Technical Blogs


1. Is Being Gas Safe Registered guarantee of carrying out competent repairs?
2. British Gas fixed price repairs - Are they real?
3. What is reverse circulation?
4. Radiator balancing - Balancing a central heating system
5. What should I do if a single radiator is not hot?
6. The pressure drops over a period, is this a problem?
7. Can a modern condensing boiler deliver an over 100% efficiency?
8. What are the pros and cons of a boiler manufacturer fixed price repair?
9. How can I flush a heating system by DIY?

1. Is Being Gas Safe Registered guarantee of carrying out competent repairs?

The answer is no! Being Gas Safe registered is only proved to certain safety standards, and it is nothing to do with boiler repair skills.

Before I registered with Gas Safe in 2009, nobody in my training centres or none of my gas safe inspectors had taught me any on boiler repairs.

One of my fellow boiler repair men said this in a closed forum: "To be able to repair modern boilers a heating engineer should be accomplished in the following area:

  • A degree in electronic engineering.

  • An understanding of how the boiler is actually doing what it does normally.

  • Years of doing my own car maintenance.

  • Common sense.

  • Then ring manufacture technical support if stuck. They are supposed to know their own boilers best and most likely better than we will."

2. British Gas Fixed Price Repairs - Are they real?

Fixed fee repairs have some interesting details in the small print as I found out recently when I didn't fancy repairing a boiler myself.

Not surprisingly, BG came up first at a search offering to sort the problem for £69. Or FROM 69 quid rather. Typing in the requested postcode, raised it to £89. Still a good price, so rang them.

After giving name and address, the girl immediately asked for credit card details. A bit reluctant, but she did give some further info. Yes, £89 was the all in price to sort the boiler out. Yes, parts labour and vat.

“So, £89 will get the boiler repaired, no matter what the problem is?”

After a slight pause, I was informed that if it took more than 30 minutes, the price would go up to £269. Not exactly a huge surprise to me, apart from the 30 minutes. If we reserve a modest 10 minutes for showing the chap the boiler, unpacking and repacking tools and doing the paperwork, that leaves not much more than 20 minutes actual working time before the extra £180 is due. Ouch.

Still, £269 as a maximum to get a bad problem sorted, didn't seem to be too bad. Let's ask, just to be sure: “Right, so £269 will get the boiler back working perfectly, no matter what the problem is?”

Another slight pause. Well, no, if the job took more than 2 hours, the price went up to £529!!!!!

Fook me sideways, that is more than the price of a new budget boiler complete with flue, clock and vat. Still, it is significantly less than a new Bosch or Vaillant, not to mention the install cost.

So I asked the same question again: “Will £529 give me a perfectly working boiler, no matter what the problem is?”

It turns out that there was no guarantee it would.

529 squid would only guarantee to bring the boiler back to work if there was only ONE fault. If after 121 minutes of work it turns out there is a second fault, the clock is reset and the bill jumps up again, presumably to £618 provided the second fault takes less than about 20 minutes to find and resolve.

Admittedly I didn't actually double check that after 31 minutes into the second fault the bill would reach £788, but as she didn't mention anything about second faults being cheaper than first faults, it seems fair to conclude there is no such discount, and it would be indeed 788 notes.

I can't wait for the next time somebody tells me I'm too expensive because BG will do it for £69.

(Copied from bengasman at the Combustion Chamber, Last edited on 28/11/2012)

3. What is reverse circulation?

In summer when you turn off central heating but you still get radiators hot when the boiler is heating the hot water. This is some times caused by "Reverse circulation".

Back flow, reverse circulationHere is the explanation of Reverse Circulation and remedial methods:

Reverse Circulation

A reverse circulation can only happen in the following conditions:

1. It is not going to happen on a combi system, ie, a reverse circulation can only happen in a system which has a cylinders for hot water.

2. It has likely happened when adding some new radiators to an existing system, eg, new radiators added after new loft conversation, so the T-off points are not correct.

