diff --git a/docs/biomass.md b/docs/biomass.md
index b075311..14fc113 100644
--- a/docs/biomass.md
+++ b/docs/biomass.md
@@ -27,6 +27,18 @@ load and realistic duty cycles:
   is what the fuel store and the monsoon
   stock have to be sized for (estimate)
 
+### Recovering heat at the boiler
+
+A fired boiler loses heat up its stack, so an economizer that preheats the
+boiler feedwater with the flue gas is the first heat-recovery measure to
+investigate, ahead of condensate return and the feedwater heat pump
+([sustainability](sustainability.md#energy-efficiency-architecture-recover-before-generating)).
+How far the flue gas can be cooled is the vendor's figure: it depends on
+the flue-gas temperature, the corrosion limit and the fuel. Rice husk ash
+is high in silica and abrasive, so the RFQ asks for fouling and cleaning
+provisions, wear protection and a bypass rather than a clean-gas
+economizer ([steam and cooling](steam-cooling.html)).
+
 ## The 60 rai: own-grown fuelwood
 
 60 rai is 9.6 hectares. Managed as short-rotation coppice (planted once,
diff --git a/docs/steam-cooling.html b/docs/steam-cooling.html
index d7f638a..b6f70b3 100644
--- a/docs/steam-cooling.html
+++ b/docs/steam-cooling.html
@@ -13,12 +13,12 @@ document.documentElement.setAttribute("data-theme",t);})();
 :root{color-scheme:dark;
   --bg:#0a0f1a;--card:#0f1626;--box:#121a2b;--stroke:#2a3a56;--head:#1a2640;--fg:#d6deeb;--mute:#8a97b0;--ink:#e6ecf7;
   --sel:#7fb2ff;--warn:#ffbf1f;--alarm:#ff3b3b;--ok:#3ddc84;
-  --st:#ff5a3c;--cd:#c96a4e;--fw:#2fb5a0;--rw:#8aa3c0;--cws:#3aa0ff;--cwr:#7fc0ff;--pri:#2e7fd0;--prw:#9c7bd9;--hr:#f5a623;--bd:#b48ead;
+  --st:#ff5a3c;--cd:#c96a4e;--fw:#2fb5a0;--rw:#8aa3c0;--cws:#3aa0ff;--cwr:#7fc0ff;--pri:#2e7fd0;--prw:#9c7bd9;--hr:#f5a623;--bd:#b48ead;--fl:#9aa4b2;
   --mono:ui-monospace,"Cascadia Mono",Consolas,"Liberation Mono",monospace}
 html[data-theme=light]{color-scheme:light;
   --bg:#eef2f7;--card:#ffffff;--box:#ffffff;--stroke:#c3cede;--head:#e3e9f3;--fg:#1b2533;--mute:#5b6880;--ink:#1b2533;
   --sel:#1f5fbf;--warn:#c98a00;--alarm:#e0352b;--ok:#16a35a;
-  --st:#e2462a;--cd:#b0573c;--fw:#14907e;--rw:#6d86a3;--cws:#1f84e0;--cwr:#4f9be0;--pri:#2468b0;--prw:#7b5cc0;--hr:#d48600;--bd:#8e6a8a}
+  --st:#e2462a;--cd:#b0573c;--fw:#14907e;--rw:#6d86a3;--cws:#1f84e0;--cwr:#4f9be0;--pri:#2468b0;--prw:#7b5cc0;--hr:#d48600;--bd:#8e6a8a;--fl:#7a8494}
 *{box-sizing:border-box;margin:0;padding:0}
 body{background:var(--bg);color:var(--fg);font:14px/1.5 -apple-system,"Segoe UI",Roboto,Arial,sans-serif}
 header{display:flex;flex-wrap:wrap;gap:8px 16px;align-items:center;padding:10px 16px;background:var(--card);border-bottom:1px solid var(--stroke);position:sticky;top:0;z-index:2}
@@ -65,8 +65,8 @@ td.low{color:var(--warn);font-weight:600}
 .p{fill:none;stroke-width:2.2;stroke-linejoin:round}
 .p.st{stroke:var(--st);stroke-width:3}.p.cd{stroke:var(--cd);stroke-dasharray:7 4}.p.fw{stroke:var(--fw)}.p.rw{stroke:var(--rw)}
 .p.cws{stroke:var(--cws);stroke-width:2.6}.p.cwr{stroke:var(--cwr);stroke-dasharray:7 4}.p.pri{stroke:var(--pri)}.p.prw{stroke:var(--prw);stroke-dasharray:7 4}
-.p.hr{stroke:var(--hr);stroke-width:2.6}.p.bd{stroke:var(--bd);stroke-dasharray:3 3}.p.opt{stroke-dasharray:3 5;opacity:.8}
-.ar{stroke:none}.ar.st{fill:var(--st)}.ar.cd{fill:var(--cd)}.ar.cws{fill:var(--cws)}.ar.cwr{fill:var(--cwr)}.ar.pri{fill:var(--pri)}.ar.prw{fill:var(--prw)}.ar.hr{fill:var(--hr)}
