MVHR with Heat Pump Recovery - Reduced Defrost

[Pincher]

I have been looking at a loft MVHR installation, but can't find quite what I want. Please have a look at the design I attached, and tell me if a heat pump packaged like this exists?

I would like to avoid getting a solar thermal panel as it doesn't seem to be that reliable as a source of hot water. Solar thermal panel is just too expensive for an intermittent hot water source. With the heat pump, I can heat hot water any time I want to, as a reliable backup should the gas boiler breaks down.
When not in use, the heat pump is switched off, so we just have a simple MVHR set up.

The differential temperature sensor works out which airflow is warmer, and make the coolant either go clockwise or anti-clockwise.


Winter

The stale air coming out of the MVHR still has some heat in it, and is warmer than the incoming fresh air.
The coolant goes clockwise, picking up the residual heat, release a little bit of it to pre-heat the incoming air, then gives up the heat energy in the heat pump, which we use to heat hot water, or even supply the underfloor heating.

If the coolant makes the outgoing stale air colder than incoming fresh air, the differential temperature sensor will reverse the flow, to extract heat from the incoming air instead, this action may even defrost the stale air radiator. When the ambient air is very cold, defrost is inevitable, but the extra two or three degrees of the airsource we are using instead of the outside air means defrost happens less often.

Summer

The incoming airflow is hot, maybe 27 degrees, ideal for airsource heating of hotwater. The blue evaporator circuit flows anti-clockwise, picking up heat from the fresh air radiator, then the stale air radiator. Note that the coolant has also pre-cooled the incoming fresh air, before it gets to the MVHR, so it acts like an air-conditioner, but not a powerful one.

By having more baths and showers, the hot water cylinder is refilled with cold water, which means we can run the heat pump as an air chiller when it's a hot day.

An alternative arrangment is to use a diverter valve to choose whichever airflow is warmer in winter, and extract heater from it. When that one freezes, the temperature sensor switches to the other radiator, while the first one defrosts, maybe by a heating element, so we don't have to wait for the defrost.

[Pincher]

I can't seem to attach the .jpg file of the design.

[Pincher]

Trying to attach the .jpg , which seems to have uploaded.

[mad fridgie]

I do not want to pour water on your dreams, but I must ask one question first, how big is your house, I presume you are looking a 1 air change every 2 hours, if so calculate the energy your are hoping to extract (remember thet your heat recovery but be doing most the work)

[Pincher]

how big is your house, I presume you are looking a 1 air change every 2 hours, if so calculate the energy your are hoping to extract (remember thet your heat recovery but be doing most the work)

Thanks for replying.

It's a two storey bungalow, 5 bedrooms, 2 shower room, one en-suite.
The main house excluding garage and kitchen (not heat recovered) is
11m x 11m x 6m (W x D x H) = 726m3: the volume to be recovered.
I was therefore looking at:
Xpelair Xcell 350V EC BP

Typical performance(m3/h) low/medium/high 100/270/360
　
I would like to have the heat pump in front of the MVHR because it can be used as an air chiller, as well as further extract the heat from outgoing stale air, which should reduce defrost time. Note that this is NOT using air inside the loft, which will create -10 degree conditions.
The hallway ceiling is only 5.4m high, and the void above is swimming pool shaped,
4m x 5m (W x D). I can stand straight up in most of the void, on the floor boards above the joists. I have made an access channel from the first floor cylinder room (1.3m x 1.8m W x D ) into the loft. The access channel has space for extra flow and return pipes from the heat pump in the loft, as well as an insulated air duct up to 200mm in diameter, for extracting moist/stale air from the en-suite and 1st floor shower room, which are on either side of the cylinder room.
The first floor cylinder is just under the roof, so a solar panel can be put in by going straight up. When I upgrade this cylinder, it will be solar compatible, just in case, but the solar coil will not be used, so the heat pump can be connected.
The gas boiler is in a lean to behind the house, with a Megaflo cylinder. When the house has lots of people, the Megaflo will supply the kitchen and en-suite, and the solar cylinder will supply the two shower rooms and the rest of the house. With three showers going simultaneously, the 30kW gas boiler will not cope, so the heat pump will improve hot water temperature recovery time: I just have to switch it on. During low occupancy, the solar cylinder will not be heated, and a diverter valve will disconnect it, and connect the Megaflow instead.
The first floor is being converted to wet under floor heating, and the manifold will be in the cylinder room as well, which is essentially plumbing central. So hooking up the heat pump to the manifold is really easy.

