This is Part 1 of a series of PDHW articles I am writing. This is a general background.
Putting a brand new, high efficiency boiler onto an S or Y plans system is like putting triple glazed windows into a rotten wooden frame. All of the performance benefits of the new window are lost as cold air enters via the gaps around it. Likewise, all of the efficiency potential of a new condensing boiler to run the heating system really efficiently, by reducing the flow temperature through the year, is lost because the boiler is fixed at 70-75ºC in order to maintain the cylinder temperature.
S&Y plan systems are a longstanding barrier to condensing boilers actually condensing and should have been regulated out in 2005 or at least in the years that followed. 17 years on, it seems almost negligent to sell a customer an A-rated boiler that is probably only ever D or E rated in practice.
There is in fact a far more efficient way to set up boilers that work with a hot water cylinder, which is no more difficult to install & wire up than S or Y plan systems, and that is ‘PDHW’ or what has also become known as ‘X-plan’. So what is PDHW? It stands for Priority Domestic Hot Water. Just to be clear, it is NOT a W plan (a W plan is a junk thing on a heat store running at 80 degrees from the 1970’s). PDHW has a confusing name as it’s not really anything much to do with hot water but far more importantly the ability to run a boiler at two different flow temps, one high to recharge the cylinder and the other as low as possible for the heating. This is to maximise the boilers ability to condense for the majority of time it is on in heating mode.
Standard SY&W plans all run at 75-80 degrees so very seldom if ever (apart from warm up & cool down) harvest any of this free energy that’s there for us to take – if we can just be bothered to make the simple changes to ‘how it’s always been done’. With the temperatures separated, PDHW heating can be controlled by WC or load compensation to really push % efficiencies into the mid 90s.
But there are other benefits. S&Y plans do both heating & hot water at the same time. This means the boiler needs to be sized for both (lets say 8kW for heating & 12kW for the cylinder). So a total of 20kW. A 28Kw boiler will still be fitted – because “that’s what we do”- But sizing the boiler in this way usually leads to excessive un needed unwanted cycling. Like a combi boiler, PDHW will only do one or the other. In this example, the bigger load is 12kW for hot water so the boiler can be sized for 12kW. This means much smaller boilers (in this case 12Kw) are needed so no gas pipe upgrades.
PDHW cylinders need larger coils to reheat faster, so the house doesn’t lose temp during a long cylinder reheat. With advanced heating controls in place to modulate the boiler’s output, PDHW is best left on 24hrs a day. As hot water is drawn off, the cylinder is being reheated at the same time.
It is also possible to fit a boiler sized for rapid hot water recover on high demand systems , say 30kW for high hot water demand households, providing the boiler can modulate down for the heating system. Big output boilers and large coil cylinders provide really really fast reheat times. Boilers are sized in the same manner as before – looking at the largest demand (nearly always DHW) – the cylinder can be smaller. So a house with 3 showers would still need only a 120ish cylinder but with a larger coil (say 30kW) and a boiler to match. If a 30kW coil cylinder is over budget the old cylinder can be plate loaded with a 30kW plate, a one way valve & WRAS pump. Most UV cylinders already have suitable sized coils.
Cylinders with 18th Century archaic mechanical bimetallic strip on off stats can still be used, but ntc controlled ones are far better & more economical if your boiler will recognise it. Any boiler with WC can be ‘tweeked’ to run PDHW using a simple 5 wire £15 relay, any OT boiler controlling either S or Y can be simply upgraded with no pipework using standard EPH controls.
Most manufacturers’ boilers will offer PDHW (anything sold on the continent), but sadly most Manu’s won’t promote it in the UK. Most reps have no idea what you are asking about. Some (large) companies refuse to offer any help on it (ffs!). To install you only need 4 wires from the airing cupboard to the boiler – 2 wires for NTC or cyl stat and 2 for the Diverter valve or X plan. On an unvented cylinder the HLS does not need to switch off the entire boiler; it simply immobilises the HW with heating still working. Cables from HLS to boiler not required.
Some boilers use internal DV (WB & Viessmann to name a couple) some use external (more flexible IMO) like Alpha, Intergas & ATAG. Even cheap boilers like the Logic offer PDHW. The Vokera range all offer PDHW via EPH controls, so the original S plan valves are not replaced, just rewired. Y plans can be upgraded using the same valve but rewired to be a diverter valve.
Its crazy to replace any boiler & cylinder to wire it up to a cold war S plan. PDHW is the same price, easier to wire up, gives better longer lasting hot water from a smaller cylinder & boiler and most importantly will allow the boiler to run in condensing mode nearly all the time in heating mode (assuming correct balancing).
Now if you add to this – a boiler that has: the lowest modulation, is range rateable, has a pressure settable pump (or better still a Dt controlled pump), OT control, and hopefully Drayton TRV4 type pressure independent TRV on all the rads, and with a bit of guidance on commissioning & balancing you have a very very efficient boiler for the next 15+ years. If the rads are oversized (they almost certainly will be) the max flow temp can be permanently lowered, further forcing the boiler further into condensing mode.
There is great and almost free software about to help with calculating heat loss (If you don’t do this its all guesswork) Look at Heat Engineer. https://www.heat-engineer.com/
There are suppliers who specialise in PDHW & will ensure you order all the right stuff & offer support ie. Richard Burrows (Mid Wales Plumbing & Heating) https://mwphs.co.uk/
Do ask your rep if your current boiler of choice has a Dt controlled pump. (Always good for a laugh). You will no doubt be told “it’s a fully automatic ERP pump” ummmmm- no.
We have a duty of care to help save the planet. Sadly most Boiler manu’s are not in the slightest bit interested in anything other than profit- by selling more new stuff. It’s down to us to make the change to install or upgrade more efficient systems & demand better from the manu’s. New home owners face a complete re-pipe & all new rads if they go down the ASHP route ( thanks to all the greedy house builders failing to future proof. ) New builds being signed off today are the same , all piped up in 10mm & handkerchief size rads.
This Blog hasn’t been very ‘technical specific’ – the next ones will explain the various different types of PDHW the different applications of it and hints tips work arounds & wiring on the various models. Hopefully others will share their experiences & knowledge.
The HHIC who recommend change & proposed new regulations don’t seem to care. Maybe they have no grandchildren – but I do – and maybe you do too?
As a footnote to this, the new Ciphe low temperature qualification (presumably to support the new low temperature regulations) doesn’t mention PDHW and still only refers cylinder systems running 75°C ?