4. Radiator Balancing - Balancing a Central Heating System

Radiator balancing or balancing radiators goes by several titles. You'll also hear it called balancing the central heating system. Unless it's referring to a warm air central heating system or underfloor heating it all means the same thing, ensuring that all the radiators get equally hot. Ideally they will all get hot together, in about the same time. If all your radiators get hot pretty evenly you don't need to balance the heating system. Adjusting radiator valves can sometimes make them leak so don't turn them if you don't need to.

Before you try balancing it's important to make sure that the radiators have been bled (that is, all the air trapped at the top of radiators has been released). If some of your radiators are hot at the bottom but cold at the top (even the very top) you need to bleed the radiators before balancing the heating system. If you bleed a lot of air out of a radiator with the system warm, the water entering the radiator to replace the air may make the radiator hot. You'll only really know whether the radiators need balancing if you start from cold.

Also, before attempting to balance radiators, make sure the pump is not set to too low a speed. Try turning it up; it shouldn't pose problems. If it causes your system to pump water over into the feed and expansion tank (known as the F+E tank or the header tank) or causes air to be pulled down the open vent pipe into the system you may have to slow the pump down again. This usually indicates a system design fault or a blockage somewhere.

Balancing radiators is simple, it just takes a little time. There are more complicated methods but the following method works fine for us. We don't use thermometers and we judge radiator temperature by touch.

There are some basic things you need to know to balance radiators but they're simple too. If you're not sure of any of the terms, follow the blue links for an explanation then come back here.

Almost all radiators have 2 radiator valves, generally at the bottom on opposite ends. (Some very old systems may have one at the top and one at the bottom on opposite ends; some newer systems have both valves at the bottom in the middle of the radiator.)

Water enters the radiator through one valve (the flow end or flow valve) and leaves the radiator through the other valve (the return end or valve).

Usually we are restricting a lockshield valve. Different manufacturers' valves turn a different number of times from fully open to fully closed. Some turn only 2½ turns and some as much as 5 or 6 turns. If your valve turns 2½ turns from open to closed, closing it by 80% means 2 full turns closed (or only ½ a turn open). If it turns 5 turns open to closed 80% closed means 4 full turns closed (or only 1 turn open).

If you close a valve on the flow end by 80% or more, the gap left open in the valve is so small that even a small air bubble coming in with the flow can become trapped in the valve and stop the flow. It may sound daft but it happens. If the flow valve is fully open and you restrict on the return valve, any air bubbles come through the flow valve and rise to the top of the radiator, safely out of the way. They don't enter the return valve which is substantially closed so they don't block it and stop the water flowing through the radiator.

The type of valve fitted is not a guarantee of which is the flow and which the return end. To be sure, start with the heating system cold and go round checking which end of the radiator (which pipe or valve on each radiator) gets hot first. Mark the end in some temporary way; a bit of tape will be fine. Sometimes slightly colder water coming along the flow pipe from a different part of the house can deceive you, making the flow end temporarily cooler than the return end. Check again as they warm up to be sure.

You may need to be quick moving round the radiators because a very small radiator connected to a powerful boiler like a combi may get hot too quickly to be sure which end is which. If necessary, check the smaller radiators first. You can check them in any order and if you've marked with tape you'll know you've checked them all.

If a central heating system needs balancing we start by opening all radiator valves fully, both flow and return. Adjusting radiator valves is not without its consequences, it can make the valves leak. If you're not sure how to deal with leaking valves, read that section first.

Different parts of the central heating circuit have different resistance to water flow. Generally, the nearer a radiator is to the boiler, the quicker it will get hot. In a well designed and installed heating system little balancing is needed.

With all the radiator valves now fully open, if all the radiators become similarly hot in a similar time there is not much more to do and your radiator balancing is completed. If not, and "not" is usually the case, we go to the radiators which get hottest quickest and restrict the flow through them. We set no fixed order in which to restrict radiator valves except to do the hotter radiators first. This pushes more flow through the remaining, slower, radiators. Where possible we always restrict the flow on the valve on the return end (the cooler end) and there's a good reason for this. To balance a poorly designed system it may be necessary to close a valve more than 80%, perhaps even 90%. Let me explain what I mean.