So the idea is to get the most out of the MVHR installation, by incorporating a heat pump which gives air chill, predictable (vs. solar) hot water heating , and back up for the gas boiler.

The position of the void is between two solid walls, and is already heavily insulated against heat loss any way, so a vibration damping mounting is all that is required. For the Xpelair Xcell 350V EC BP:

Noise(dB(A)@3m) low/medium/high 10/30/36

Condensation will be pumped into the no longer in use old radiator loft expanion tank, which has an overflow.

[mad fridgie]

I understand what you are trying to achieve, but you have a very small airflow, thus very little energy to play with. (both for cooling and heat recovery)

[frank]

I can see a couple of problems with this set up.

The airflow on the MVHR unit is 0.1m3/s (360m3/hr) at high speed, so we can assume that this will be the volume of fresh or stale air passing over the cooling coils in the ductwork. If the cooling coils are to change direction (i.e. 1 in use at any one time) then they would need to be on a water circuit, not refrigerant, for obvious reasons, so a plate HE would be needed.

Assuming a 6K dt across the cooling coil, the energy absorbed would be 1.032kW. If the heat pump had a COP of 3:1 in heating then the total heat energy available for DHW heating would be 1.376kW.

To heat a 300ltr DHW cylinder from say, 12C incoming to 55C in 30 minutes,(so the heat pump will improve hot water temperature recovery time) would take 30.03kW - not really any improvement in recovery time.

One other thing to consider is how the HP would control. If it tries to heat up a cylinder from cold there would be a chance of liquid flood back due to excessive sub cooling, also, how would the low limit be controlled on the incoming air on low ambient days?

Seems like a non starter to me, unless you have large amounts of spare cash you don't mind wasting

[Pincher]

Thanks to Mad Fridgie and Frank for a very useful explanation.

I am not an air-conditioning engineer, so the quantity of energy available was not in my consideration. I just thought I could add a ready made heat pump that will provide limited air chilling and some hot water as an extra bonus.

So, back to the standard MVHR, and the solar thermal panel when I can get a grant.

An extra air chill unit might be added in the loft, inline with the fresh air supply to the whole house. From what you are saying, I will need to install bigger exhaust ducts to expel the waste heat. This is possible by going through the roof, but I presume it's no good piggy backing on to the outgoing duct from the MVHR? I assume the waste heat would require quite an airflow, which also explains why the necessity of having an outdoor unit.

[freddie]

I might have what you are looking for although cannot attach here!!!

SL Services - 07919 444452

MVHR with integral heat pump for heating and cooling with optional DHW tank 180L

[freddie]

Oh, and optional solar coil in 180L tank

[MikeHolm]

Go back to the original statement of not having enough solar to make it viable. I don't know where you are living but i know many people in the UK who use solar only 4-5 months of the year for DHW (no back up). I do as well but I am in Canada so things are different...but, if your only reason for doing this to to be able to keep up with DHW load, get a bit bigger tank with a more efficient HX. A 30kw boiler is plenty big enough to supply the loads you have noted. Most gas fired tanks available here are 10-12kw and 55% efficient so I think your issue is with the tank.

For the solar, you need to have a minimum of 1.5m2 surface area per person and it should be the primary source of heat backed up in the same or in sequential tanks (solar feeding the back up heated tank). One 400-500L dual coil or 2-300L single coil tanks will do. Unless you use rainforest shower heads, you should be fine.

As Frank says, you will need a big HP to service the load as you desire and it will cost a lot more.