{"id":9967,"date":"2025-12-19T00:00:00","date_gmt":"2025-12-18T23:00:00","guid":{"rendered":"https:\/\/reventongroup.eu\/why-is-the-unit-heater-blowing-cold-air-5-hydraulic-errors-and-how-to-fix-them\/"},"modified":"2026-07-16T23:57:18","modified_gmt":"2026-07-16T21:57:18","slug":"why-is-the-unit-heater-blowing-cold-air-5-hydraulic-errors-and-how-to-fix-them","status":"publish","type":"post","link":"https:\/\/reventongroup.eu\/en\/why-is-the-unit-heater-blowing-cold-air-5-hydraulic-errors-and-how-to-fix-them\/","title":{"rendered":"Why is the unit heater blowing cold air? 5 hydraulic errors and how to fix them"},"content":{"rendered":"\n<p>&#8220;<strong>The new LPHW unit heater is blowing cold!<\/strong>&#8221; \u2013 that is a sentence I hear every winter. The controller shows heating mode, the fan is running at full speed, and the workers on the shop floor are wearing their jackets. Before you pick up the phone to make a warranty claim, read this. In 90% of cases the unit is functioning correctly, and the problem lies in physics and hydraulics.<\/p>\n\n\n\n<div id=\"ez-toc-container\" class=\"ez-toc-v2_0_85 counter-hierarchy ez-toc-counter ez-toc-grey ez-toc-container-direction\">\n<div class=\"ez-toc-title-container\">\n<p class=\"ez-toc-title\" style=\"cursor:inherit\">Table of Contents<\/p>\n<span class=\"ez-toc-title-toggle\"><a href=\"#\" class=\"ez-toc-pull-right ez-toc-btn ez-toc-btn-xs ez-toc-btn-default ez-toc-toggle\" aria-label=\"Toggle Table of Content\"><span class=\"ez-toc-js-icon-con\"><span class=\"\"><span class=\"eztoc-hide\" style=\"display:none;\">Toggle<\/span><span class=\"ez-toc-icon-toggle-span\"><svg style=\"fill: #999;color:#999\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" class=\"list-377408\" width=\"20px\" height=\"20px\" viewBox=\"0 0 24 24\" fill=\"none\"><path d=\"M6 6H4v2h2V6zm14 0H8v2h12V6zM4 11h2v2H4v-2zm16 0H8v2h12v-2zM4 16h2v2H4v-2zm16 0H8v2h12v-2z\" fill=\"currentColor\"><\/path><\/svg><svg style=\"fill: #999;color:#999\" class=\"arrow-unsorted-368013\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"10px\" height=\"10px\" viewBox=\"0 0 24 24\" version=\"1.2\" baseProfile=\"tiny\"><path d=\"M18.2 9.3l-6.2-6.3-6.2 6.3c-.2.2-.3.4-.3.7s.1.5.3.7c.2.2.4.3.7.3h11c.3 0 .5-.1.7-.3.2-.2.3-.5.3-.7s-.1-.5-.3-.7zM5.8 14.7l6.2 6.3 6.2-6.3c.2-.2.3-.5.3-.7s-.1-.5-.3-.7c-.2-.2-.4-.3-.7-.3h-11c-.3 0-.5.1-.7.3-.2.2-.3.5-.3.7s.1.5.3.7z\"\/><\/svg><\/span><\/span><\/span><\/a><\/span><\/div>\n<nav><ul class='ez-toc-list ez-toc-list-level-1 ' ><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-1\" href=\"https:\/\/reventongroup.eu\/en\/why-is-the-unit-heater-blowing-cold-air-5-hydraulic-errors-and-how-to-fix-them\/#Key_conclusions\" >Key conclusions<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-2\" href=\"https:\/\/reventongroup.eu\/en\/why-is-the-unit-heater-blowing-cold-air-5-hydraulic-errors-and-how-to-fix-them\/#LPHW_unit_heaters_require_bleeding_%E2%80%93_have_you_done_this\" >LPHW unit heaters require bleeding \u2013 have you done this?<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-3\" href=\"https:\/\/reventongroup.eu\/en\/why-is-the-unit-heater-blowing-cold-air-5-hydraulic-errors-and-how-to-fix-them\/#Where_did_you_install_the_three-way_valve_The_%E2%80%9CCold_Start%E2%80%9D_problem\" >Where did you install the three-way valve? (The &#8220;Cold Start&#8221; problem)<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-4\" href=\"https:\/\/reventongroup.eu\/en\/why-is-the-unit-heater-blowing-cold-air-5-hydraulic-errors-and-how-to-fix-them\/#Flow_or_return_%E2%80%93_have_you_confused_the_spigots\" >Flow or return \u2013 have you confused the spigots?<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-5\" href=\"https:\/\/reventongroup.eu\/en\/why-is-the-unit-heater-blowing-cold-air-5-hydraulic-errors-and-how-to-fix-them\/#Is_the_circulation_pump_generating_adequate_pressure\" >Is the circulation pump generating adequate pressure?<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-6\" href=\"https:\/\/reventongroup.eu\/en\/why-is-the-unit-heater-blowing-cold-air-5-hydraulic-errors-and-how-to-fix-them\/#Have_you_replaced_the_boiler_with_a_heat_pump_but_kept_the_old_industrial_LPHW_unit_heaters\" >Have you replaced the boiler with a heat pump but kept the old industrial LPHW unit heaters?