Central Heating Design Tips

[Malc]

I thought I'd share my central heating design in case anyone finds it is useful. I should add I'm not just blowing my own trumpet here, when the boiler manufacturer's engineer saw the setup he was impressed so much that he shared it with a colleague too.

Picture Key:

B: Gas Boiler (condensing, system boiler)
F: CH Feed tap
G: Gas tap
R: CH return tap
FL1: Fill Loop 1
FL2: Fill Loop 2
CD: Condensate Drain
PV: Pressure (expansion) Vessel
MF: Magnetite Filter
PVI: Pressure Vessel Isolator (service valve)
PVD: Pressure Vessel Drain

Gas boiler (B), isolation taps (F, G, R), Fill loop (FL1), Pressure Vessel (PV) and Magnetite Filter (MF) are pretty standard stuff. PV is on return side so that boiler pump (internal to boiler) increases the pressure so no chance of negative pressure to suck in air (e.g. if PV was on feed side it would be PV - pump head on return side and if this was negative then air could be sucked in, promoting corrosion). Likewise MF is on the return side to remove magnetite before it gets to the pump. And FL1, the normal fill loop is on the return side so the flow is going to be through the pump in the normal dirrection when filling.

The purpose of PVI and PVD is twofold. Firstly, in the event that the PV fails and needs to be replace you can isolate it and just drain off a small amount of water rather than the entire system. Secondly, in order to check the static air pressure in PV you need to drop the CH system side pressure, otherwise you are just measuring the CH pressure. Again using the isolation valve and drain reduces the water you have to lose.

For example, my PV is 12 l, the static pressure is 1 bar, my CH/HW system is ~100 l. I set my cold pressure to 1.2 bar hence my PV air volume is now 10 l (12x1/1.2). When the CH is hot the water expands by a maximum of 4%, e.g. 4 l in my case so air in PV is now 6 l and pressure is 2 bar (12x1/6) which is well within the 3 bar limit for the boiler, the point where pressure relief valve operates.

CD is perhaps a bit unusual. My gas fitter persuaded me to change from having the CD going into a second trap, which is how I originally had it, into a direct connection to the inside sewer. CD is after the the boiler trap of course. The maker's installation instructions allow either type of connection. The benefit of 2 traps is that if the sewer backs up then this floods into the house not inside the boiler. I don't think in my case that is a serious risk, the boiler is high up and there are plenty of lower traps to let any sewage escape should the sewers ever back up to that extent. The benefit of direcly connecting CD is that if the boiler trap dries out or gases leak through the boiler trap (very strong wind directly onto flue perhaps) then the poisonous flue gases doesn't escape into the house but go into the sewer. That seems the higher risk to me which is why the gas fitter persuaded me to redo it.

FL2 functions with FL1 as follows. British standards allow a mains water pressure cold flush of new installations. That's cheaper than paying for power flushing. Hence I isolate the boiler with F and R, then I connect a drain pipe to FL2 and use the filling loop FL1 to drive mains pressure water backwards through the radiators and out of FL2. Of course only one radiator is turned on at a time during the flush so each gets a the full force of the flush. Then by swapping the filling loop to FL2 and the drain pipe to FL1 I can flush in the normal direction. And I can flush the boiler by conecting the filling loop to FL1, the drain to FL2 and turning on F and R. I don't think it would be a good idea to reverse flush the boiler although you could do that too with FL1/FL2. And of course any time in the future the system needs a flush this is easily done with no need for the cost of a power flush.

 

[Madrab]

Not wishing to rain on anyone's parade but unfortunately there are a couple of things on that setup that aren't strictly correct.

 

[Malc]

2 Gas fitter's have seen this and not had a problem, quite the opposite, so why don't you say what you think is wrong and why, rather than just posting "something is wrong but I'm not saying what"?

 

[Madrab]

So why don't you say what you think is wrong and why

OK - normally I don't like to criticise someone else's work but if you wish.

