Communal DHW generation is decided in two steps: the distribution architecture first, the generator second. The architecture sets the permanent losses, and they are heavy: the SOCOL 2021 sheet records a ratio between distribution losses and DHW demand of between 0.6 and 1.5, with working defaults of 0.6 for new buildings and 1 for existing ones. On an older building, in other words, the network consumes as much as the showers. Choosing the generator never recovers a badly set architecture.
The four communal DHW architectures
What the architecture decides, and what it does not
The architecture sets the length of pipework held at temperature, and therefore the permanent losses, the number of generators to maintain and how finely you can meter. It sets neither the demand, which is calculated separately, nor the cost per kWh, which depends on the fuel. Demand itself is determined with the method developed in calculating DHW demand for a multi-occupancy building.
The comparison table of the four schemes
| Architecture | Where generation sits | Length held at temperature | Metering available | Watch points |
|---|---|---|---|---|
| Centralised | A single plant room | Full risers and secondary return | Building level, or per dwelling with sub-meters | Maximum return losses, balancing essential |
| Sub-station | One per block or per core | Limited to the zone served | Per sub-station | More plant rooms and more service contracts |
| Per riser | One generator per riser | The riser alone | Natural, per riser | Access inside ducts, repeated maintenance |
| In-flat | One generator per dwelling | Close to nil | Direct, per dwelling | Electrical or gas capacity per unit, maintenance devolved |
Moving down the table cuts permanent losses and increases the number of service points. That is the central trade-off, and it is settled on lengths surveyed in the ducts, not on a preference.
The requirement that locks the choice
Once the volume of water held between generation and the furthest outlet is more than a few litres, holding temperature across the whole distribution system becomes unavoidable, which in practice means a secondary return or trace heating. It is that held volume, not the length in metres, that triggers the requirement. Establishing it means reconstructing diameters and lengths, not estimating by eye.
Choosing the generator once the architecture is set
Gas or biomass plant with a plate heat exchanger
A plant room feeding a store through an exchanger absorbs peaks without an oversized store and is easy to control finely. It brings flue requirements, periodic inspections and, for biomass, a fuel supply chain and a fuel store. It remains the simplest solution to operate on buildings with high simultaneity, where the morning peak dictates everything.
A heat pump dedicated to DHW
A DHW heat pump only holds its performance if the generation temperature stays low. The hygiene constraint pushes the other way, and peak reheats push harder still. The arrangement that works has the heat pump preheat to a moderate temperature, then hands the finishing and the temperature boosts to a backup sized on the peak. Check the output available at design outdoor temperature, not the rated output.
Solar thermal as preheat
Solar input is preheat: there is always a backup to reach the setpoint. Solar cuts the bill without ever guaranteeing the service, which makes it a complement to an architecture rather than a standalone system. Its sizing follows its own ratios, developed in communal solar DHW, and in particular the rule that the collector array is set on the low end of the demand range.
The secondary return, the main loss centre
Costing the losses before installing anything
The SOCOL 2021 sheet suggests using, in the absence of measurement, a distribution energy to DHW demand ratio of 0.6 for a new building and 1 for an existing one, the observed range running from 0.6 to 1.5. On an existing building the measurement is quick: loop flow and return temperatures, circulation rate, and the energy lost follows. These losses are systematically underestimated at design stage, because the loop accumulates connections and therefore thermal bridges.
The levers that actually work
Three levers matter, in this order: cutting the length held at temperature, insulating the entire circuit including valves and fittings, and balancing the riser returns with proper regulating devices. Circulator control comes after, and never compensates for bare pipework. The orders of magnitude and the levers are set out in DHW secondary return.
The measurement trap on hot water metering
A slow, badly set or leaking mixing valve, or a backup running too hot combined with an excessive loop flow, causes heavy cold water ingress at the mixer during draws. The hot water meter is then bypassed on part of the draws, up to 60% of them according to the SOCOL sheet. An abnormally low demand measured on a building should therefore raise suspicion about the mixing arrangement before it is taken at face value.
Temperatures, hygiene and operation
The table of temperatures to hold
| Point in the installation | Value | Nature |
|---|---|---|
| Bath outlets in new dwellings | 48°C | maximum |
| Other outlets, after thermostatic mixing | 43 to 46°C typical | maximum |
| Hot water leaving the store | 60°C | minimum |
| Return leg of the secondary circuit | 50°C | minimum |
| Temperature reached at outlets within one minute | 50°C | minimum |
The most frequent confusion on site is reading 50°C as a target at the tap. It is a minimum across the network and a maximum at the outlet once mixed. The two are reconciled by terminal mixing, never by lowering generation.
Commissioning and recorded settings
At commissioning, record the generation setpoint, the loop return temperature, the temperature at the least favourable outlet before and after mixing, the circulation rate and the setting of each balancing device. Record the thermal treatment cycle and its frequency. That document is what separates a compliant installation from one merely asserted to be compliant. The devices themselves are covered in legionella safety devices.
What the service contract has to cover
The contract has to name the readings expected and their frequency, not only the operations. Flow and return temperatures, circulator running hours, pressure, anode condition, safety group check, descaling according to the measured hardness. A contract that produces no series of measurements cannot detect a drift, and leaves the building owner with nothing to rely on in a dispute.
Costing and defending the project
What the costing has to contain
Supply and installation price is not the cost. Add the surveys, the plant room adaptation, the strip-out and disposal, the rework of the secondary return and its insulation, the metering, the commissioning and the balancing. On the operating side, set a consumption assumption, an energy price and a replacement budget for the short-lived components: circulators, controls, exchangers.
Metering as an argument, not an option
Metering turns a promise into a verifiable commitment. A heat meter on generation and sub-meters close to the risers let you demonstrate the saving after the works and apportion charges without argument, the same logic collective heating applies through heat cost allocators and their accuracy limits. Installation and use are covered in thermal energy metering.
Presenting the case with Argile
What carries a decision is a costed, dated, comparable file. With Argile you prepare several work plans from the same survey, with distribution losses made explicit, the available funding built into the costing and the net cost per unit. You present argued cost differences rather than a single quotation, and that is what triggers the decision.