<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-7\" href=\"https:\/\/reventongroup.eu\/en\/why-is-the-unit-heater-blowing-cold-air-5-hydraulic-errors-and-how-to-fix-them\/#Summary\" >Summary<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-8\" href=\"https:\/\/reventongroup.eu\/en\/why-is-the-unit-heater-blowing-cold-air-5-hydraulic-errors-and-how-to-fix-them\/#FAQ_%E2%80%93_Technical_questions\" >FAQ \u2013 Technical questions<\/a><\/li><\/ul><\/nav><\/div>\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Key_conclusions\"><\/span>Key conclusions<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Air is an insulator:<\/strong> Even a small amount of air in the heat exchanger can reduce heating capacity to zero.<\/li>\n\n\n\n<li><strong>The &#8220;Cold Start&#8221; effect:<\/strong> Incorrect positioning of the three-way valve causes cyclical blasts of cold air.<\/li>\n\n\n\n<li><strong>The Heat Pump trap:<\/strong> Old installation + new heat source (low temperature parameters) = drastic reduction in unit heater output, unless you replace the unit heaters with 2- or 3-row models.<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"LPHW_unit_heaters_require_bleeding_%E2%80%93_have_you_done_this\"><\/span>LPHW unit heaters require bleeding \u2013 have you done this?<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p>This sounds trivial, but it is the <strong>most common installation error<\/strong>. LPHW unit heaters (e.g. <a target=\"_blank\" rel=\"noreferrer noopener\" href=\"https:\/\/reventongroup.eu\/nagrzewnica-wodna-reventon-hc-3s-230v\/\">HC series<\/a>) are installed several metres below the ceiling (typically on pillars in the hall) \u2013 they are often the highest point of the installation. According to the laws of physics, air bubbles rise.<\/p>\n\n\n\n<p>If you bled only the manifold in the boiler room and not the unit itself at 4\u20135 metres height, an &#8220;air pocket&#8221; forms in the heat exchanger. Water will not flow through it.<\/p>\n\n\n\n<p><strong>Symptom<\/strong>: The top of the casing is cold, the bottom is warm (the unit heater is not heating).<\/p>\n\n\n\n<p><strong>Solution for the installer<\/strong>: The installation must be bled using an air vent valve. This should be installed at the highest point of the installation \u2013 in this case, at the upper spigot of the unit heater.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Where_did_you_install_the_three-way_valve_The_%E2%80%9CCold_Start%E2%80%9D_problem\"><\/span>Where did you install the three-way valve? (The &#8220;Cold Start&#8221; problem)<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p>The cold start phenomenon occurs when the flow of hot water through the heat exchanger is cut off by a valve. The hot water, unable to continue flowing, stagnates in the pipe at the valve supplying the heating medium to the <a href=\"https:\/\/reventongroup.eu\/nagrzewnica-wodna-seria-ec\/\">LPHW unit heater<\/a> and cools down. When the valve opens, it is the cooled medium that first enters the unit heater \u2013 this is known as the cold start effect.<br>This phenomenon occurs when <a href=\"https:\/\/reventongroup.eu\/zawor-dwudrogowy-z-silownikiem-hc-3-4%e2%80%b3\/\">two-way valves<\/a> are installed in the pipework supplying the heating medium to the unit heater.<\/p>\n\n\n\n<p><strong>Solution<\/strong>: Use <a href=\"https:\/\/reventongroup.eu\/zawor-trojdrogowy-z-silownikiem-hc-3-4%e2%80%b3\/\">three-way valves<\/a> in a mixing arrangement, or install them so that hot water circulates in a loop immediately adjacent to the unit (via a bypass). Hot water must &#8220;wait&#8221; right next to the unit heater.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Flow_or_return_%E2%80%93_have_you_confused_the_spigots\"><\/span>Flow or return \u2013 have you confused the spigots?