+.p.hr{stroke:var(--hr);stroke-width:2.6}.p.fg{stroke:var(--fl);stroke-width:3;stroke-dasharray:1 4;stroke-linecap:round}.p.bd{stroke:var(--bd);stroke-dasharray:3 3}.p.opt{stroke-dasharray:3 5;opacity:.8}
+.ar{stroke:none}.ar.st{fill:var(--st)}.ar.cd{fill:var(--cd)}.ar.cws{fill:var(--cws)}.ar.cwr{fill:var(--cwr)}.ar.pri{fill:var(--pri)}.ar.prw{fill:var(--prw)}.ar.hr{fill:var(--hr)}.ar.fw{fill:var(--fw)}
 .vv{fill:var(--card);stroke:var(--fg);stroke-width:1.2}
 .trap{fill:var(--card);stroke:var(--cd);stroke-width:1.4}
 .tt{fill:var(--fg);font:bold 7px var(--mono);text-anchor:middle}
@@ -132,8 +132,9 @@ td.low{color:var(--warn);font-weight:600}
       <tr><td>P-1</td><td>Feed pumps, duty/standby</td><td class="n">~1 m³/h, ~13 bar</td><td>launch</td><td>estimate</td></tr>
       <tr><td>B-1, B-2</td><td>Boiler modules, lead/lag</td><td class="n">2 × 500 kg/h, 10.5 bar</td><td>B-1 launch, B-2 with Duplex 2</td><td>proposal</td></tr>
       <tr><td>B-3</td><td>Third module for N+1</td><td class="n">500 kg/h</td><td>option</td><td>RFQ option</td></tr>
+      <tr><td>ECON-1</td><td>Flue-gas economizer, fired boilers</td><td class="n">feed in 90 to 95 °C, out by vendor</td><td>with a fired bank</td><td>1st measure</td></tr>
       <tr><td>ACC-1</td><td>Steam accumulator</td><td class="n">by cycle overlap</td><td>option</td><td>Newbilt option</td></tr>
-      <tr><td>HP-1</td><td>Heat-recovery heat pump</td><td class="n">feed to 90 to 95 °C</td><td>launch or later</td><td>proposal</td></tr>
+      <tr><td>HP-1</td><td>Heat-recovery heat pump</td><td class="n">feed tank to 90 to 95 °C</td><td>if it pays</td><td>3rd measure</td></tr>
       <tr><td>CH-1 to CH-3</td><td>Air-cooled chillers at 40 °C ambient</td><td class="n">3 × 250 kW</td><td>one per press stage</td><td>proposal</td></tr>
       <tr><td>CH-4</td><td>Fourth module for N+1</td><td class="n">250 kW</td><td>option</td><td>RFQ option</td></tr>
       <tr><td>HX-1</td><td>Plate heat exchanger</td><td class="n">~650 kW, ~2 K approach</td><td>launch, sized for 5</td><td>estimate</td></tr>
@@ -150,12 +151,13 @@ td.low{color:var(--warn);font-weight:600}
       <li>Classic: steam, cooling, power, weight; can it run from the central hydraulic plant?</li>
       <li>Press return temperature over the cycle: 53 l/min at ~60 °C out of 20 to 25 °C is ~140 kW, more than the stated 125 kW peak. The real profile sizes HP-1, HX-1 and TC-1.</li>
       <li>Is the accumulator needed with a modular boiler bank, and how big?</li>
-      <li>Steam trap and condensate return at the press: Newbilt's or ours?</li>
+      <li>Condensate: the 2024 offer has one steam/water outlet valve and a steam trap per mould. How much cooling water reaches the trap per cycle, and can that condensate go back to boiler feed?</li>
     </ul>
     <h3>Boilers (Miura, Bangkok Industrial Boilers, STE)</h3>
     <ul>
       <li>2 × 500 kg/h against 3 × 500 kg/h and 1 × 1 t/h: price, footprint, lead/lag control, service from Chiang Mai.</li>
       <li>Fuel: electric, biomass or LPG (sustainability.md, biomass.md); the electric bank draws ~520 to 625 kW.</li>
+      <li>Economizer on a fired bank: outlet feed and stack temperature, corrosion limit, pressure drop; rice husk ash is high in silica and abrasive, so fouling, cleaning, wear protection and a bypass.</li>
