When old meets new

MHS Boilers, refurbishment, replacement, plate heat exchanger, PHE
When new meets old — Peter Gammon.

New replacement boilers could easily be damaged if they are directly interfaced with old heating systems. Peter Gammon takes a look at how to approach such retrofit work.

Although the dust is starting to settle on the October 2010 revisions to Part L of the Building Regulations, the efficiency of heating systems used in existing properties is still a hot topic.

A lot of building owners and system designers are looking at renewable energy — given the prospect of the Renewable Heat Incentive — and it may well be possible to introduce some solar thermal panels to help produce domestic hot water.

However the introduction of heat pumps into an existing system is very often impractical, except with and underfloor wet heating system.

In addition, people are considering replacing their existing boiler plant with condensing boilers. Upgrading boiler plant in existing buildings is not without its difficulties because the heating system can often be as old as, or even older than, the boiler that is being replaced.

However, there are a couple of choices when it comes to heating system refurbishment — replacing the whole system and boilers or retrofitting a new boiler to the existing system.

Replacing the whole system is very often cost prohibitive and, if the rest of the building is not being refurbished, can create all sorts of problems with closures due to structural work or removal and replacement of pipe work and heat emitters. The expense of replacing the whole system is not only due to the capital outlay of the new equipment and its installation but also the associated closure of the facilities.

Adding a new boiler to an old system is not without its risks either. It is not that the old system will break down and cause problems because a new high performance boiler has been added — rather the opposite. It is much more likely that the new boiler will be damaged by detritus and deposits from the old system that have accumulated over years.

It is highly likely that an older system will have had its water treated and will in most instances have suffered ongoing corrosion. We often see lots of debris in an old system, particularly in low-velocity parts (at the bottom of heat emitters for example). As the debris has accumulated over a number of years, it may not have caused any problems yet. That said, as soon as you drain down the system, you create the unfortunate situation of letting silt dry into a fine dust. This is not necessarily a problem in older boilers, which tend to have large, low-velocity waterways, so the silt is spread thinly. But, with newer condensing boilers you can have real headaches. Cleansing the old system may not be a realistic option because of the large amounts of chemicals and the immense flushing volumes and velocities needed, along with a full-bore drainage facility.

Allowing silt from the old system to contaminate new boilers may create thermal drift in flue gas temperatures and reduce the efficiency of the boiler. Even a 1 mm-thick deposit can mean a thermal drift of 80 to 90°C which reduces the efficiency of the boiler by more than 4%.

MHS Boilers, refurbishment, replacement, plate heat exchanger, PHE
New high-efficiency boilers could easily suffer damage from debris in old heating systems. A plate heat exchanger provides an effective way of separating new boilers from an old heating system — and can be included within the case of MHS Boilers’ Ultramax PB boilers.

So how do you solve the problem?

The ideal solution is separating the old system from the new boiler plant. The best way of doing this is to use a plate heat exchanger to positive isolate the old from the new, allowing the creation of a brand new mini-environment on the primary side of the heat exchanger onto which the new boiler is installed. Separating the new boiler plant from the old system enables the water to be treated on the boiler side and extend the life of the boiler. It also allows you to use a modern, high-efficiency, sealed and pressurised boiler while the old system can remain open vented and as dirty (and leaky maybe) as it ever was!

For consultants and contractors, using a plate heat exchanger in this way is considered best practice and is now simple to do. For example, at MHS Boilers our high performance Ultramax PB appliance has an option for a built-in plate heat exchanger within the case. This makes installation easy because you do not even have to find additional space in the boiler room. As long as you have a temperature difference between the primary and secondary sides across the heat exchanger, it will work. If the temperature difference is not sufficient, all you need to do is run the new boiler at a slightly higher temperature than the system or marginally reduce the operating temperature of the existing load circuits — or perhaps a bit of both.

 

I believe that using a plate heat exchanger as a separator is not only cost effective but also acts as an insurance policy for the new components of the heating system. You can operate it safe in the knowledge that it should give you many years of hassle-free operation.

Peter Gammon is technical manager with MHS Boilers.

 

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