"> PDHW Pt 2 - Heating Academy

There are various ways to implement PDHW. Some  manus  get  the boiler to control the DV situated  inside the case just like a combi does. (will talk later about ways of doing PDHW with a boiler that doesn’t control the DV) In fact its often the same combi DV  that you add to a system boiler to make it PDHW. Instead of aiming the DHW at the plate heat exchanger it aims it directly to the coil in the cylinder. These are generally known as 4 pipe systems.  Both the flow to heating & cylinder and returns from the  heating & cylinder start & end at the boiler. (Perfect if the boiler & cylinder are next to each other in a garage plant room or Loft etc.)

Boiler controlled DV are not always sited inside the boiler case – many are external from the boiler (my preferred choice) They just offer a switched live to control the external  DV as & when PDHW calls.  The external DV type are  further  broken down into spring return DV (Honeywell type) and MOMO (Motor On Motor Off)  Valves .  To make things more interesting you can use a standard 3 port DV or a ‘X’ plan (NOT a W Plan) using  NO + NC  zone valve working simultaneously. More about those later. Another way of providing PDHW is via OT controls and OT boilers. In theory any ‘pure’ OT boiler will work with the EPH controls (very competitively priced) and  Evo Home (and possibly others like OT Nest but I have never done one that way) . For boilers that aren’t PDHW ready and are not OT,  PDHW can still be sorted with any remaining boilers that offer WC with the addition of a  £15 5 terminal relay.

The external DV or X plan options are loosely called single or twin pipe systems. These are great for retro fit as you don’t need any extra pipes running anywhere.

The last detail is the cylinder stat. Here there are 2 sorts – the bimetallic strip from the 18th century or the vastly superior and more accurate energy saving communicating NTC.  Ideally the NTC needs to fit in a cylinder pocket in the cylinder, but they seem to work fine just held in place by the old stat elastic tucked under the original cyl stat with a bit of heat conducting paste. PDHW is not really about the hot water and far more about getting the return temp down on the heating to force the boiler into condensing mode.

Even the clunkiest PDHW is better than the best S or Y plan.  Some PDHW are really sexy (ATAG-my fav) and manage to keep the boiler in condensing mode during most of the water heat up time.  PDHW boilers do not have to be expensive – Alpha Vokera & Ideal Logic all offer PDHW but of course there are other criteria to consider.

Recharging the cylinder can also be done in a variety of ways – the most common is via the coil in the cylinder assuming its large enough,  or plate loading a small coiled cylinder (using it like a buffer)  or even using the cylinder with a small unsuitable coil again as a buffer and using the hot water from a combi to deliver direct hot water into the buffer. This method imo is the best and fastest reheat but goes against everything you understand about cylinder delivered hot water lol. This way is how some storage combi’s work.

You can use small coil cylinders but as the eheat time is slow  they need to be timed ‘off’ rather than ‘on’ so it doesn’t interfere with initial house warm up morning or evening.

PDHW can easily be achieved with cascading boilers if required using the lag heating boiler as the lead DHW boiler. More (much) later on that.

Boilers that have been designed from new to take PDHW like V100/200 the WB 4000/8000 and the ATAG to name a few are still seen fitted to S/Y plan. (Aghhhh)   Is like using a racehorse to pull a milk cart. Even so these jobs can be upgraded, all be they a bit clunky rather than the streamlined way  had it been installed PDHW from conception.

It all sounds as complicated as can be, but it really isn’t once u get your head around the concept. Everything (yes everything) is noticeably more efficient  and easier to wire and no different to pipe up. . You only need 4 wires from the airing cupboard to the boiler, namely the 2 wires from the ntc or cyl stat and 2 wires from boiler to DV to operate the valve. Its that simple.

The only advantage (IMO) of 4 pipe is that both branches can not only be range rated according to load, but also the pump pressure balanced to both loads. Most twin pipe systems can be range rated for both loads but the pump on DHW simply ramps up to max. Some (naff) boilers cant be range rated anyway but PDHW is still the best option. The sexy 2 pipers only ramp up to about 15 degrees higher than the real time cylinder temp to keep the boiler in condensing mode as long as possible during re-heat. The more basic models will ramp to max boiler output  (not its range rated value)  & set the pump to max.  Not perfect but still better than any S/Y plan. some  boilers can have the max temp set manually in parameters.

I will aim to break all these different methods down one blog at a time with pics & diagrams.   I certainly don’t know everything about PDHW and there will be other ways – if you know one do speak up. This certainly isn’t a pissing contest.  At the moment everything associated to this will soon be found here.  http://www.heatingacademynorthampton.co.uk/pdhw   I am the Anti Christ of all things IT so do bear with!   I have added a few PDHW vids I have seen. I am not endorsing or criticising them – just trying to get them all in one place.   Do you think a PDHW forum worthwhile?  I think I could just manage that.


Author Kimbo

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