> I don't believe a slotted ISO valve fitted as an isolation point inline on the EV feed, that has the ability to be easily shut down, is best practice on a critical safety device - any valve designed and supplied for that purpose are typically tamperproof/lockshield types, this avoids an accidental isolation of the EV by the client and should only really be used by the service engineer if/when needed.
> Whilst the idea behind 2 access points to allow flushing is a positive one - a standard install should include 2 drain off's as best practice - these would then facilitate the ability to mains flush in both directions - that also being said a properly installed, flushed, treated and serviced system would hopefully not ever have the need to be mains flushed.
> As per their MI - IG specify an open ended connection for the condensate into the waste run - that basically means an air break - ideally that would be facilitated by a tundish or similar transition point
> Whilst not really an issue - shame it wasn't an IG filter that had been installed when the boiler was - it would have added another 2 years onto the warranty

 

[Razor900]

To be honest there is very little there that I haven't seen on thousands of Intergas installs. Almost everyone connects the condensate directly to the discharge pipe. There is actually a built in air break in the boiler trap to facilitate this.

No issue with the EV having an isolation valve and drain off. It's really useful when you need to check the precharge although those penny isolation valves aren't really suitable for the job as they tend to leak when operated a lever valve would be much better.

You would be better leaving the filling loop connected to the flow rather than the return, it suits the Intergas boiler better.

The only real issue I can see is the pipe clips being incorrect for gas pipe usage.

You did make me laugh though as I know for a fact that two of the Intergas direct labour engineers were sweeping up in the warehouse having no experience with gas, plumbing or heating work until their manager put them through the gas course. Most of their current engineers aren't that great which is a shame because the boilers are very good

Oh Yes I almost forgot. I really hope it's running priority hot water or you are wasting so much energy....

 

[Malc]

EV valve is actually a full bore type so a bit better than 10 in bag types. The intergas fitter's preference was for a second trap as he'd seen a backed up sewer fill up a boiler with.... BTW IMO the intergas fitter knew his stuff. My fitter preferred direct because of the potential flue gas escape if the boiler trap dried out. Both arguments have merits.

It's a system boiler not a combi. All the boiler knows is heat demand. External controls select heating or HW cylinder.

The problem with connecting via the flow side is that when you add water you get flow to radiators, i.e. compressing any air in them. If the TRVs are off then that flow is via the lockshields so water flows backwards through the pump which is not a good idea. Hence is better to put it on the return side so whichever way the water wants to flow it will be in the normal pump direction or nothing.

 

[Razor900]

You seem very defensive for someone that put a very simple install up as an example of what to do.

Firstly if you compress air in the rads its easier to bleed them.

In addition the Intergas heat exchanger is a great design how ever if someone tops the boiler up whilst its running you can introduce ambient tap water into a very hot heat exchanger which can cause cracking issues. There is absolutely nothing to worry about if the water flows backwards through the pump.

You mention a second trap. I would expect there isn't one and the flexible pipe connects onto the pipe that goes directly to the soil. You aren't supposed to have two traps in line as you can get an air column trapped between them which airlock the pipe and prevents it draining. Its one of the reasons Intergas introduced an air break into the trap.

Full bore or not those valves have a habit of leaking if not regularly used.

Stating the boiler only knows a heat demand is completely wrong. The boiler has the ability to differentiate between a hot water or a central heating demand.

This means the boiler can be set up with multiple power and temperature outlets or it can use weather compensation to automatically adjust the flow temp for space heating.

Most properties will heat very well with the flow temperature below 60 degrees but hot water reheat would be too long at that temp so when the boiler detects that the hot water needs reheating it shuts off the space heating and puts the flow temp much higher so you get very fast hot water reheat then it switches back to super efficient lower temperature space heating.


I never said the installer didn't know what he was doing its fine apart from the pipe clips just a standard installation - as I said I've seen thousands.

It really makes a massive difference to the overall efficiency. If the boiler is running at a flow temp over 70 degrees it can drop the efficiency down to that of an older band B boiler.
With a few tweaks you could probably save getting on for another 10% on your gas usage

 

[Gasguru]

It's pointless having an isolating valve or drain-off on the vessel.
When checking the vessel pressure just bleed the rads and drop the system pressure to zero on the filling loop valve.
Whenever you have vertical pipe upstands there's never normally a need to drain down the whole system or isolate the boiler/vessel when working on them...the water will happily stay in the rads/pipework with air pressure.
You're just adding reliability issues with those redundant parts.

