What is a heat-only boiler? How do they work? What’s the difference between a heat-only boiler and a combi boiler. Or even a system boiler for that matter? This guide will explain everything you need to know what a heat-only boiler is and how they work.
What is a heat-only boiler?
Commonly known as ‘regular boilers’ or ‘conventional boiler’. All three are exactly the same, they’re just different terms.
A heat-only boiler is a unit that does exactly what its name implies. It only creates heat. Now, I know you’re thinking all boilers create heat, don’t they? Well yes, they do. But heat only boilers don’t do anything other than creating heat. Heat only boilers have no moving parts. Whereas other boilers like combi boilers and system boilers have all the moving parts on the inside.
Heat only boilers have all their moving parts external to the boiler. The main two parts are the pump and the zone valves/three-way valves. Also, unlike combi boilers, heat only boilers usually work in conjunction with a hot water cylinder to provide hot water.
How do heat only boilers work?
Feed and expansion tanks
What’s the difference between a combi boiler and a heat-only boiler?
There a quite a few differences between a combi boiler and a heat-only boiler. The main difference being the lack of a hot water cylinder or the feed tanks. Also, combi boilers heat water instantly when there is a demand for it. Unlike heat-only boilers that heat up a hot water tank. You can find out more about combi boilers in our guide – how do combi boilers work?
What’s the difference between a system boiler and a heat-only boiler?
System boilers work in a similar way to regular boilers, heating up a cylinder indirectly via a coil. But there are a few fundamental differences. Unlike regular boilers, systems boilers have the pump inside the unit.
Also, system boilers work on a pressurised system instead of a gravity-fed system. So instead of having a feed and expansion tank in the loft, they have an expansion vessel inside the boiler that deals with expansion. Also, water has to be entered manually through a filling loop.
Although system boilers are mainly installed with an invented cylinder they can be installed with gravity fed hot water cylinders.
Heat only boiler advantages
Can provide lots of hot water
Hot water back up
Works well with low cold water pressure
Unlike combi boilers, gravity-fed systems don’t need good water pressure to work. Due to them being fed by gravity. This makes them perfect for regions where the water pressure is too low for a combi boiler to work.
Heat only boilers have very few parts inside. Added to the external parts like the pump and zone valves, this makes problems relatively easy to diagnose. Unlike combi boilers, there isn’t much that can go wrong with a heat-only boiler system.
Cheaper to replace
When it comes to replacing an old heat only boiler it is a relatively easy job. Due to it being a straight swap, generally, there won’t be very much upheaval caused to the home. Another benefit to this is the installation costs are considerably less than converting the system to a combi system.
Only need a small output
Because heat only boilers provide hot water indirectly instead of directly like a combi boiler. They don’t have to work as hard, and as a result, don’t need a high kW output. The average 3 bedroom home in the UK would need a 25-30kW combi boiler, if this home had a heat-only boiler it would probably only need 15-20kW. As a result, buying heat only boilers is generally much cheaper than buying combi boilers.
Heat only boiler disadvantages
Low water pressure
Due to heat only boiler systems being gravity fed, the water pressure from the cylinder is generally poor. Unless you have mains fed electric showers or a shower pump the flow rate from a shower will be very low.
Takes a lot of space
With heat only boilers needing a cylinder and two cold water tanks, alongside the boiler. They take up a lot of space, especially the cylinder. Usually found in airing cupboards in the bathroom or next to a bedroom their cupboards take up valuable space. In my own heating company, our customers have removed the airing cupboards to create bigger bathrooms or bedrooms once the cylinder has been removed. In a small house, this can make a big difference in how a bedroom or bathroom feels.
Warranty only applies to the boiler
Due to most of the parts being external. The boilers warranty doesn’t cover them. Meaning when things start to go wrong it can get expensive. A new pump or zone valve replacement can cost between £120-£200 so it’s not pocket change.
Not as efficient as combi boilers
Most only heat only boilers are about 60% efficient. Which means for every £1 you spend running it you only the equivalent of 60p’s worth of heat from it. Modern regular boilers are much more efficient. All modern boilers have to be at least 90% efficient. But that doesn’t take into account the cylinder. Generally poorly insulated, gravity-fed cylinder tend to lose a lot of heat.
Ever noticed how warm airing cupboards are? That warmth comes from heat lost from the cylinder. Due to the heat lost, the boiler has to work harder to keep the cylinder warm. And, as a result, costs you more on your energy bill.
Hot water can run out
If a cylinder is undersized, or if a home uses a lot of hot water. Usually ones with teenagers or kids that have long showers or baths. The hot water can run out completely. This leaves you with an unwanted wait for the cylinder to warm up. Not ideal after a long day at work when you’re desperate for a shower.
Heating system corrodes quicker
Constantly filled with aerated water, due to them being gravity fed. The water inside reacts with the steel from the radiators, causing sludge. Aerated water accelerates the corrosion. This means gravity-fed systems deteriorate much faster than the sealed systems used with combi and system boilers. The sludge formed from the corrosion blocks up pipework and can stop entire sections of the system from working. Along with clogging up moving parts like the pump and zone valves. Furthermore, the corrosion can begin to attack the radiators causing small leaks.
Heat only boiler systems are prone to airlocks. Usually caused by leaks or aerated water in the system. Air can get stuck in radiators, pipework or the pump, stopping them from working correctly. Cold spots at the top of radiators are usually caused by air. Released easily through the bleed, this isn’t a big job.
But, airlocks in pipework can be much more difficult to remove. This can be difficult and very time-consuming, as working radiators have to be turned off. Forcing the air to move to a point where it can be released.
Bad selling point
When buying a house, the majority of people look for combi boilers. People see combi boilers as modern and efficient. Unlike their traditional counterparts. Most buyers would see replacing a heat-only boiler as an additional expense they will have to factor into their budget. Which is always offputting.
We hope this guide has helped you understand what a heat-only boiler is and how they work. As well as showing you all the advantages and disadvantages. Regular boilers were a great invention back in the day. But now compared to the more modern combi and system boilers they are largely inferior. If you have any questions regarding heat only boilers or if you feel that we have missed anything out let us know at – firstname.lastname@example.org.