       <li>Blowdown cooling to the effluent limit; boiler registration and licensed operator (facility-security.md).</li>
     </ul>
     <h3>Chillers (Trane, Carrier, Daikin Applied, Hitachi)</h3>
@@ -182,11 +184,12 @@ td.low{color:var(--warn);font-weight:600}
       <div><i class="d" style="border-color:var(--prw)"></i>Chiller loop, warm</div>
       <div><i style="border-color:var(--hr)"></i>Heat recovery feedwater loop</div>
       <div><i class="d" style="border-color:var(--bd)"></i>Blowdown, drains</div>
+      <div><i class="d" style="border-color:var(--fl)"></i>Flue gas</div>
       <div><span class="tpl v">°C</span>Temperature, Newbilt figure</div>
       <div><span class="tpl e">°C</span>Design target or estimate</div>
       <div><span class="tpl t">TBD</span>Set by the vendor or by measurement</div>
     </div>
-    <p class="note">Dashed boxes are options. Bow-ties are isolation or control valves (a stem marks a control valve), squares strainers, T a steam trap. Circles are instruments: PT pressure, TT temperature, FT flow; every one is logged to the plant data platform (digital-factory.md). Flow times supply and return temperature on each branch gives that press's cooling load; steam flow against records pressed gives steam per record.</p>
+    <p class="note">Dashed boxes are options. Bow-ties are isolation or control valves (a stem marks a control valve), squares strainers, T a steam trap. Circles are instruments: PT pressure, TT temperature, FT flow, AT conductivity (off-spec condensate goes to the warm tank); every one is logged to the plant data platform (digital-factory.md). Flow times supply and return temperature on each branch gives that press's cooling load; steam flow against records pressed gives steam per record.</p>
   </div>
 </div>
 <script>
@@ -227,9 +230,11 @@ box('B3',780,198,190,64,'B-3 · boiler',['500 kg/h · for N+1','RFQ option'],{op
 box('SEP1',1030,120,170,64,'SEP-1 · separator',['FT-101 · PT-101','dry steam to header']);
 box('BD1',1030,230,170,64,'BD-1 · blowdown',['flash, cooled to drain','safety valves vent out']);
 box('ACC1',1240,40,170,64,'ACC-1 · accumulator',['option (Newbilt)','rides cycle peaks'],{opt:true});
-box('HP1',400,196,170,74,'HP-1 · heat pump',['heat recovery','feed to 90 to 95 °C','from warm press return']);
+box('HP1',400,196,170,74,'HP-1 · heat pump',['3rd measure, if it pays','feed to 90 to 95 °C','from warm press return'],{opt:true});
+box('ECON1',605,196,145,76,'ECON-1 · economizer',['fired boilers only','feed in 90 to 95 °C','out, stack: vendor'],{opt:true});
 pipe('M170,102 H200','rw');pipe('M370,102 H400','fw');pipe('M570,112 H600','fw');
-pipe('M740,112 H760 M760,82 V156 M760,82 H780 M760,156 H780','fw');pipe('M760,156 V230 H780','fw opt');
+pipe('M720,144 V196 M750,234 H765 V82 H780 M765,156 H780','fw');pipe('M765,234 H780','fw opt');pipe('M740,112 H765','fw opt');arrow(720,172,'s','fw');
+pipe('M820,262 V286 H700 V272','fg');lbl(704,300,'flue gas from the boilers');
 pipe('M970,82 H1000 V156 M970,156 H1000 M1000,152 H1030','st');pipe('M970,230 H1000 V156','st opt');
 pipe('M970,104 H985 M970,178 H985 M985,104 V250 H1030','bd');pipe('M1200,262 H1240','bd');lbl(1246,266,'to drain, cooled');
 pipe('M1325,104 V152','st opt');
@@ -249,7 +254,7 @@ PR.forEach(([id,t,s],i)=>{const px=640+i*220,c=id==='C1';
   pipe(`M${px+130},650 V540`,'cws');valve(px+130,600,true);
