Temperature equalisation throughout the house

Question

We have been living in our home for a number of years now, so have seen several summers and winters, but we are still looking to solve the heating and cooling problems we have.

Heating

The walls and roof of the house are well insulated, and upstairs only has relatively small Velux windows, so upstairs is always warm enough, even in the depths of winter.

The open plan downstairs however is laminate flooring on concrete with minimal under floor insulation, so the floor is always cold, and it's always a struggle to get downstairs to feel warm in winter.

Even when downstairs is warm enough and the upstairs radiators cold, upstairs is almost always too hot, so a huge temperature differential exists.

Cooling

So far we haven't had many problems with downstairs being too warm in summer, but upstairs can get uncomfortably hot, especially in our bedroom.

Neither of us sleep too well if too hot, so we are currently looking at options for air-conditioning. I am concerned about our environmental footprint however and would prefer a more sustainable solution if possible.

Some Notes

All radiators except the upstairs bathroom towel radiator (which acts as a bypass radiator) are controlled by TRVs. Originally they had thermostatically controlled valves, but switching to smart valves just confirmed what we already knew, the logs show that upstairs radiators never turn on.

Downstairs consists of a small toilet room (2m²/22ft²), plus a large open plan area consisting of living and dining areas, stair well and kitchen which is also open to the conservatory (a total of 39m²/416ft²), where all external doors open into this area.

The central heating is relatively new, so the condensing gas combi-boiler is quite efficient. It has a SEDBUK rating of 90% and a heat output of 24.9kW, which should be more than enough to heat a to heat 77m² / 826ft² house.

In the UK gas is much cheaper for heating than electricity, so we currently pay 8.3p/kWh for gas (at 90% boiler efficiency) and 28p/kWh for electricity (that's 10 & 36 cents/kWh at current exchange rates) so we would prefer the bulk of the heating load to be carried by the gas central heating.

Down the line we would like under floor heating downstairs, but that would be a big, disruptive job, teh same as addadding more radiators. We will also eventually get around to replacing the patio doors and the conservatory, which should help with insulation downstairs, but again those are longer term plans.

Question

How can we equalise the temperature between upstairs and downstairs?

I've already added a vent and duct fan to pump cold air from downstairs to upstairs, but this doesn't help with extracting the hot air from upstairs.

I thought of just fitting an insulated duct and a fan which extracts air from upstairs and pipes it downstairs, but the only place I could fit such a duct would be in the stair well, and it feels like the air would just end up circulating straight back upstairs. Other than that we would need to take a duct outside, which feels like a lot of work for an indeterminate benefit.

We have sought out quotes for a standard split air conditioner unit to cool upstairs, but that won't help with heating downstairs and I'm not happy about the environmental impact.

I was hoping that there may be solutions which could pump heat from upstairs to downstairs, but normal split air conditioner systems can't be configured to take advantage of temperature differentials like this.

Since upstairs is almost always too hot, even in winter, and downstairs is almost always too cold, even in summer, the absolute last thing I want is to be using electricity to cool upstairs all year round, at the same time as we are burning gas to heat downstairs. This would have an unacceptable environmental impact to us.

_{Note, I'm not looking for product recommendations, or someone to do a complete analysis of our situation, the detail is more to illustrate an example of the more general problem. What I am looking for is what would be considered a sustainable solution to the more generic problem of equalising temperature throughout a house with differing levels of heat load and desired temperatures.}

Answer 1

The story so far

There are three different things that can cause an upstairs to be hotter than a downstairs, and you've mentioned each briefly in your question and comments:

• Convection. Hot air rises to the higher parts of the house. It sounds like you have reason to believe this is only part of your problem, and you've either taken steps to mitigate it, or doing so would not be possible.
• Solar radiation. The sun hits the top of your house, adding more warmth there than to the other parts. Since your problem is year-round (and there's less sun in winter) this may not be the main factor, but long-term you might think about switching to a light-colored roof, and planting shade trees (particularly deciduous on the south side, so they shade in summer but not in winter).
• Internal loads. People, pets, and anything that uses electricity, generate heat. It sounds like this is a big part of your problem, as you've got more of your stuff upstairs, and there isn't much that can be done about that.

All of these are compounded by another problem which hasn't been addressed yet -- poor thermal mass distribution. You have a large downstairs with a concrete slab floor. This is a huge thermal mass which takes a long time to heat up, and releases heat for a long time once it's warm. Upstairs, there's no concrete, and less space, so only minimal thermal mass. This means the space heats up quickly.

One solution to this is to heat the concrete directly with the underfloor heating, but you mention that's not in the cards right now. Since removing the concrete floor probably isn't an option either, the other solution would be to add more thermal mass upstairs.

Enter PCM -- Phase Change Materials

Per Wikipedia, a phase change material is:

[...] a substance which releases/absorbs sufficient energy at phase transition to provide useful heat/cooling. Generally the transition will be from one of the first two fundamental states of matter - solid and liquid - to the other. [...]

The energy released/absorbed by phase transition from solid to liquid, or vice versa, the heat of fusion is generally much higher than the sensible heat. Ice, for example, requires 333.55 J/g to melt, but then water will rise one degree further with the addition of just 4.18 J/g. Water/ice is therefore a very useful phase change material and has been used to store winter cold to cool buildings in summer since at least the time of the Achaemenid Empire.