Having restricted the return valves on the hottest radiators by 50% or 60% to start with, we wait to see what happens. Cooler radiators will start to get hotter. Some previously cool radiators may get fully hot. If some are still cool we go round again, restricting all the hotter radiators, some which we restricted before get closed down even more (always, if possible, on the return end) and some which weren't restricted first time round are restricted this time because they are now hot. Again we wait to see what effect we've had and again, if necessary, we restrict the hotter radiators. We'll do it yet again if necessary.

Hopefully this process brings the radiator system into balance; you'll only know for sure when you heat the system up again from cold. If you close a valve down too far, that radiator will cool down and may go cold. Just open the restricted valve a little (10% of its total travel between closed and open) and see if that sorts it. Open it progressively more if you need to.

If balancing radiators doesn't resolve the problems, and no matter how you try you still have some cold or very slow radiators, there are two likely causes. First is a failing pump (but make sure someone hasn't partially closed one of the pump isolation valves). Check to see whether you can turn up the pump speed. Second likely cause is sludge build up and partial blockage of pipework or of an air separator. We would always opt to try changing the pump before trying a process like power flushing to clear possibly blocked pipework.

5. What should I do if a single radiator is not hot?

If all other radiator are hot but only one is not, it is probably a problem only related to this particular radiator. You need to do the followings:

  1. Set boiler out put between 80% to 100% of full scale.

  2. Open both valves on each side of the radiator. For lockshied valves, use a spanner if necessary.

  3. For TRV valves, often TRV pins are sticking at closed poistion, you need to free its pin, following YouTube clips (TRV pin stuck). Here is one.

  4. Bleed air out from the inside of a radiator. After bleeding please don't forget to top up some pressure if it is a sealed system.

  5. Balancing the whole system. See above.

Above procedures are for general house holders to complete, otherwise you are expecting to pay min one hour labour to call a plumber.

6. The pressure drops over a period, is this a problem?

In a system with good quality radiator valves and no microscopic leaks it is possible for the system pressure to hold up adequately between annual services. Most people seem to find that they have to top up a few times a year, and some people more often. If you find that you are needing to top up more than once a week it might be worth trying to find the problem or see if the relief valve is leaking and expansion vessel is flat. If topping up annoys you (or it’s awkward) then you might be able to get the system to hold pressure better by adding an internal leak sealer such as Fernox F4. This seals very small leaks but it is worth checking that the boiler manufacturer allows its use especially if the boiler is still under guarantee.

7. Can a modern condensing boiler deliver an over 100% efficiency?

It is impossible! As flue gas temperatures are still higher than ambient air temperatures, some energy is lost through flue gases. You can’t beat law of physics, conservation of energy.

On average, a modern boiler has around 90% overall energy efficiency.

But why do we sometimes get figures saying a boiler’s efficiency is over 100%? That is from a misleading calculation by some boiler manufacturers.

In St Albans, we have another good gas engineer, Ian Braithwaite. He is not only good on boiler repairs, also good on boiler theory. Here is one of his writings about boiler efficiency calculation, which explains the way how an over 100% energy efficiency calculation is achieved.

8. What are the pros and cons of a boiler manufacturer fixed price repair?

Most of boiler manufacturers are offering boiler repair service for their own boilers, by around £300, so we called it as Fixed Price Repair.

The advantages are: 1. They own engineers are competent on their own boilers, they are much better than most of our independent gas operatives. 2. These engineers have most of parts or spares in their vans, so more likely they will fix the boiler by first visit. 3. The price from a manufacturer is capped. Even a part plus labour are exceed £300, they still carry out the repairs agreed.

And the disadvantages are: 1. If you have a simple problem, they will still charge you £300. 2. In winter times it will take days for them to come out. 3. They only repair boilers but not for heating which is outside the boiler.

9. How can I flush a heating system by DIY?

As a boiler repair man I don't often suggest a powerflush as I am able to fix most problem on boilers and heating. Powerflush is very expensive (around £500), but sometimes it is necessary. If one is good on DIY, he or she can always try to flush their heating system by a chemical flush.

Here is a good DIY guide on chemical flush: 

Mobile: 07944 168888 / Email:
Telephone: 01727 537688 / Fax: 01727 537689
Address: 77 Batchwood Drive, St Albans, Herts, AL3 5UF

Copyright© 2009-2018 Herts Boilers. All rights reserved.