Combi boiler versus hot water tank

Back in April I posted about How to Save on Hot Water and I promised to post again about heating losses from hot water systems. Losses are considerable - at least 20% and up to half the total hot water energy. There are big differences between combi boiler and regular boiler systems regarding how much of the hot water energy losses are usable as heat gains. It turns out that if you have a well insulated hot water tank then you are likely to save energy by having a regular boiler rather than a combi with similar efficiency rating, even though with a combi you only heat the water you use, more or less. Also if you have a hot water tank you can take advantage of solar hot water panels (see Optimising your solar water heating).


It is extremely difficult to measure in an actual house how much energy is used for water heating and how much for space heating because your boiler does both and it doesn't tell you how much gas it uses for each. Theoretically you can measure the temperature and flow rate in water pipes leaving and entering the boiler but there aren't any consumer products available which do this. The numbers I use in this post come from a model based on the government's Standard Assessment Procedure for dwellings (SAP). This is only an approximation but hopefully the assumptions in the model are broadly based in reality.

The SAP model considers 4 types of heat loss from hot water supply systems:

• Heat lost from pipes which feed taps around the house - the distribution loss
• Heat lost from the hot water tank if you have one - the storage loss
• Heat lost from the pipes going from the boiler to the hot water tank - the primary circuit loss
• Combi loss from combi boilers due mainly to losses on starting and stopping

Distribution loss.

When you start running a tap, first of all you get water which has been sitting in the pipes getting cold. Those pipes are not usually insulated at all. The amount of heat loss depends on how much you use, compared to the run of the pipe. In my house I get 4 litres of cold/warm water from the kitchen tap before it runs hot. This cold water was hot once but it's been sitting in the pipes too long. If I only use 4 litres at a time, I run through 8 litres to get 4 litres of hot so overall I have lost 50% of the heat. However, that is a particularly bad case. SAP assumes the distribution loss is 15% of water usage. For a 2-person household with 'average' water use this would be about 200 kWh/year.

Storage loss

The storage loss depends only on your tank, not how much water you use. In practice it depends on

• the size of your tank (larger tanks have a larger surface area so lose more heat)
• the temperature of the water in your tank (the hotter the water the more the losses)
• the insulation you have around your tank

The insulation makes a huge difference and modern boilers all come with thick foam insulation which is much better than loose jackets. With 50mm of foam insulation around a 140 litre tank your storage loss would be about 440 kWh/year. The same thickness in a loose jacket would more than double the losses - to 960 kWh/year. That means starting from 60 C and leaving the heating off, within 24 hours the water would be only just warm enough to shower in. The tank with 50mm foam insulation would have cooled by only 7.5 C.

Primary circuit loss

You are recommended to insulate the pipes between the boiler and the hot water tank. If you have this done then SAP estimates the loss at 360 kWh/year. Otherwise it will be nearly double: 610 kWh/year. In practice the losses would also depend on factors such as how you set the timer for heating your tank. When the tank isn't being heated there is no flow in these pipes so much less heat loss.

Combi loss

Every time you turn on the hot tap the boiler has to start the burners going and warm up the heat exchangers so that the water is heated properly. Then when you turn the tap off it takes a moment for the boiler to notice this and to shut off. It clears out the hot gas from the combustion chamber (for safety reasons) and the heat exchanger cools down. Some boilers are better at this than others but in the absence of specific test data SAP assumes combi loss comes to 600 kWh/year. In practice it will depend on how often you turn the taps on and off.

Keep hot facilities

Some combi boilers have a keep hot facility which is effectively a small hot water tank. The losses from that should be small however, since the tank is so small.

Allowing for heat gains

Based on what I have said so far you would think that even in the best case the combi boiler was better than the regular boiler because the combi loss - 600 kWh/year - is lower than the losses with a hot water tank.  In my example the storage loss plus primary circuit loss (440 + 360) is 800 kWh/year even in the well insulated case. However, the heat lost from the water tank and pipework mostly warms up the house whereas the combi loss mostly goes up the boiler flue to outside. In the SAP model, 80% of the storage loss and primary circuit loss counts towards heat gains whereas only 25% of the combi loss is retained. In practice you don't need those heat gains all the year round, and in any case they may not be where you want the heat so let's suppose that 2/3 of the heat gains are actually useful for space heating. That gives you a net loss from the hot water tank of 370 kWh/year while the net combi loss is 500 kWh/year. In the end the combi boiler uses more heating energy than the regular boiler system in the well insulated case.

If your hot water cylinder and primary circuit pipes are not very well insulated then your losses will be much greater and since you don't need that heat in summer overall you lose out very considerably. The 140 litre tank with 50mm loose jacket plus an uninsulated primary circuit gives you a whopping 1570 kWh/year loss and even if 2/3 of the heat gains are usable you have still wasted 730 kWh/year - considerably more than the combi case.


These calculations are summarised in the table below.

Heat losses in kWh/year	Regular boiler with well insulated tank	Combi boiler	Regular boiler with poorly insulated tank
Losses	800	600	1570
Usable gains factor	80% x 2/3	25% x 2/3	80% x 2/3
Usable gains	430	100	840
Net loss	370	500	730

The distribution loss is the same for the combi boiler and the regular boiler case so it isn't relevant to deciding which is better. Also it depends a lot more on your habits so I would not trust the SAP model very much. However, SAP thinks that 80% of the distribution loss can be counted as heat gains which means that the 200kWh/year loss for the 2 person calculation gives a net loss of only 90 kWh/year. By the way, the hot water demand would be about 1300 kWh/year, of which SAP assumes that 25% counts towards heat gains (think of the steam rising from your bath or shower) so the net heat requirement for actual water is about 1100 kWh/year.



Differences in water usage between regular and combi boilers

A survey conducted by the EST in 2006 found that households with a combi boiler tend to use more water than those with a regular boiler, especially from the kitchen tap. The average for combi boiler households was 20% more than the average for households with a regular boiler. However, the water delivered by the combi boilers was not quite so hot - 3C lower on average - which partly makes up for the extra water use. It may be that combi people use more partly because of the lower temperature - after all, in your mixer shower and bath if the water isn't so hot you simply mix it with less cold. However, it may also be because of the length of time, and hence water flow, that it takes for the combi boiler to come on properly when you open a tap.

What you can do to reduce your losses.

If you have a hot water tank:

• Insulate the primary circuit pipes.
• If you only have a loose jacket think about getting a new tank with proper foam insulation
• Try reducing the thermostat level on the tank so it is not so hot (but don't go below 60C because you could end up growing bugs like Legionnaires' disease.
• Try reducing the heating time so you only heat the water just before you need it to be hot.

If you have a combi boiler you need to avoid turning the hot taps on and off so often so:

• Only use the hot tap if you really want hot water.
• If you are washing up, run a bowl full of water for rinsing rather than continually turning the tap on and off
• From a comment on 'Getting the best from your condensing boiler' if you turn the hot tap down to a trickle you can fool the boiler into thinking you have turned it off, allowing you to draw off another few litres of hot from the pipes, albeit slowly.

That's the theory, but it is hard to prove in practice because it is so difficult to monitor hot water energy usage. Especially in the winter time, variations in space heating demand due to the weather will swamp the savings you are likely to make on hot water.

References
The Government's Standard Assessment Procedure for Energy Rating of Dwellings (BRE, 2010)
Measurement of Hot Water Consumption in Dwellings (EST, 2008)
Minimising boiler short cycling losses US Dept. of Energy