On a normal S Plan or Y Plan the boiler has one flow setpoint. That setpoint is almost always the cylinder’s — typically 65–75 °C — because the coil will not satisfy a 60 °C cylinder stat if the flow is cooler. Heating then runs at that same high temperature all winter, so a condensing boiler spends most of its life above the dew point. That is the problem PDHW is there to fix.
Priority domestic hot water is for a system or regular boiler and a stored cylinder. The cylinder call takes the full boiler output at a high flow. When the store is satisfied, heating returns at a lower weather-compensated or fixed flow. Output (kW) can also drop with the heating load, instead of sitting at the figure you needed for a combined coil-plus-radiator duty.
What it is not
A combi uses the same idea internally: a diverter throws the heat exchanger onto the plate when a tap opens, then back to heating. That is not what installers mean by PDHW. PDHW is stored hot water — open-vented regular or sealed system boiler — with a separate coil and a second temperature.
S Plan that can heat the cylinder and the radiators at once, on one boiler knob, is not PDHW either. Both 2-ports can be open; the boiler never learns there are two jobs.
Two-pipe, four-pipe, and X-Plan
There is more than one hydraulic way to get two temperatures. The software in the boiler is as important as the valves.
- Two-pipe S/Y with a clever boiler — keep the existing 2-port or 3-port layout if the boiler has a DHW sensor or tank-stat input and can lift flow (and often kW) on a cylinder call, then drop it for CH. Not every make does this. Ideal and Vokèra system boilers, for example, often have no diverter-drive terminals; you can still do a form of priority with valves, but you do not get combi-style internal diverting. Confirm the handbook before you promise dual setpoints.
- Four-pipe — separate flow and return to the cylinder coil and to heating. Some system boilers (Viessmann Vitodens 100-W is the usual example) have an integral diverter like a combi. Cylinder and heating pipes land on the case; external zone valves can come off. DHW is priority. Viessmann’s system wiring notes want a coil of more than about 10 kW so reheats do not drag on. An outdoor sensor is the normal partner so CH can weather-compensate while DHW still gets the high setpoint.
- Intergas X-Plan — two independent target flows (Intergas’s leaflet uses examples such as heating 55 °C for Part L and hot water around 65–70 °C). On a cylinder NTC call the boiler goes to the DHW target at a parameterised output matched to the coil. Heating uses a normally open 22 mm zone valve, not a standard S-Plan normally-closed 2-port. Intergas quote a training-room 175 L unvented with a 30 kW coil recovering 10–60 °C in about 12 minutes — that coil is not a typical domestic spec; size coil and cylinder to the house.
- W Plan — Honeywell V4044C 3-port diverter (not mid-position). Flow is cylinder or heating, never both. Cylinder is the default port; heating only if the cylinder stat is satisfied. That is genuine priority, but it is crude: a cylinder stat set too high, or stuck calling, means no heating. It is uncommon on new work because of that inconvenience. DIY Wiki and Hive’s W-Plan notes cover the same trap.
Y Plan mid-position sharing both ports at once is not priority. Convert it (W, S with cylinder lockout, four-pipe, or X-Plan) if you want dual temperatures.
Temperatures that actually work
Cylinder water: 60 °C at the stat (legionella). Primary flow to the coil: at least about 5 °C above that, or the cylinder never satisfies and the boiler sits on the programmer. Fast reheats use 70–80 °C for that window only.
Heating after the reheat: as low as the emitters allow. Weather compensation or a room sensor is the clean way. Fixed 45–55 °C is the usual retrofit if the radiators were not oversized. Building Regulations Part L now expects new heating systems to be designed around a 55 °C flow; PDHW is how you still get a hot cylinder without running 75 °C through the rads all day. Without dual setpoints see system / regular boiler flow temperature.
Coil rating is the recovery time. Undersized coil + large cylinder = a long priority period and a cooling house. If reheats regularly run past 20–30 minutes, check coil kW, pump, and primary sludge — not just the control wires.
What you gain
- Heating return can sit in the condensing band most of the year.
- Cylinder gets max boiler kW for a short time instead of sharing with radiators.
- The boiler’s heating output can be set to the house load (a 35 kW system boiler might only need ~19 kW on the rads once the cylinder is off the combined duty — that is a real Worcester example from retrofit write-ups, not a design rule).
- A well-rated coil can allow a smaller cylinder because recovery is short.
Heat pumps
A heat pump still runs a dedicated DHW cycle and pauses space heating. That is not gas PDHW. Flow is capped by the refrigerant; copy 80 °C boiler numbers onto it and you will not get them. Coil or plate area has to be for low-temperature heat.
What goes wrong
- Heating 2-port still opening on a HW call — no lockout, no real dual temperature.
- No cylinder sensor / DHW input on the boiler — flow never lifts for reheat (or never drops for CH).
- Fitting a normally-closed S-Plan valve where the maker wants normally-open (X-Plan).
- No pump overrun after DHW — heat left in the exchanger with nowhere useful to go.
- W Plan cylinder stat too high — heating never gets a port.
- Promising PDHW on a boiler that cannot do two setpoints, then leaving one knob at 75 °C.
Gas work and unvented cylinder work stay with the correctly registered person. Use the manufacturer’s pipe and wiring sheet for the actual terminals — this page is the logic, not a substitute for the Intergas X-Plan leaflet or a Viessmann 4-pipe diagram.