Mixing down water temperatures

[brunstar]

Hey Guys, It has been bugging me to find out about mixing down hot water of about 55 deg c to 35 deg c and what losses there would be in doing that.
In my thinking i would say it would be minimal as you are using less hot water when you are mixing it down but i was wondering if it was less efficient to do so.
I know it would be better to only produce the water in a hydronic circuit to 35 deg c and use that for the under floor loop but if you had a mixed circuit with high temperature radiators and underfloor on the one circuit.
Another thing that could be done is to pass it through the radiators first to reduce the temp of the water through dissipation and not mixing it down, this would be better but then both circuits will have to be in series.

Does anyone know of the losses incurred when mixing down water?
I look forward to your comments.

[Brian_UK]

Are you talking about mixing fresh cold (cool) water with the heated water or simply using a mixing valve to maintain the temperature?

[Stev]

Higher temperatures have higher rates of loss so lower temperatures would have fewer losses but you need larger storage for the same amount of thermal energy.

[brunstar]

i am talking about mixing down the return water to give a constant 30 -35 deg c for underfloor heating so that you can have 2 different flow temperatures running in one circuit, i know it is possible but it would be ideal to split the two circuits but this means double the heat pumps. Just wondering how in efficient this will be?

[Brian_UK]

Make the low temperature underfloor circuit a pumped circuit with a mixing valve connected to the main high temperature circuit.

That way you can have two temperatures running off the one heat pump.

[brunstar]

i was also thinking if it would be more efficient to pump through the radiators first to drop the temp of the water, then mix it down once some of the heat has been dissapated out of the radiators, i think this will be more efficient to do it this way.
what do you think?

[Brian_UK]

It could be but will there be changes to the flow rates when the TRVs close down?

Another option is simple zone pumps such as shown here..
http://www.fluidhandlingreps.com/index.php/the_fhra_network/water_heating_articles/the_passive_injection_loop

[nike123]

i am talking about mixing down the return water to give a constant 30 -35 deg c for underfloor heating so that you can have 2 different flow temperatures running in one circuit, i know it is possible but it would be ideal to split the two circuits but this means double the heat pumps.


There is no need for double heat pumps.
You just need more than one circulation pump.
Check this example with more than one circuit and different flow requirements on only one heat source.

2626

[SteinarN]

Whether you are mixing down the temp or not it is very inefficient to produce 55C water out of a heat pump.
Do you really need that high temp to the radiators?
What control logic is it on the system?

[brunstar]

i know that the COP is not god on 55 deg hot water, but if you are doing a retro fit from boiler to heat pump soe sites require higher temperatures to get sufficient exchange, otherwise low temperature radiators will be used.
timeclock control will be used with weather dependant control.

my main concern is the losses in mixing water down.

I think a stratified buffer tank will do the two different temperatures more efficiently.

[SteinarN]

You dont have any energy loss in mixing down the temp. The only "loss" is the heat pump delivering the energy to the 35C under flor loop at 55C. That "loss" is however considerably.

I suppose the rooms with radiators don't have under flor loops?

[brunstar]

yeah so it will be a mixed circuit, upsatirs radiators and down stairs underfloor, this is the only reason for taking the water flow up that high otherwise it will always be at 30 -35 deg. it is for retro it situations really and i am doing different types of designs.

[desA]

Whether you are mixing down the temp or not it is very inefficient to produce 55C water out of a heat pump.


On that topic. What are typical temperatures folks run from their AWHP (air-to-water heat-pump) internationally?

Over here, in Asia, folks generate hot water up to 65'C, then back-blend with cold water using mixing taps. This takes some of the load off the heat-pump capacity, but, certainly lowers the average COP of the heat-pump.

I've seen German heat-pumps specced on R-407C, with a top-end temperature of 50'C, and others on R-134A at around 65/70'C.

[nike123]

On that topic. What are typical temperatures folks run from their AWHP (air-to-water heat-pump) internationally?


It is usually 45/40°C regime for R407C and fan-coils setup!

[desA]

^ Much obliged. Thanks very much.

What do folks run their R-134a systems at?

[SteinarN]

On that topic. What are typical temperatures folks run from their AWHP (air-to-water heat-pump) internationally?

Over here, in Asia, folks generate hot water up to 65'C, then back-blend with cold water using mixing taps. This takes some of the load off the heat-pump capacity, but, certainly lowers the average COP of the heat-pump.

I've seen German heat-pumps specced on R-407C, with a top-end temperature of 50'C, and others on R-134A at around 65/70'C.

In Norway it is common with under floor loops, often in concrete. Temps may be as low as 30/25C or at most 35/30C.