   pipe(`M${px+160},540 V690`,'cwr');valve(px+160,566,true);bubble(px+180,598,'FT',px+160,598);bubble(px+180,622,'TT',px+160,622);});
 pipe('M620,350 V580','cd');trap(620,372);pipe('M1740,350 V580','cd');trap(1740,372);
-pipe('M1740,580 H590 V135 H570','cd');arrow(1110,580,'w','cd');lbl(642,598,'condensate return to T-1');
+pipe('M1740,580 H590 V135 H570','cd');arrow(1110,580,'w','cd');lbl(642,598,'condensate return to T-1');bubble(566,430,'AT',590,430);
 box('EX1',30,400,180,76,'EX · extruder cooling',['~4 l/min · 2 to 3 kW each','2 EX 50, 2 EX 35, Classic']);
 box('HY1',230,400,180,76,'HY · oil cooler',['central hydraulic plant','~8 l/min per aggregate']);
 pipe('M90,650 V476 M270,650 V476','cws');pipe('M170,476 V690 M350,476 V690','cwr');
@@ -302,14 +307,15 @@ temp(1240,957,'TBD','t');                               // chiller loop return
 const D={
  RW:{t:'RW · Raw water',w:'Mains or well water feeds the boiler make-up and the cooling loop make-up. Which one depends on the site, and the water analysis decides how much treatment both need.',f:['Boiler make-up ~0.75 to 0.9 m³/h at full output before condensate return, far less with it (estimate)','Cooling make-up is small: the chillers are closed, losses are the open return tank and side-stream blowdown','Raw water ~28 to 30 °C: the ground sits at 28 to 30 °C year-round (sustainability.md, estimate)'],q:['Water analysis (hardness, silica, TDS, chloride) for the site, before the boiler and chiller RFQs']},
  WT1:{t:'WT-1 · Boiler feedwater treatment',w:'A duplex softener (one regenerates while the other works) and chemical dosing (oxygen scavenger, pH) protect the boilers. RO only if the analysis demands low TDS; it is an option row, not the baseline.',f:['Duplex softener 120k to 300k THB, sized up for the five-press fleet (utilities CSV, market estimate)','RO skid 200k to 500k THB, option only (utilities CSV)'],q:['Softener alone or softener plus RO, after the water analysis']},
- T1:{t:'T-1 · Feed tank and hotwell',w:'Collects the condensate returning from the presses and the treated make-up, warmed by HP-1, and holds the feedwater at 90 to 95 °C so little oxygen stays dissolved. The boiler then only does the final lift to 180 °C.',f:['Feedwater at 90 to 95 °C and the boiler only doing the final lift (sustainability.md)','Heat recovery plus condensate return save roughly 10 to 15 % of steam energy (sustainability.md)','~1.5 m³, about 1.5 h of feed at full output (estimate); level LT logged','Condensate leaves each trap near saturation (~180 °C at the press) and flashes to about 100 °C in the atmospheric return (estimate); the flash steam is heat that could be recovered'],q:['Atmospheric hotwell or pressurised deaerator: boiler vendor\'s call at this size']},
+ T1:{t:'T-1 · Feed tank and hotwell',w:'Collects the condensate returning from the presses and the treated make-up, warmed by HP-1, and holds the feedwater at 90 to 95 °C so little oxygen stays dissolved. The boiler then only does the final lift to 180 °C.',f:['Feedwater at 90 to 95 °C and the boiler only doing the final lift (sustainability.md)','Heat recovery plus condensate return save roughly 10 to 15 % of steam energy (sustainability.md)','~1.5 m³, about 1.5 h of feed at full output (estimate); level LT logged','Condensate leaves each trap near saturation (~180 °C at the press) and flashes to about 100 °C in the atmospheric return (estimate); the