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.


  • 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 !!)


  • 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.


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).


Akva Solar plus coils diagram   Akva Solar diagram key

Akva Solar Plus coils + baffle plate     Akva Solar Plus coils






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




Heating System Schematic

Dual Cylinders?


  • 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.

Thermal Scanner

There are a lot of conversations on the GreenBuilding Forum:

That include using a thermal scanner for during and post build use of thermal scanners to check for thermal efficiency (leaks, bridging, U-values).

Wait for the heating to be working, so that it’s warmer inside than outside by 8C or more (so probably winter !) and start using a scanner.

From inside, to look in all directions (floor, walls, ceiling) and from as many external aspects as you can. You could find where (for example) insulation in the wall has perhaps sagged and left a less insulated section.

Yes you’ll possibly find problems when it’s too late (ie not during the build) but better late than never, as you may still be able to improve where these problems are.
– if it’s during the build, but post final hand over, you can get the builder(s) in to sort out the problems.

It’d also be interesting to 2, 3, 5, 10, 20 years on to do the same and see how the building has held up.
– yes it’d be good to also get an air test several years in.

Cost Implication / Problem

One problem with this plan is that an air test is about £300 at the moment. That’s a lot, unless you believe there is a big reduction in the building efficiency and you want to check, to confirm (and if the case) resolve what has failed over time.

FLIR Thermal Image cameras start at around £1,000 and what training / learning do you need to use one properly?


FLIR i3 / i5 / i7 camera model comparison


 FLIR i3 FLIR i5 FLIR i7
Thermal image quality:
60×60 pixels
Thermal image quality:
100×100 pixels
Thermal image quality:
140×140 pixels
Field of View:
12.5°(H) x 12.5°(V)
Field of View:
21°(H) x 21°(V)
Field of View:
29°(H) x 29°(V)
Center spot Center spot Spotmeter, area with max./min. temperature, isotherm above/below
Thermal sensitivity: 0.15°C Thermal sensitivity: 0.10°C Thermal sensitivity: 0.10°C

Hot water heat recovery

What device(s) to put where to recapture as much of the heat from waste / grey water needs a decision.

The solutions from are clearly all very efficient and appear to be the same or similar to those that are well used in the US, where a lot of properties have their heating systems in the basement.

They are based on an “instant” transfer of the waste water heat to the mixer in the shower and also to the cold water feed to the water heating system.

BUT, the planned house will have clothes washing machines, a dishwasher and 2 showers on the ground floor. Being on the ground floor they wouldn’t work with all the recoupenergy solutions. Also a washing machine, dishwasher, bath (or hot tub) generates the waste water, some time after the hot water tank has been re-filled with mains cold water.

So in those circumstances, the Heat Squirrel (scroll to the bottom) could be better and could provide a single (so a lot cheaper) whole house solution for all waste / grey hot water heat recovery. They are about £399 (not installed). The heat squirrel has a 120 litre capacity.

A key consideration / idea will be:

Can the waste water input be regulated so that only waste water that is warmer than the water in the heat squirrel is let in to it?

It seems that for a shower, the recoupenergy solutions will be the most efficient, but for the whole house, and the total cost, a single heat squirrel could be better than a heat squirrel and one or more recoupenergy solutions.

Heat Squirrel Schematic

Heat Squirrel - schematic
Heat Squirrel - installed

Net Zero Vs Passivhaus


 in Passivhaus, you want to put in so much insulation and such high quality windows that you barely need to heat or cool at all, minimizing energy use; In Net Zero, you want to generate enough power on site to heat, cool and light your house; it could be a draughty barn,

OK, with the proviso that the cost (actual and environmental) of creating an insulated and well sealed house is sensible, this gives a long term resilient property, that should be liveable  even if the power generation systems fail.

I’m with the bias being a super insulated and sealed house first and then minimum tech to supply on-site power (elec and heating).

Cold water tap temperature

I was pondering the temperature of the cold water tap!

To get hot water, a domestic hot water system needs to take the cold water up to around 50 degrees C.
– I know there are also considerations to periodically take it above 60 degrees C to prevent the Legionella bacterium growing in the water system.

Looking at the chart below from


Seasonal variations in domestic hot water energy requirements


I assumed that the mains water pipes were deep enough to cause less seasonal variation in the output temperatures, between 4°C in the winter and up to 20°C in the summer.

I’m not sure why the graph has a lower summer temperature for domestic hot water (DHW).


Initial SAP Calc’s look great

Although the project is a long way off being ready for a full and final SAP Calculation (SAP background and definition below) I wanted to put the planned building, with the current details, past an experienced SAP consultant to see where it comes out.

This meant that for a lot of figures, there are assumed values, which are a lot worse than the planned values.

For example the SAP calc’s used a figure of 4.0m³/m²/hr at 50Pa.

  • The aim is a target air permeability of 1m³/m²/hr at 50Pa.
    (Current UK Building Regulation Standard is 10m³/m²/hr at 50Pa.)

Even with these default values, the house exceeds current UK requirements.

The initial value has come out as 74 and an emissions (t/year) figure of 5.53.
Adding in the planned 4KW PV system gives figures of 82 and 3.71 (respectively).

That is an annual saving of 1.82 tonnes of carbon. This highlights the level of carbon created at generation (power plants) and the transmittal loss to point of use, PV removes this element and has such a positive affect in reducing CO2.

