Hot water heat recovery via the Thermal Store?

Instead of, or in addition to a heat squirrelhow about sending the domestic hot water (DHW) waste via a coil in the thermal store?

It should be easy to have a thermostat that checks if hot water being “thrown away” is above the temperature of the lower section of the thermal store, and if so, sends the water through a heating coil in the thermal store, so it passes over some of it’s heat before it goes down the drain.

From www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=8005

  • water leaving the shower head at 42 deg C will have cooled to around 37 deg C by the time it gets to the drain.
  • The 5 deg heat loss will already have been recovered to the house (assuming effective MVHR on the shower room exhaust).
  • The heat squirrel holds 120 litres of hot waste water, to pre-heat any incoming cold water that is heading to the heat store.
  • By holding the hot water, there is more time for the heat transfer to any incoming cold water, especially any that sits in the coil inside the heat squirrel.
  • Hopefully this 120 litres of water is regularly self flushing as “grey water very quickly turns manky when stored, even for short periods.”

Hot Tub Hot Water Heat Recovery

One key thing here, is my plan to have a log fired hot tub.

  • I’m looking at hot tubs that use logs to get the water up to temp and then keep the temp there.
  • BUT to accelerate getting it up to temp after it has been left a while, I want the option to top it up with hot water from the domestic hot water supply.
    • Reading elsewhere, hot tubs tend to run around 36° degrees.  Although friends with hot tubs say they tend to run theirs between 37° degrees and 39° degrees.
  • I also want the option of taking water from the hot tub, when it’s not going to be used for a while and sending this via a heat recovery option back to the heat store.
    • I accept that sending “lumps” of less than 120 litres at a time will optimise this in terms of giving the water time to transfer it’s heat to the heat store / cold water coming in to the house.

Thermal Store

This is part grabbing historical Thermal Store notes to one page, and part adding new notes.

Click here for notes on Thermal storage – pros & cons.

Looking at the Akvaterm thermal stores (which can go up to 5,000 litres !!)

From www.stovesonline.co.uk/wood_burning_stoves/Akvaterm-Geo-Thermal-Stores.html:

  • have a stratification baffle plate about a third of the way down the tank. (see photo below). This can be optionally upgraded to an insulated baffle to further improve performance but the added benefit is not huge as it is very good already.
  • Once the water above the baffle has been heated to a high temperature by the heat pump (50ºC-60ºC) it then shifts to heating the bottom two thirds of the tank to a much lower temperature suitable for underfloor heating (around 40ºC).
  • OR if / when the heat pump is generating lower temp water (often more efficient COP) it only targets the bottom section.
  • larger than normal lower domestic hot water coil. This is to ensure that the incoming mains water picks up as much heat from the bottom of the tank which holds the ‘cheaper’ heat produced at a high C.O.P.

From www.ecoangus.co.uk/Akvaterm_Solar_Plus_Accumulator_Tanks.html:

The AKVAir Solar Plus is available from 300-2000 litres and is 3 bar pressure rated. The tank has 4 coils, two for solar input and two for domestic hot water (DHW) and is divided by a baffle plate, approximately 60% below and 40% above the baffle. Each section contains one solar and one DHW coil and all coils are positioned vertically.

The AKVAir Solar is available from 300-2000 litres and is 3 bar pressure rated. The tank has 3 coils, one for solar input and two for domestic hot water (DHW).

From www.accumulatortanks.co.uk/Solarplus.htm:

Akva Solar plus coils diagram   Akva Solar diagram key

Akva Solar Plus coils + baffle plate     Akva Solar Plus coils

 

From www.akvaterm.fi/eng/Accumulators/AKVA_SOLAR.41.html:

akvasolarplus_460

 

From www.akvaterm.fi/eng/Accumulators/AKVA_GEO.206.html:

  • AKVA GEO is suited to all heat sources (others seem to be solar or something specific).
  • example layout:

akvageo_solar_kaavio_459

 

From www.navitron.org.uk/forum/index.php?topic=14183.0:

Heating System Schematic

Dual Cylinders?

From www.chelmerheating.co.uk/dual_cylinder_thermal_store_systems.html:

  • For larger domestic and commercial projects, our dedicated heating buffer cylinders are used in conjunction with our high-gain unvented cylinders to allow greater variation between heating and hot water demand.
  • The separate low-temperature heating buffer allows the small, infrequent heating demands of a property that is “up to temperature” to be met by stored renewable energy before activating the heat pump/boiler to reduce wasteful on/off cycling.

Thermodynamic Panels & a heat store or heat recovery system

Looking at one of the Thermodynamic Panel system PDFs:

One set up has a thermal store (a tank that heats up, and your heating is delivered by coils that go into this store, heat up and take that hot water to where you want it), and a second, linked to a pool seems to have a form of heat recover system, in that the colder water from the pool is going back in the loop for re-heating via the Thermodynamic panels and the thermal store tank.

 

For the thermal store the Akvaterm thermal store water tanks looked good at the 2012 Eco Expo in London.

  •  The Akvaterm Akvantti thermal stores are oblong which could be a better shape for the plant room. It’s available as 1400lt, 2000lt or 2400lt volumes. The 1,400 litre unit is £3,757.00 + £85 carriage.

Akvantti-Accumulator-Heat-Store-Tanks-4

 

A chunk more information on the concept and benefits of a thermal store (and their version of one) at http://www.greenspec.co.uk/thermal-storage.php:

Thermal storage – pros & cons

+ Provides effective buffering
+ Reduces boiler cycling
+ Allows for integration with low temp heating systems eg underfloor
+ Adds mains pressure to hot showers
+ Provides potable hot water
+ The use of a heat exchanger means that in most cases, thermal stores can be integrated with existing pressurised boiler circuits
+ Requires much smaller cold water tank then standard vented systems
+ Thermal storage is recognised by NHER software
– Heat can be lost through inefficient heat exchangers
– Storage temperature will usually have to be 10 deg C higher than required DHW temperature
– Cannot be used with existing DHW power showers and pumps
– Expensive and unvented storage, very expensive
– Vented stores require a header tank to be located above the heating systems

Points to consider when specifying a Thermal Store

  • The design of the heating system should be matched to the calculated peak heat load.
  • When including solar heating, ensure that there is extra capacity within the store to accommodate fluctuations.
  • Where a biomass boiler is being used, consider sizing the store to provide for the heat capacity generated in a load / firing
  • Consider designing not only for short-term anticipated capacity but possible future extensions to the system.
  • Consider stratification of water temperatures within the store, particularly where low-grade heating is provided. Effective separation between the hot water at the top of the tank and the cooler water at the bottom, can increase the time between charges.
  • Ensure that there is adequate insulation to the store (100mm + PU foam)
  • Ensure that there is adequate pipework insulation