flash steam is heat that could be recovered','Steam and cooling water share the mould channels, with one outlet valve and steam trap per mould (Newbilt offer 240620-04): the condensate is only as clean as the cooling loop, so AT conductivity on the return diverts off-spec condensate to the warm tank (proposal)','Order of heat recovery for the feedwater: flue-gas economizer, condensate return, then HP-1 (sustainability.md)'],q:['Atmospheric hotwell or pressurised deaerator: boiler vendor\'s call at this size']},
  P1:{t:'P-1 · Boiler feed pumps',w:'Duty and standby multistage pumps feed every boiler module through one manifold, so a pump failure never stops steam.',f:['~1 m³/h each against ~13 bar (estimate, above the 10.5 bar boiler pressure)'],q:['Supplied with the boiler bank or separately']},
  boiler:{t:'B-1 and B-2 · Boiler modules, 500 kg/h each',w:'Two modules run lead and lag at ~10.5 bar so the presses see 9.5 to 10.5 bar after the line. B-1 comes with the launch press and covers all of Duplex system 1 (300 to 360 kg/h); B-2 comes with Duplex system 2 and also carries the Classic. Fuel is open: electric, biomass or LPG.',f:['Newbilt: 150 to 180 kg/h per press, Duplex up to 360 kg/h, min. 10 bar, 9.5 to 10.5 bar operating (site requirements 3.2.3, vendor)','Five presses: 750 to 900 kg/h, against 1,000 kg/h installed (111 to 133 %)','The 2024 Newbilt quote excludes a ~400 kg/h steam generator: one Duplex system (pressing-line.md)','Electric alternative ~350 kW input per module, 520 to 625 kW at full demand (0.68 kWh/kg at 98 %, estimate)','With one module down, 500 kg/h runs about three presses','Boiler bank 4.5M to 7M THB installed (utilities CSV, estimate)'],q:['2 × 500 kg/h against 3 × 500 kg/h (B-3) and a single 1 t/h boiler: price, footprint, the cost of a lost shift','Fuel decision (biomass.md, sustainability.md)','Service response from Chiang Mai: the candidates are Bangkok-based']},
+ ECON1:{t:'ECON-1 · Flue-gas economizer (fired boilers)',w:'On a fired boiler bank (biomass such as rice husk, or LPG) the flue gas leaves hot. An economizer between the feed pumps and the boilers recovers part of that heat into the feedwater: the first of the three heat-recovery measures to investigate. An electric boiler has no flue gas and runs on the bypass.',f:['Feedwater in at 90 to 95 °C from T-1 (sustainability.md)','Outlet feed and stack temperatures are vendor figures: how far the gas can be cooled depends on its temperature and the corrosion limit','The gas-fired package row in the utilities CSV already includes an economizer; the biomass row does not yet'],q:['Outlet feed and stack temperature, pressure drop, corrosion limit','Rice husk ash is high in silica and abrasive: fouling, cleaning, wear protection and a bypass']},
  B3:{t:'B-3 · Third boiler module (option)',w:'A third 500 kg/h module keeps all five presses running with any one module out. Without it a boiler fault costs about two presses until it is fixed.',f:['3 × 500 kg/h = 1,500 kg/h, full demand with one module out'],q:['Priced as an option in the boiler RFQ; decided against measured cycle overlap and the cost of a lost shift']},
