Waste water heat recovery

[Stones]

I can see the benefit of preheating input water into a point of use heater for your hot water, but lets assume you are taking DHW from a cylinder to a thermostatic shower valve. Will raising the temperature of the cold supply have an impact on the operation of the thermostatic mixer? 

[Jeremy Harris]

3 minutes ago, Stones said:

I can see the benefit of preheating input water into a point of use heater for your hot water, but lets assume you are taking DHW from a cylinder to a thermostatic shower valve. Will raising the temperature of the cold supply have an impact on the operation of the thermostatic mixer? 

Yes, it will, so just like we do with the Sunamp PV, you have to arrange for a non-preheated cold supply to be fed to the TMV.  Our system preheats the water to the Sunamp to around 30 to 35 deg C, and this isn't that much different to preheating using waste water in principle.

[PeterW]

4 minutes ago, Stones said:

I can see the benefit of preheating input water into a point of use heater for your hot water, but lets assume you are taking DHW from a cylinder to a thermostatic shower valve. Will raising the temperature of the cold supply have an impact on the operation of the thermostatic mixer? 

Most mixers need 5-10c difference between hot and cold so unless you're getting a big increase in temperature then it's unlikely you will get close to the differential limit 

[Stones]

1 hour ago, JSHarris said:

Yes, it will, so just like we do with the Sunamp PV, you have to arrange for a non-preheated cold supply to be fed to the TMV.  Our system preheats the water to the Sunamp to around 30 to 35 deg C, and this isn't that much different to preheating using waste water in principle.

I understand that.  My question was how the performance of a TMV would be affected given a rise in the temp of a preheated cold supply.

1 hour ago, PeterW said:

Most mixers need 5-10c difference between hot and cold so unless you're getting a big increase in temperature then it's unlikely you will get close to the differential limit 

As Peter says, I suppose all that happens is the valve will take less hot water, and more preheated cold to achieve the output temperature.

Is preheating the cold supply direct to the TMV the best way to it (in applications where you are not using a Sunamp or TS)?  Any reason why you couldn't preheat the cold feed into a UVC to assist in its replenishment as DHW is drawn off?

[Jeremy Harris]

Any pre-heating from waste heat seems to be a good thing to me, and I can't see any downsides at all with the principle.  It's better to pre-heat the really cold incoming water for the hot system, as the bigger the temperature difference the easier it is to transfer heat energy and so the greater the potential benefit.

I can't think of a hot water system where preheating wouldn't work, and preheating the feed to any hot water tank or cylinder is going to give much the same benefit,

[Crofter]

For my own use, I would be looking to use the waste-warmed water right away at the shower, rather than storing it.

Current setup is a TMS so I guess I would be feeding this into the cold side, at header tank pressure.

However I really need to redesign the entire system and in the long term would be looking to feed at mains pressure into either an electric shower or an instantaneous water heater. Jeremy, what sort of input temperature can your Stiebel Eltron handle?

[ProDave]

Probably an even better idea up here. Our drinking water comes from a loch about 600 feet above sea level, so it's the run off from surrounding mountains, and in the spring, snow melt. By the spring, it comes out of the taps so cold I wonder it's not coming out in lumps. One winter I must measure the incoming temperature.

[Jeremy Harris]

1 hour ago, Crofter said:

For my own use, I would be looking to use the waste-warmed water right away at the shower, rather than storing it.

Current setup is a TMS so I guess I would be feeding this into the cold side, at header tank pressure.

However I really need to redesign the entire system and in the long term would be looking to feed at mains pressure into either an electric shower or an instantaneous water heater. Jeremy, what sort of input temperature can your Stiebel Eltron handle?

The lower limit is set by the temperature you want, the flow rate and the incoming temperature of the cold supply, the upper limit is anything that's safe to come out of the hot taps.  If the incoming water to the instant heater is hotter than the set temperature it just doesn't turn the heating element on and lets the water go straight through.  It does appear to come on, as it detects the flow and the light comes on, but if you measure the power being used it's near zero when the water coming in is above the set temperature.

If you want 10 litres/minute for a shower, at 42 deg C, then the incoming supply water has to be at least 29 deg C, below that and the instant heater doesn't have the power to boost the temperature to the set level.  If you lower the flow rate then it will deliver the set temperature down to a lower input temperature. 

It modulates the amount of electrical power it uses depending on the difference in temperature between the incoming and outgoing water, and will always try to heat to whatever the set temperature is.  I have ours set to 42 deg C, as that's just about hot enough to get the shower to work comfortably.  I have the thermostatic mixer on the Sunamp PV set to 45 deg C, so as long as that is delivering water above 42 deg C the instant heater stays off.  The preheat from the ASHP can warm the water to between 30 and 35 deg C, so if the Sunamp PV has exhausted its charge, then for as long as the preheated water is above 29 deg C we can still have shower flow rates at 42 deg C OK.  If the Sunamp PV has exhausted its charge and the preheat tank is cooler than 29 deg C, then we can still get hot water, but the flow rate is lower.  This means we can use all the charge in the Sunamp PV in the morning for showers, but still have hot water for hand washing etc through the day, even if there is no sun to re-charge the Sunamp and we're relying on the backup charge from the time switch in the early hours of the next morning.

[Stones]

On 11/06/2016 at 11:39, jack said:

We have one of these installed, connected to the drains of our two most frequently used showers.  I was slightly concerned that our thermostatic valves (Mike Pro by Crosswater) might have trouble coping with warm water being fed to the cold side, especially as the feed temperature changes over the first couple of minutes, but it's been perfectly fine.  

There are two main types: one with three or four parallel copper pipes spiralling down the outside of a central copper pipe, and another with two concentric pipes such that the incoming cold water passes through the space between them.

You also need to look into connection types.  You can send the pre-heated water to just the shower(s), or tee off after the unit and send the pre-heated water to both the shower(s) and the cold feed to your DHW system.  There are apparently some efficiency gains to be had with the latter system.

These devices are in SAP and I understand some developers throw them in where they need easy SAP points (well, that's what the suppliers suggest!) 

There are question marks over payback, but with four very active people in our house, we seem to take a lot of showers, often in quick succession (the best scenario for heat recovery).  We only have a 250L tank, so my thinking is that we probably get a bit more effective hot water than we would without the recovery unit.  We probably don't save that much money, each year, but I see no reason why the unit shouldn't have a long life, so it'll keep contributing a little bit for many years to come.

Do you mind saying how much you paid for the unit you installed? 

[Crofter]

Thinking about this some more.

I have to have a horizontal run, both on my own house and if I were to fit something like this on the new house. As Steamy points out upthread, this means the water flowing only along the bottom of the pipe, so probably too little contact area for effective heat transfer.

So... fit a simple filter and overflow arrangement. If everything is running clear, then the waste water goes down through the narrowest diameter copper pipe that will do the job (need to work that out of course)- or perhaps multiple pipes in parallel. These are overwrapped with microbore to form the HE. In the inevitable event of a blockage, there is a separate high level waste pipe which can just be normal plastic. Ideally you would have some sort of sensor (float switch?) to let you know when it was backing up, and that is your cue to clean the filter. Or just make the filter easily accessible and remember to check it.

[jack]

4 hours ago, Stones said:

Do you mind saying how much you paid for the unit you installed? 

I can't remember, sorry.  It was the largest Recoup HE+, I think, but I don't believe the installation cost was broken out in the plumber's invoice.