The SAP consultant (Grant Williams, Tel: 01249 650051) was also able to give his general feedback:

The thermal values issued by ARCO2 are as good as I get to see in all of my clients ….

You seem to be on top of the project with regards to the air tightness this is very refreshing as a vast number rely on mastic post construction.

Also you are very aware of the importance of the thermal bridges I have entered a standard figure for this at a Y Value of 0.08 however if someone has completed the calculation I can alter this at a later date.

The final rating will be improve upon just by issuing the make and model of the Ventilation, Boiler and Closed Log Burner as I have used default figures which are at the bottom of the required efficiency to pass the Compliance.

Standard Assessment Procedure (SAP) calculations show details of the Target Emission Rate (TER) versus the Dwelling Emmission Rate (DER).

To be comparable for all properties, the SAP rating is adjusted to floor area, so that it is essentially independent of dwelling size. It takes into account the following range of factors, which contribute to energy consumption:

  • Thermal insulation of the building fabric.
  • Efficiency and control of the heating system.
  • Ventilation characteristics of the dwelling
  • Solar gain characteristics of the dwelling
  • The fuel used for space and water heating

SAP has been adopted by government as part of the UK national standard for calculating the energy performance of buildings.

Every new house has to have a SAP rating.  It provides a simple means of reliably estimating the energy efficiency performance of your home.
SAP ratings are expressed on a scale of 1 to 100 – the higher the number, the better the rating. Thus it is similar to the fuel consumption of a car under standard driving conditions. SAP is calculated by a procedure which is specified in Building Regulations, and which predicts heating and hot water costs.These costs depend on the insulation and air tightness of the house, and on the efficiency and control of the heating system. The calculation uses the Building Research Establishment‘s Domestic Energy Model (BREDEM).
The ecofab panels and general construction has:
  • Floor, walls & roof are all the same box panels with 0.12 w/m2K U-Value (or better)
  • Air permeability target is below 1 m³/(h.m²) @ 50 Pa

SAP calculations (Air leakage, U-values & thermal bridging)

As we head towards sending in the planning permission (we’ve had 2 positive pre-planning meetings), the design has been sent for a preliminary SAP analysis.

A fundamental objective is to create a thermally efficient building, so that over it’s lifetime, the amount of energy to keep the interior at a comfortable temperature and humidity more than offsets the cost (money and environmental cost) to achieve this efficiency.

In crude financial terms, the cost of heating an uninsulated house is nearly three times that of heating a modern well insulated property of the same living area.

Heated buildings loose energy in 3 ways:

  1. Air leakage through holes (hence an airtest and an “air-tight” building).
  2. Through the fabric of the building. The u-values of the materials measure how much heat is lost through them. This is primarily the walls, floors, windows, doors and roof of the building.
    The lower the U-value, the better that section of the structure. For example, a wall with a U-value of 1.0 will lose heat twice as fast as a wall with a U-value of 0.5.
  3. Through the cold bridges between the different elements. These are the Ψ (psi) values.
    – “Thermal bridging occurs where the insulation layer is penetrated by a material with a relatively high thermal conductivity.”

The SAP assessor will look at all of these. They will multiply the Ψ (psi) values by the total length of their construction in the building to get a y-value. The y-value is analogous to an aggregated u-value for all the junctions in the building.

What y-values are used in the SAP calculations can have a big impact on the end figure.


  1. SAP assessor can use default value of 0.15, or
  2. Calculated value using the Ψ (psi) values listed by the Building Regulations for Accredited Details (normally 0.8 or higher, or
  3. A calculated value using thermally modelled junction Ψ (psi) values, which can come out as low as 0.04 depending upon construction details used.
The difference can, apparently be the equivalent of an open garage door on the side of the building ! (best to worst).

Exhaust air into a hot water heat pump ?

I’ve been sent some info by Cernunnos Homes (“Renewable Energy specialists for the domestic & commercial sector.”) in praise of the  ESP Ecocent system. Peter one of the Cernunnos founders has put this into his own house:

If you not decided on a hot water system – take a look at the ESP Ecocent.

No RHI (exhaust air source is not considered green)

Can be integrated with RegaVent MVHR system (so in the summer the Ecocent can cool the house by recycling the air from which heat extracted back through the ventilation system).

When I got back to them about this meaning that the MVHR system isn’t re-directing captured warmth from air being expelled from the house to the cold air being pulled back into the house, they replied:

“…. normal MVHR is transfering heat out to  heat in.
However in the summer you want heat out and cold in, which is what the Ecocent delivers by cooling the air via the compressor.
In the winter we can either bypass (so extract and expel from the outside and leave the MVHR as a traditional system) however normally people either shower in the morning (before they go out to work), or in the evening (before they go out to socialise) or at end of night (when you want to let the temperature fall in the house). At these points (when the bathroom is over heated) the MVHR then kicks more heat back into the house when one would naturally be comfortable with it not recovering the heat to recycle to space heating, but there is a demand for water heating. So whilst either use (space or water) for the outgoing heat is an efficient use of that heat it can be argued that the water heating (with very low heat loss) is a slightly more efficient use of that heat! It is energy efficiency at the extremes, ie being efficient in the most efficient manner possible!

Carbon Neutral Woodstove!


Some models are carbon neutral. European woodstoves using the Nordic Ecolabelsystem are so efficient that the carbon emitted when they’re used properly is equal to the carbon a tree naturally emits while decomposing in the forest.