 

[Malc]

I'm not a bungalow. The water in the upstairs radiators is a pressure so only if I drain all the water out of the upstairs radiators and any tall downstairs rads would the pressure at the pressure vessel be zero. Far easier to have an isolator and then drain the PV IMO. With my tall upstairs rad I have about 3m head or 0.3bar, so not zero and a significant error when trying to measure 1 bar in PV.

 

[Madrab]

The water in the upstairs radiators is a pressure so only if I drain all the water out of the upstairs radiators and any tall downstairs rads would the pressure at the pressure vessel be zero. With my tall upstairs rad I have about 3m head or 0.3bar, so not zero and a significant error when trying to measure 1 bar in PV.

Nope - All that needs to be done is isolate the rads at a valve (count the turns if it's a lockshield) and the system can be de-pressurised @ the loops or mag filter. Even then some systems will de-pressurise (not drain) without the rads being touched. As long as the system pressure @ the boiler drops to 0 then that's all that's required, as the pressure @ the EV will then be 0.

 

[Malc]

Even if I isolate all the rads, which is a lot more work than turning one service valve, I'd still have to drain all the water in the pipes running in the ceiling space above the PV. Surface tension won't hold ~1.5m head of water in an open 22 or 15mm pipe. Any air in the pipe will allow the full water head to be seen at the PV. The only way to be sure you don't have any head of water and that the PV wet side is empty so the air side is at the static pressure is to ensure the water side is empty of water. The way to do that is to have a drain at the PV so you can test that, or empty the whole system. I guess you could fit a pressure gauge point on the water side at the PV and measure it to be zero. But why do that when it is just easier to isolate and drain off the PV? Either way you still have to drain off the PV wet side water so there is no way to save that.

 

[Gasguru]

The water in the upstairs radiators is a pressure so only if I drain all the water out of the upstairs radiators and any tall downstairs rads would the pressure at the pressure vessel be zero....

Try this...
Turn off the boiler.
Bleed out all air from all the radiators & boiler and if necessary top up the system pressure to say 1 bar.
Now isolate the vessel with your ball valve.
Disconnect the loop hose at the cold main valve end and hold the loop facing upwards.
Slowly open the filling loop valve on the return pipework.
Note how much water comes out and what the pressure is with your thumb over the end.

 

[Razor900]

Having an isolation valve and drain off on the EV is good practice. Last week I changed a 600L EV at a hotel, not sure you would want to shut all the emitters and drain things down there...

 

[Madrab]

I must be honest - a commercial 600L vessel - certainly makes sense and only something an experienced engineer would ever interact with - not sure it's the same on an 8L domestic tho TBF.

We were always taught there should never be a means of isolation to/from a key safety device that a client could inadvertently interact with, lockable/tamperproof for the service engineer, potentially yes. For all the effort it is to isolate a depressurise on most sealed domestic systems just to service an EV.

 

[Razor900]

I must be honest - a commercial 600L vessel - certainly - an 8L domestic - not sure that's a fair comparison

We were always taught there should never be a means of isolation to/from a key safety device that a client could inadvertently interact with, lockable/tamperproof for the service engineer, potentially yes. For all the effort it is to isolate a depressurise on most sealed domestic systems just to service an EV.

That is very common advice and it's also wrong I'm afraid.

An expansion vessel is not a safety device in any way shape or form.

If it was than a boiler with a flat internal vessel would be classified as ID. I would be interested to see the paperwork on that one.

Tesla do a couple of nice solutions - the bracket version isn't for potable water as it's steel though:

https://www.screwfix.com/p/tesla-expansion-vessel-mounting-bracket-kit/299RE

https://www.screwfix.com/p/tesla-expansion-vessel-service-valve-3-4-x-3-4-/442fj

It's not a must for domestic as it's generally easy enough to depressurise the system but it's definitely a nice to have as long as it's a decent valve