  BD1:{t:'BD-1 · Blowdown vessel and safety-valve discharge',w:'Boiler blowdown and drains flash in a vessel; the flash steam vents outside, the water is cooled before it reaches the drain. Safety valves discharge to a safe place outside the boiler hall.',f:['Boiler in its own room, Department of Industrial Works rules, licensed boiler operator (facility-security.md)','Detached boiler hall of ~66 m² with a 14 m flue (scale.md)'],q:['Drain temperature and quality against the industrial effluent limit']},
  SEP1:{t:'SEP-1 · Separator and steam metering',w:'Removes entrained water so the moulds get dry steam. FT-101 and PT-101 log steam flow and pressure: steam per record pressed becomes a plant KPI next to the traceability record.',f:['Telegraf/MQTT acquisition into the plant data platform (digital-factory.md)','Saturated steam: Newbilt states 10 bar and 180 °C; 180 °C is saturation at 10 bar absolute, at 10 bar gauge it would be ~184 °C (to confirm which Newbilt means)'],q:['Meter type and turndown for 150 to 1,000 kg/h']},
  ACC1:{t:'ACC-1 · Steam accumulator (option)',w:'Each press draws steam in bursts during its steam phase. An accumulator rides those peaks so the boilers see a steadier load. It does not add capacity: the bank must still cover the hourly demand.',f:['Already a Newbilt option (sustainability.md)'],q:['Needed at all with two modules? Size from the presses\' cycle overlap (Newbilt re-quote)']},
- HP1:{t:'HP-1 · Heat-recovery heat pump',w:'A water-to-water heat pump takes heat from the warm press return before it reaches the chillers and lifts the boiler feedwater to 90 to 95 °C. Each kWh it uses cuts boiler work and chiller load at the same time.',f:['Press return ~60 °C, feedwater 90 to 95 °C, 10 to 15 % steam saving with condensate return (sustainability.md)','Heating all make-up from 20 to 90 °C at full output is only ~70 kW thermal, small against ~650 kW of cooling (estimate)','Source side in up to ~60 °C from the return header, out set by the heat pump vendor; feed loop back to T-1 at up to 95 °C (sustainability.md)'],q:['Real return temperature profile from Newbilt: it sets the heat pump\'s lift and COP','Second use for recovered heat: label oven, HVAC reheat (sustainability.md)']},
+ HP1:{t:'HP-1 · Heat-recovery heat pump',w:'Third of the three heat-recovery measures for the feedwater, after the flue-gas economizer and condensate return. A water-to-water heat pump takes heat from the warm press return before it reaches the chillers and heats the feed tank. It runs only where the tank still needs heat, the press return has enough of it, and the combined saving on boiler fuel and chiller power beats its own electricity. With most condensate returned the remaining tank load is small, so HP-1 may not pay.',f:['Press return ~60 °C, feedwater 90 to 95 °C, 10 to 15 % steam saving with condensate return (sustainability.md)','Heating all make-up from 20 to 90 °C at full output is only ~70 kW thermal, small against ~650 kW of cooling (estimate)','Source side in up to ~60 °C from the return header, out set by the heat pump vendor; feed loop back to T-1 at up to 95 °C (sustainability.md)','About 15 % of the condensate flashes to steam when it drops from ~180 °C to atmospheric pressure (estimate); with the rest returned the feed tank needs only a fraction of the ~70 kW'],q:['Real return temperature profile from Newbilt: it sets the heat pump\'s lift and COP','Second use for recovered heat: label oven, HVAC reheat (sustainability.md)']},
  press:{t:'Duplex presses · HRP 12 (Newbilt)',w:'Each press takes steam from the top of the header through an isolation valve, strainer and control valve, with its own pressure transmitter, and returns condensate through its own trap. Cooling water comes from the 10 bar supply header and drains open to the warm tank; flow and return temperature on each branch give that press\'s cooling load.',f:['Steam 150 to 180 kg/h, min. 10 bar, G 1 connection (site requirements 3.2.3, vendor)','Cooling water up to 100 l/min, ~53 l/min used, in 20 to 25 °C at 10 bar, out max. 2 bar open, average outlet ~60 °C, up to 125 kW (site requirements 3.2.2, vendor)','Water quality 7 to 8 °dH, pH 7 to 9, particles ≤ 200 µm, ≤ 500 µS/cm (site requirements 3.1.2, vendor)'],q:['53 l/min warming from 20 to 25 °C to ~60 °C is ~140 kW, above the stated 125 kW peak: ask for the measured return profile over a cycle']},
  classic:{t:'Classic · automatic press (Newbilt)',w:'Fully automatic, with its own extruder. Newbilt has published no steam, cooling or power figures, so it is drawn and counted as one HRP 12 press plus one extruder.',f:['Toolex Alpha leaflet (the Classic\'s base): 1.2 kg steam and 15 l cooling water per cycle, ~144 kg/h of steam at 2 cycles a minute (fleet calculator, cross-check)','Monitors steam temperature, cooling water temperature and flow; cooling by time or delivered volume (2024 flyer)'],q:['Steam, cooling, power and weight for the re-quote; whether it runs from the central hydraulic plant']},
  EX1:{t:'EX · Extruder cooling',w:'Every extruder needs a little cooling water at the same 20 to 25 °C. The EX 35 and the Classic\'s built-in extruder are counted like the EX 50 until Newbilt states theirs.',f:['~4 l/min, 2 to 3 kW, in 20 to 25 °C at 5 to 10 bar, outlet nearly without pressure (site requirements 3.3.3, vendor)','Five extruders at the start fleet: ~20 l/min, ~15 kW','Outlet ~30 to 35 °C: 2 to 3 kW at 4 l/min is a 7 to 11 K rise (estimate)'],q:['EX 35 and Classic extruder cooling figures']},
diff --git a/docs/sustainability.md b/docs/sustainability.md
index d702fba..76a2955 100644
--- a/docs/sustainability.md
+++ b/docs/sustainability.md
@@ -137,6 +137,21 @@ reusing it before generating anything new:
   (electric, LPG, or biomass), it supplies just the last step to 180 °C /
   10 bar from already-hot feedwater.
 
+**Order to investigate the feedwater heat recovery (October 2026).** For a
+fired boiler (biomass such as rice husk, or LPG): first a flue-gas
+economizer, recovering combustion heat into the boiler feedwater where the
+flue-gas temperature and corrosion limits allow; second, as much clean, hot
+condensate back to the feed tank as possible; third, the feedwater heat
+pump above, where the press cooling return has enough heat and the
+combined heating and cooling economics justify it. Each step shrinks the
+next one's job: with most condensate returned the feed tank needs only a
+fraction of the ~70 kW that heating all make-up would take (estimate), and
+an electric boiler has no flue gas at all. One caution on condensate: the
+press moulds take steam and cooling water through the same channels, with
+one outlet valve and steam trap per mould (Newbilt offer 240620-04), so the
+returned condensate is only as clean as the cooling loop; it is monitored
+and diverted when off-spec ([steam and cooling](steam-cooling.html)).
+
 Two honest exclusions, checked and ruled out:
 
 - **Heat pumps cannot make the final steam**: commercial steam-generating