<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<figure class=\"wp-block-image size-large\"><img decoding=\"async\" src=\"https:\/\/reventongroup.eu\/wp-content\/uploads\/2025\/12\/instalator-montaz-i-zastosowanie-nagrzewnic-wodnych-1200x650.png\" alt=\"LPHW unit heaters for central heating \u2013 installation and application\" class=\"wp-image-10773\"\/><\/figure>\n\n\n\n<p>Our <a target=\"_blank\" rel=\"noreferrer noopener\" href=\"https:\/\/reventongroup.eu\/en\/lphw-unit-heaters\/\">LPHW unit heaters with heat exchanger<\/a> are designed for a <strong>counter-flow<\/strong> arrangement. This means that the hottest water should meet the outlet air, and the cooled water should meet the inlet air.<\/p>\n\n\n\n<p><strong>Error:<\/strong> Connecting the flow in place of the return (swapping the spigots).<\/p>\n\n\n\n<p><strong>Effect<\/strong>: Reduction in the log mean temperature difference (LMTD) and a decrease in the unit&#8217;s heating capacity of several, and in extreme cases over ten, per cent.<\/p>\n\n\n\n<p><strong>Verification<\/strong>: Check the markings on the unit or in the technical documentation \u2013 the spigots are colour-coded with red caps (flow) and blue caps (return).<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Is_the_circulation_pump_generating_adequate_pressure\"><\/span>Is the circulation pump generating adequate pressure?<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p>A frequent scenario in large halls: the designer specified pipe diameters to the minimum, and the contractor used PEX fittings with significant restriction.<\/p>\n\n\n\n<p><strong>Effect:<\/strong> The hydraulic resistance is so great that the pump is unable to provide the nominal water flow rate (e.g. 4,000 l\/h for a large unit heater).<\/p>\n\n\n\n<p><strong>Diagnostics<\/strong>: Measure the temperature differential (Delta T) between the flow and return with the fan running. If water enters at 70\u00b0C and exits at 30\u00b0C (Delta 40K), the flow rate is drastically too low (water &#8220;stagnates&#8221; in the unit heater and releases all its heat at the beginning). The correct Delta T is typically 10\u201320K.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Have_you_replaced_the_boiler_with_a_heat_pump_but_kept_the_old_industrial_LPHW_unit_heaters\"><\/span>Have you replaced the boiler with a heat pump but kept the old industrial LPHW unit heaters?<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p>This is a critical point for building owners refurbishing their premises. An old gas\/coal boiler delivered water at 80\/60\u00b0C. A heat pump delivers a maximum of 45\/35\u00b0C.<\/p>\n\n\n\n<p><strong>Problem:<\/strong> The output of a unit heater is directly linked to the temperature of the heating medium. A unit heater that had 50 kW on a gas supply may have only 15 kW when connected to a heat pump. That is physics, and it cannot be bypassed.<\/p>\n\n\n\n<p><strong>Symptom:<\/strong> Everything is functioning, there is no air lock, and the unit heater is blowing lukewarm air (approx. 25\u201328\u00b0C), which at the velocity of the airstream is perceived as cold.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Summary\"><\/span>Summary<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p>Bear in mind that LPHW unit heaters with heat exchangers are simple in construction \u2013 if supplied with an adequate quantity of hot water, they must heat. Before reaching for the phone, check and answer the following questions:<\/p>\n\n\n\n<ol class=\"wp-block-list\">\n<li>Has the system been bled at the highest point (directly at the unit heater spigot)?<\/li>\n\n\n\n<li>Is the three-way valve installed so as to eliminate the &#8220;cold start&#8221; effect?<\/li>\n\n\n\n<li>Are the flow and return connected in accordance with the spigot colour coding (counter-flow)?<\/li>\n\n\n\n<li>Are the pipe diameters not throttling the pump (check Delta T)?<\/li>\n\n\n\n<li>Have the unit heaters been recalculated for the new, lower temperature parameters of the heat pump?<\/li>\n<\/ol>\n\n\n\n<p>A correct diagnosis saves time and frustration \u2013 for you and for the client freezing on the shop floor.<\/p>\n\n\n\n<p><\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"FAQ_%E2%80%93_Technical_questions\"><\/span>FAQ \u2013 Technical questions<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p><strong>Q: How can I check whether an LPHW<\/strong> <strong>unit heater<\/strong> <strong>is air-locked without climbing a ladder?