 ºF. If the controller in the unit operates as described on previous slide, will the burner fire under the stated conditions? Thermal load on heater: Minimum heat transfer rate of heater: Because Qthermal QHTmin THEN burner doesnʼt fire heat exchanger outlet! temperature! sensor inlet! temperature! sensor flow sensor controller hot isolation &! flushing valves premix gas burner cold gas or propane supply ASSE 1070 mixing valve

14 One work around is to mix down the inlet temperature using a second thermostatic valve. This works - BUT... It also lowers the net preheating effect supplied by the solar subsystem. hot cold thermostatic MIX DOWN valve! (set 20 ºF lower than setpoint of heater) For example: Assume the setpoint of the tankless heater is 120 ºF. Solar preheated arriving at the tankless heater at 110ºF would have to be mixed down to 100 ºF before entering the heater. Assuming a desired delivery temperature of 120ºF and a cold temperature of 50 ºF, this 10 ºF mix down decreases the solar supplied portion of the heating load from 86% to 71%. preheated

15 One work around is to mix down the inlet temperature using a second thermostatic valve European product by Caleffi

16 Another option is to divert around the tankless heater if temperature is above a specified setting. PRV hot cold solar preheated electric diverter valve

17 Thermostatically controlled electric tankless heaters Typical point of use ETWH 3-6 KW maximum possible temperature rise always depends on flow rate 20KW unit Typical whole house ETWH KW

18 Thermostatically controlled electric tankless heaters 12KW unit, 50Amp / 240VAC element enclosure overtemp switch contactor outlet! temperature! sensor flow! sensor PRV hot ETWH cold heating! element controls 240 VAC! electrical supply inlet! temperature! sensor isolation &! flushing valves HOT out COLD in 240VAC input relay coil contacts setpoint adjustment electronics (PCB) flow switch

19 Thermostatically controlled electric tankless heaters Electric tankless heaters are HIGH AMPERAGE devices. 3.5 KW Requires 15 amp / 240VAC breaker Amps = KW 0.24 Minimum 200 Amp breaker panel recommended. May be an issue in some older retrofits. 23 KW Requires TWO, 50 amp /240VAC breakers

20 Thermostatically controlled electric tankless heaters Thermostatically controlled electric tankless heaters use a TRIAC to vary the wattage to their heating elements from 0 to 100%. outlet! temperature! sensor flow! sensor PRV hot ETWH cold heating! element controls 240 VAC! electrical supply inlet! temperature! sensor isolation &! flushing valves gates for TRIACS They can therefor handle situations where preheated needs a small temperature boost without short cycling.

21 Instantaneous DHW subassembly air vent temperature! sensor(s) from heat! source space heating! supply to heat! source space heating! return from storage small circulator with check valve HX flushing! valves space heating! electric supply tankless! heater ASSE 1017! anti scald! mixing valve return to storage stainless steel heat exchanger flow switch hot cold space heating! return

22 Instantaneous DHW subassembly TRV Leverages the thermal mass for stabilizing DHW delivery. 1/2" PEX or PEX-AL-PEX tubing pressure-regulated! variable speed circulator electric tankless! heater ASSE 1017! anti scald! mixing valve Brazed plate heat exchanger provides very fast response (1-2 seconds) Fully serviceable heat exchanger (unlike an internal coil heat exchanger) Can be cleaned or replaced if necessary. Predictable heat exchanger performance hot buffer tank flow switch cold electric tankless! heater Very little heated domestic is stored (reducing potential for Legionella growth). ASSE 1017! anti scald! mixing valve hot Very low wattage circulator needed on primary side of heat exchanger flow switch cold

23 Instantaneous DHW subassembly 3 x5 5 x12 Brazed plate stainless steel heat exchangers are widely available. They have very high ratio of surface area to volume. Response time to quasi steady state = 1 to 2 seconds 10 x20 Response time of this subassembly is likely under 5 seconds. (assuming short, insulated piping b/w HX and storage tank)

24 Sizing the brazed plate heat exchanger Suggest a maximum approach temperature difference of 10 ºF under max. anticipated demand, and minimum preheat inlet temperature. maximum suggested! approach temperature! difference from! storage! tank <= 10 ºF domestic FG5x wide x12 long -30 plates

25 Instantaneous DHW subassembly air vent temperature! sensor(s) from heat! source space heating! supply to heat! source space heating! return from storage small circulator with check valve HX flushing! valves return to storage stainless steel heat exchanger space heating! electric supply tankless! heater flow switch ASSE 1017! anti scald! mixing valve hot cold space heating! return B&G Vario circulator operating at 33 watts yields10 gpm as required to raise 4 gpm of from 60 to 110 ºF through FG5x12-30 heat exchanger supplied by 120ºF from thermal storage. To deliver 60 gallons per day at average draw rate of 2 gpm, this circulator would operate for 30 minutes, and consume KWHR. Operating cost of this circulator would be $0.78 per YEAR.

26 Other tankless heater options combi-boiler anti-scald! valve combi-boiler cold hot flow! switch stainless steel! brazed plate! heat exchanger to / from! space! heating internal! circulator motorized! diverter! valve gas-fired tankless Response time from cold start to near steady state at delivery temperature (20-30 seconds) All gas fired equipment needs gas supply, venting,and electrical connections

27 Instantaneous DHW subassembly piping circulator with check valve circulator with check valve to / from heat source(s) electric tankless! heater! mixing valve hot NOTE: Check valve required on tank-to-hx circuit to prevent reverse flow during space heating to / from! other loads cold flow switch storage tank! (with varying temperature) HX flushing valves swing check valve stainless steel! heat exchanger

28 Using extra terminal on ETWH contactor to operate circulator This eliminates the need for the flow switch. Contactor inside Eemax EX012240T extra terminal on coil circuit of contactor thermostatically! controlled ETWH 120 VAC relay! 240 VAC coil! in junction box 240 VAC! electrical supply A B storage to! HX circulator PRV N

29 Add 2nd thermostatic valve if the high limit switch canʼt be set higher than 140 ºF. from storage thermostatically controlled! tankless electric! heater set for 130 ºF set for max 120 ºF DHW return to storage flow switch cold insulated piping

30 Why not just use internal heat exchanger coils? These are all examples of European multi-coil thermal storage tanks.

31 Why not just use internal heat exchanger coils? These tanks cost over $4000 What happens when a pinhole leak develops anywhere inside the tank? Internal coils are limited by natural convection heat transfer. No one offers tanks like this in North America Many European tanks - even if available in North America, may not meet codes (ASME).

32 This concept is already being used... Thermal energy substations in European flats

33 Parting thoughts Keep it neat...

34 Parting thoughts Recognize opportunity...

35 Parting thoughts Donʼt get hung up...

36 "Best Practices in Modern Hydronic Heating - THE DETAILS Thank you for attending today s session... Thanks also to the planning committee, and the sponsors of this session. Please visit our website for more information (publications & software) on hydronic systems: