Waste Water Heat Recovery for a Combi Boiler Okay?

a1234555

Looking to do some home improvements (extension) and exploring ways I can save in the long term. Thinking to install Waste Water Heat Recovery for a shower and bath but not sure whether it's useful since we have a combi boiler. Anyone know if these work just as well with Combi boilers does it make more sense to have these with a traditional central heating water tank?

tacpot12

 There is no reason why a Combi boiler should not be able to benefit from the recovered heat, but Waste Water Heat Recovery systems works best when they are close to the boiler (so that the recovered heat isn't lost as it is transferred to the boiler). Insulating the cold feed between the recovery system and the boiler would be sensible. Having the boiler close to the shower/bath is also helpful. 

The further the recovery system is from the boiler and the longer the hot water has to travel back to the shwoer/bath, the less benefit you will see. I would imaging that the savings would be negligible if there are long distances between the recovery system and the boiler. 

FreeBear

During the summer months, I am noticing a considerable reduction in gas consumption (20% ?) when heating water via the combi boiler. This is down to the fact that the incoming cold water is some 4-5°C warmer. I can see there is a potential saving to be had, particularly in winter when using a combi with a shower (there would also be a saving to be had with an electric shower). But if you regularly use a bath, there will be no savings to be had.

Some rough back of fag packet calculations and assuming a 10kW shower - A 10 minute shower with heat recovery might save 25% and return a saving of 400Wh. That is about 10p based on a cost of 25p/kWh. A WWHR unit is going to cost something in the region of £1000, so it is going to take a very, very long time to see any return on investment. Using gas to heat the water, the RoI is going to be four times longer..

If a WWHR unit could be used to heat air as part of a ventilation system, there would be greater savings to be had especially if you use a bath. Even better if the waste heat could be stored and released when needed - Such a system would be complex and difficult to retrofit in many buildings. Personally, I'd target the £1K+ cost at other areas of heat loss within a building such as doors, windows, and walls.

casper_gutman

There are various ways to fit waste water heat recovery units. If you use it to preheat the cold feed to the boiler, then your shower and boiler ideally need to be close together as @tacpot12 says. You can also install units to preheat the cold water feed to a mixer shower, the idea being that your thermostatic shower would then use less hot water from the combi boiler to provide the same temperature and flow rate to the shower. So, and the boiler will then moderate down and use less gas. With this sort of setup I can't see why it should matter if the shower is located at longer distance from the boiler.

The bigger issue is probably the return on investment. Some units can be bought for about £600 (see e.g. Waste Water Heat Recovery Systems (WWHRS) | Shower Heat Recovery UK | City Plumbing), but that's not including fitting which is likely to take the cost much closer to the £1k @FreeBear mentioned. I'm guessing your money would probably be better spent on improving insulation, or solar panels if you don't have those yet.

GDB2222

If you have a “grey” waste water tank within the house, you can recover around half the heat energy in the shower water simply as room heating. You then end up with room temperature water, at around 20c that you can use for washing the car or watering the plants.