<\/strong><\/p>\n\n\n\n<p>A: Touch the flow pipes at the manifold. If the flow pipe is hot but the return is unnaturally cold (much colder than on other circuits) with the valve open, the lack of flow is most likely caused by an air pocket at the highest point.<\/p>\n\n\n\n<p><strong>Q: Can I fill the installation with glycol to prevent freezing?<\/strong><\/p>\n\n\n\n<p>A: Not entirely. Our heat exchangers are not designed to operate on glycol alone. The maximum concentration is 50% glycol and 50% water. Bear in mind, however, that glycol has poorer heat transfer properties than water. The heating capacity of a unit heater using a water-glycol mixture can fall by as much as approximately 15%.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>&#8220;The new LPHW unit heater is blowing cold!&#8221; \u2013 that is a sentence I hear every winter. The controller shows heating mode,&#8230;<\/p>\n","protected":false},"author":2,"featured_media":10098,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_kad_blocks_custom_css":"","_kad_blocks_head_custom_js":"","_kad_blocks_body_custom_js":"","_kad_blocks_footer_custom_js":"","_kad_post_transparent":"","_kad_post_title":"","_kad_post_layout":"","_kad_post_sidebar_id":"","_kad_post_content_style":"","_kad_post_vertical_padding":"","_kad_post_feature":"","_kad_post_feature_position":"","_kad_post_header":false,"_kad_post_footer":false,"_kad_post_classname":"","footnotes":""},"categories":[58,66],"tags":[],"odbiorca":[],"produkt_blog":[],"class_list":["post-9967","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-lphw-unit-heaters","category-guide"],"acf":[],"taxonomy_info":{"category":[{"value":58,"label":"LPHW unit heaters"},{"value":66,"label":"Guide"}]},"featured_image_src_large":["https:\/\/reventongroup.eu\/wp-content\/uploads\/2025\/12\/why-is-your-unit-heater-blowing-cold-air-1024x800.jpg",1024,800,true],"author_info":{"display_name":"S\u0142awomir Bulandra","author_link":"https:\/\/reventongroup.eu\/en\/author\/sbulandra\/"},"comment_info":0,"category_info":[{"term_id":58,"name":"LPHW unit heaters","slug":"lphw-unit-heaters","term_group":0,"term_taxonomy_id":58,"taxonomy":"category","description":"","parent":0,"count":6,"filter":"raw","cat_ID":58,"category_count":6,"category_description":"","cat_name":"LPHW unit heaters","category_nicename":"lphw-unit-heaters","category_parent":0},{"term_id":66,"name":"Guide","slug":"guide","term_group":0,"term_taxonomy_id":66,"taxonomy":"category","description":"","parent":0,"count":10,"filter":"raw","cat_ID":66,"category_count":10,"category_description":"","cat_name":"Guide","category_nicename":"guide","category_parent":0}],"tag_info":false,"_links":{"self":[{"href":"https:\/\/reventongroup.eu\/en\/wp-json\/wp\/v2\/posts\/9967","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/reventongroup.eu\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/reventongroup.eu\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/reventongroup.eu\/en\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/reventongroup.eu\/en\/wp-json\/wp\/v2\/comments?post=9967"}],"version-history":[{"count":3,"href":"https:\/\/reventongroup.eu\/en\/wp-json\/wp\/v2\/posts\/9967\/revisions"}],"predecessor-version":[{"id":10100,"href":"https:\/\/reventongroup.eu\/en\/wp-json\/wp\/v2\/posts\/9967\/revisions\/10100"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/reventongroup.eu\/en\/wp-json\/wp\/v2\/media\/10098"}],"wp:attachment":[{"href":"https:\/\/reventongroup.eu\/en\/wp-json\/wp\/v2\/media?parent=9967"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/reventongroup.eu\/en\/wp-json\/wp\/v2\/categories?post=9967"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/reventongroup.eu\/en\/wp-json\/wp\/v2\/tags?post=9967"},{"taxonomy":"odbiorca","embeddable":true,"href":"https:\/\/reventongroup.eu\/en\/wp-json\/wp\/v2\/odbiorca?post=9967"},{"taxonomy":"produkt_blog","embeddable":true,"href":"https:\/\/reventongroup.eu\/en\/wp-json\/wp\/v2\/produkt_blog?post=9967"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}