DIY Doctor UK DIY Forums

Hello Doctors!
My first post on this forum, please be gentle.
I have a conventional/ heat only non-condensing boiler which is going to need changing sooner rather than later. My existing boiler has 2 sets of I/O pipes, one pair pumped, for the radiators and one pair gravity operated for the hot water.
I’ve noticed all modern condensing boilers only have one pair of pumped pipes, so how will my current set up have to be modified?
I ask because I will be getting in quotes and want to be forearmed when the CH engineer gives the sharp intake of breath while scratching his head, and tells me how much extra this is going to cost me!
In case you’re wondering why I’m not going for a Combi I have two friends who, like me, have old pipework and after having a Combi fitted suffered at least two leaks each because their pipework was so old (Although not as old as mine), this apparently being a common factor with Combi’s.

Hi royort
As a general guide - from your new boiler there will be a T, one leg will go to the heating circuit and the other will go to the hot water circuit. Both legs will need motorised valves and the control circuitry and controller. Suggest full flow lever ball valves are fitted so the motorised valves can be replaced without a full system drain down.
Regards S

There is no reason why a new system has to be sealed, however the problem is the gravity domestic hot water (DHW) in the main today it is pumped, and temperature controlled.

I have a C Plan in this house (normal plan for gravity hot water) the C Plan has been developed over the years, it started as my house still is, with no control over the temperature of DHW when boiler runs the DHW is heated, this progressed over the years, and a tank thermostat was added so during the summer the boiler only runs when required, but over winter no control, and last stage was to add a motorised valve so DHW fully controlled, however this could only be done with some boilers, as some relied on the thermo syphon to cool boiler when switched off.

The Y plan replaced the C plan and used a three port valve, and like the C plan the boiler could cool on switch off with thermo syphon, then we got the S plan with two motorised valves and a by-pass valve, and the boiler took over some of the control so it could cool.

The combi boiler is really a W plan where DHW gets priority, but also DHW ends up as mains pressure, which is a mixed blessing, in that a thermostatic mixer shower can work well, but unless re-plumbed you have loads of over sized pipes, so it takes an age for hot water to reach the taps.

The boiler must not run out of water, so some where in the boiler there is either a flow switch or pressure switch, clearly with the latter the system needs pressurised, again mixed blessing, a leak with a header tank can go on for years undetected, but with a pressurised sealed system there is a limit to how much water can leak before the boiler stops. Also with a header tank it can absorb air, so you get more corrosion, with a sealed system the inhibiter's used means water is clean even after many years, radiators last longer, etc.

I am sure you can today buy a brand new oil boiler and fit it to your system without a problem, or of course a solid fuel boiler, but with gas the law required that they are condensing and to be condensing they also must modulate, so the whole system is turned on it's head.

The big change is the return water needed to be cool, with the old system hot water could be pumped around and around and as long as the water leaving the boiler was under a set temperature the boiler would run, that is no longer the case, for the latent heat to be used the return water needs to be cool, so to get all rooms warm the flow through each radiators has to be set with a set in and out temperature so on start up all radiators heat up at same rate, and once running each room is independently controlled with a thermostatic radiator valve (TRV) and because they all could close also a by-pass valve, there are a few variations with control some just return water, and some more complex, but this is not really going to work with thermo syphon heating of the domestic hot water, the return water could end up shutting down the boiler.

So if using gas than some plumbing changes will have to happen, it will not be simply swap the boiler, as to what that varies.

I am not a heating engineer, I am an electrical engineer but I have in the past fitted and repaired and altered central heating, and the more you learn the more you realise how little you know.

An engineer in my view is taught over level 3, he has to have a very good command of maths, and also other skills, I looked at mothers old house and tried to get the central heating to work better, it was a combi boiler and condensing so also modulating (that is the flame height varies) so basic idea is the radiators hold as little water as possible, so they can heat and cool fast, so when the sun shines the radiator can stop heating the room so it does not over heat, ideal is the fan assisted radiator, but rather expensive. This again is reverse of old idea of large cast iron radiator that retained the heat so room still heated when boiler turns off, because modern boiler does not turn off it turns up/down.

The TRV needs to act fast but also not too fast it gradually adjusts the flow, the old *123456 on the TRV head is not really fit for the job, we want electronic heads marked in degs C and programmable so vary the rooms temperature through the day.

So looking at a package, the electronic head starts at around £10 for a stand alone, up to £70 for a wifi linked with algorithms to allow fast warm up using geofencing.

Anyway don't even know if you have gas, so I will stop there.

Thanks for the reply. I have to say that it went a little over my head but goes some way to explain this is not going to be an easy install!

We are gas by the way

Sorry, I went a little OTT, point is system working is not the same as working efficiently.

Some boilers have a pressure sensor, and some have flow sensor, it pressure sensor you can't have open system, if flow sensor you can.

So existing boiled can be simply replaced with some models, and it will work, but not really in an efficient way.

So you need to consider how much you pay for installation v how much for fuel and maintenance.

It is easy to say fit top of range Drayton controller and TRV heads, but they are not cheap. And likely up stairs much cheaper heads can be used.

In fact it is possible even old set up can be made cheaper to run.