Boiler Replacement

In that case, there's new pipework for hot water which means the price you originally quoted seems much more reasonable.

Why not use existing pipework?

If the old Potterton isn't a combi then you have a hot water cyclinder. But with a combi hot water comes direct from the boiler. Some existing pipe work for hot water will need altering. See:

I'm not planning to go for a combi boiler.

Even so there may be other changes that are required / worth doing. For use with a cylinder then a system boiler is the obvious choice. That may require converting from a vented system to a sealed one (getting rid of expansion tank in the loft etc, removing the external pump).

Was the existing Potterton fully pumped (i.e. for CH and DHW)? If not, then that needs to be sorted.

Modern boilers will be able to run in condensing mode for most of the DHW recharge - however that also depends on the cylinder being of a fast recovery type such that it can absorb enough boiler power. Older slow recovery cylinders could only absorb power at around 5kW. That means that systems were often configured as Y plan so that the boiler could run both CH and DHW at the same time without efficiency sapping short cycling (however that was often unavoidable in the summer). A modern setup would use W plan or S Plan with a fast recovery cylinder so that the boiler can stuff its full (close to its full) output into the cylinder.

(e.g. My boiler max output is 24kW, the Unistor cylinder can draw up to

OOI what model range of Vaillant did they quote? (i.e. 400 series, or 600)

Modern boilers can modulate down to cater for low demand. Our house is set up as one zone for the hot water cylinder and 8 zones (each of one room) for the heating. So time and temperature can conspire for a single room to require heat. The boiler will then short cycle (except for the living room), but two rooms or the hot water cylinder is enough to prevent short cycling. Although they quote a modulation range of 6:1, I have actually seen the boiler modulate down to almost 7:1, with a little over 2kW.

That heating sounds like it would be very flexible in use.

I would like to have such a thing the next time I change my heating. Was a lot of extra material and work needed for such a configuration?

What sort of programmer are you using to select the rooms for heating?

Haven't gone into that detail at this stage. The current boiler is a 33 year old Potterton, I had a new green cylinder and TRV's fitted last year and the pump was relocated to make it easier to replace. I fitted a Honeywell TR31 which is programmable. The quote is for a straight replacement boiler but allows for a soakaway for condensate, the flue to be fitted higher and a new programmer. Subject to that it will just plumb into the existing system.

Assuming that it's not oversized in the first place. Specify a boiler with too much output and it may not be able to modulate down far enough.

It's not wise to replace a 30 year old boiler with one of the same output unless the heat loss calculation for the property has again been performed. If since the boiler was installed double glazing, cavity and loft insulation added a boiler of a lower output is probably required if it is to be run in the ideal condensing mode.

It seems that it was also common practice years ago to over specify the output of a boiler. A lazy installer may just look at the old boiler and erroneously assume it was correctly sized in the first place or erroneously assume that the house still needs that amount of heating.

Our house is

My boiler has an inhibit to prevent very short cycling. If the on time is short the boiler will keep the pump running but impose an inhibit time before it will allow firing again.

Although they quote a modulation range of 6:1, I

This is one thing I slightly overlooked - my boiler will only modulate around 4:1 and for hot water only just the wrong side of being able to modulate down and stay continuously on. I do however have my boiler set for a 60C flow which gets the water in the cylinder to the low to mid

Unless the soakaway is problematic you're being ripped off. Is there no other way to drain the condensate to internal waste? You can get condensate pumps to pump upwards if that makes things easier.

I put it in in 2003, ready for our first son's birth, so his room could be kept warm independently of the other upstairs rooms and similarly for the living room, as these were the two rooms he would be spending most of his time in.

Each room has a combined timer/thermostat, allowing it to be independently programmed. The timer/stats are battery powered and are switching 12V ac (chosen because I could run alarm cable up door architrave, rather than channelling the walls and having to redecorate and also as I could also use it in the bathroom).

Each timer/stat switches a 12Vac relay, which powers a motorised valve. All the motorised valves' microswitches are paralleled to switch on the pump and request heat from the boiler.

Upstairs, I produced a PCB which holds the 240V/12V safety, isolating transformer, the relays and terminals. Downstairs I just used DIN rail mounted relays, terminals and a transformer.

The hot water cylinder has a mains timer, in series with the cylinder stat, directly driving the motorised valve.

There is also a timer and relay for using the immersion heater in emergencies.

It is an expensive set up these days, but then, my local B&Q were selling off Honeywell motorised valves for £18 each and Maplin were selling timer/stats for £14.

They are just battery operated, combined timer/thermostats, with volt-free relay contacts.

I am currently looking at a change to the system. I have been slowly resizing radiators to allow for lower temperature operation and so have bought a comms unit that will allow me to tell the boiler to run at the lower temperature for room heating, but a higher temperature for the hot water cylinder. It will still link to the same valves and timer/stats though.

Indeed modulation can stop short cycling, although with DHW you really want it to be as fast as possible - especially if you are giving it priority over the CH. You don't want to freeze your nadgers off for several hours while the boiler dribbles a few kW into a cylinder!

That's impressive. Presumably you're running an electrical cable to each motorised valve. Are these valves each close to the rad they serve or are they located together centrally?

I've been pondering something similar, at least for additional emitters.

It's not too hard using an underfloor heating manifold and actuators. On an UFH system you put the manifold centrally and run flow and return to each room, with the return controlled by a 240v or 24v actuator. There's then a room thermostat cabled back to a wiring centre which switches power to the actuator. Effectively it's like a bank of centralised TRVs.

I don't think there's any showstoppers to doing this with radiators instead of UFH. The main thing is you'd have to plumb each radiator back to the manifold - which is doable using flexible plastic pipe, assuming you have space. You couldn't tee off rads from an existing pipe run, without having a valve locally. Although you could fit actuators in place of your TRVs.

Theo

You could, but it makes the wiring far more complex. You really want a room stat that drives the motorised valves locally, and if you need to wire OR them back to a call-for-heat at least there is only a twin cable to run, not a pipe and two wires

The upstairs valves are located in the airing cupboard, together with the pump, hot water cylinder and control box and the downstairs valves are located under the floor, along with the relay box. Effectively, the valves are where a manifold to feed the radiators might normally have been placed.

The valves are close enough to the respective boxes, that their supplied cables are long enough.

Radiators are often fed from and return to manifolds anyway (my parents' system was put in like that in 1972).

I inherited part of an existing system when I bought this house.

Under the downstairs floor, I re-plumbed, so the motorised valves are all together and the returns are all to a manifold.

Upstairs, the returns connect at different points to the return pipe, but all the feeds are brought to the airing cupboard, where the valves are located. The tank and two radiator connections were already in there, two others had to be extended slightly (about 8 feet).

Or, do it properly, and size each radiator correctly for the heat required for that area, then you only need one thermostat, and no valves at all. Weather compensation would be a real benefit in such a set up too. Individual rooms could have lockshields turned off if you want that room colder, but, doing that could actually use more power than if it was left on in that room.