By melting and solidifying at the phase change temperature (PCT), a PCM is capable of storing and releasing large amounts of energy compared to sensible heat storage. Heat is absorbed or released when the material changes from solid to liquid and vice versa or when the internal structure of the material changes; PCMs are accordingly referred to as latent heat storage (LHS) materials.

In daily life we are most likely to encounter these in hot and cold packs for treating injuries or keeping drinks cool. However, there are also products now on the market for HVAC applications in buildings. The PCM can be "tuned" to an appropriate temperature in the range that's comfortable for humans, plus or minus a few degrees depending on whether the climate is cooling dominated or heating dominated. This results in shifting the thermal setpoint of the space toward the more comfortable temperature, providing thermal stability and reducing the amount of energy needed to heat or cool the space.

In your case, adding PCM to the upstairs with a temperature tuned just below what you want means that as heat is added to the space, it is being used to melt PCM, rather than heating the air and other mass in the space. As long as the space cools enough overnight for the PCM to refreeze, this cycle repeats again the following day (and if it doesn't cool enough, the addition of a simple exhaust fan at night to draw cooler outdoor air across the PCM could be sufficient).

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Further reading:

• Hussein Akeiber, Payam Nejat, Muhd Zaimi Abd. Majid, Mazlan A. Wahid, Fatemeh Jomehzadeh, Iman Zeynali Famileh, John Kaiser Calautit, Ben Richard Hughes, Sheikh Ahmad Zaki, A review on phase change material (PCM) for sustainable passive cooling in building envelopes (pdf), Renewable and Sustainable Energy Reviews, Volume 60, 2016.
• Schmerse, Erik, Charles A. Ikutegbe, Amar Auckaili, and Mohammed M. Farid. 2020. Using PCM in Two Proposed Residential Buildings in Christchurch, New Zealand, Energies 13, no. 22: 6025.

Answer 2

Enclosing the stairwell would be a big help with the heating if it can be done with your floorplan.

Answer 3

You seem to be having issues with convection taking warm air upstairs, especially if you have extractors or vents taking air from upstairs bathrooms to the outside world. I have similar issues resulting in upstairs being too warm at the same time as downstairs is cooler than ideal if the heating has been on for some time, again in a well-insulated house. Like you I've got to the point of considering building in a fan to take air from the landing to downstairs (which is a hall plus an open plan living area in my case), but I can't find a way to fit it nicely.

Thick carpets or rugs and closing downstairs curtains will help a little with the winter temperatures. Shutting the bedroom door in the evening with the TRV in there low or off can reduce the amount of warm air convected in there, especially if it has an en-suite. The conservatory will be costing you a lot of heat. If you can't close it off with doors, consider (thermal) curtains across the opening, or adding any sort of window insulation you can in there

In summer, shading south-facing windows and opening windows only when outside is cooler than inside goes quite a long way. UK weather of course isn't consistent enough to experiment thoroughly. Opening downstairs windows at night makes a huge difference to convection but also to security so I can't recommend it for you. I'm ~100  km west of the location in your profile so similar climate but wetter.

Answer 4

Issue 1: Cold floor downstairs:

Installing a few ceiling fans that kept the air near the floor moving would help a lot.

Place area rugs where people's feet park helps a lot too.

If the slab is insulated below, then removing the laminate floor, adding radiant heat tubing to the floor, and casting another inch of concrete may be the best, but more expensive solution.

Issue 2: Overheated upstairs.

My grandparents house had grates in the floor upstairs over the kitchen. This allowed warm kitchen air (wood stove) to circulate to the bedrooms. The downside was the decrease in noise isolation. Many secrets were discovered at these grates.

Siting these grates is a balancing act: If you can put them over the ceiling fans, the those fans can be used to augment the circulation.

You have the opposite problem it's too hot upstairs. With a return path through grates, putting a ceiling fan in the stairwell may be sufficient. Failing that, put a fan in each grate. Not sure if these fans would work better blowing up with air returning through the stairwell, or blowing down. You want some form of speed control on these fans otherwise they can be obnoxiously noisy.

If the grate is above a ceiling fan, the ceiling fan may provide sufficient air pressure to make air flow through the grate.

You can also tap into the power use by lights for an embedded fan -- think of a 14" box fan between the floor joists with a grill on the ceiling below, and the floor above. A smaller fan is noisier than a large fan, and uses more power per thousand cubic feet of air moved.

Answer 5

It looks like what we need is a small VRF system with heat recovery. These are incredibly efficient, but only normally found in commercial and larger installations.

VRF is Variable Refrigerant Flow, and heat recovery means that refrigerant can be pumped between different indoor units. So when one room is too hot and another is too cold, the heat taken out of the hot room can be used to warm another room rather than both units pumping heat to and from their own outside units. This makes both the heating and the cooling more efficient.

Unfortunately, most VFR-HR systems require specialist external units. These have 3 pipes rather then two (High pressure liquid, High pressure gas and low pressure gas), require heading mode change units inside, require a 3 phase electric supply and come in capacities far higher than an average house needs and they are huge (1-2 cubic meters). One manufacturer does a VFR-HR system utilising a 2 pipe reversible flow external unit, but again their smallest configuration for this is huge, and unlikely to be viable for all but the largest homes.