Jeremy Harris

Is this a Sunamp PV? If so, then there was an issue on early models that led to two potential problems. The primary issue seemed to be that the over-run time on the pump was a bit too short, leading to heat soak in the small heater chamber. This could cause the resettable over-temperature cut-out to trip (I had this problem a fair bit). The second problem was that the over-temperature cut-out on some early models had a manufacturing defect, that lead to the terminals over-heating. Another possible issue could be that the heater was running at more than its rated power, due to a high local supply voltage. This was one concern that was raised whan we were having problems with the thermal trip operating on our old Sunamp PV. During sunny weather our local supply voltage runs right up to the maximum allowable of 253 VAC. At that voltage the 230 VAC rated heating element will be delivering around 3.08 kW, rather than 2.8 kW. I had several conversations with the Sunamp technical guru, who was very helpful at the time, and we worked through options to fix things, but unfortunately I believe that he's either left the company, or no longer communicates directly with customers (no idea why, as he was very helpful). I have the name and contact details for their current customer service chap, that I can pass on to you by PM if you wish. He's not quite as familiar with the technical side of their products, and may not know much about the Sunamp PV, but he does seem keen to communicate, which is a start. The control board switches the heating element with a relay, and IIRC that relay has 16 A contacts. Should be OK for a 3 kW heating element, but it's easy enough to whip the cover off and check the relay rating to be sure. Best check the supply voltage, though, as an element rated at 3 kW at 230 VAC would be well over that at the maximum allowable grid voltage.

That's most probably because the traditional place to put radiators was underneath windows, so the area in front of windows would often feel fairly warm compared to elsewhere. There's no "cold radiation" from windows, as "cold" doesn't flow anywhere, by any means. All we have is heat flow, and that can be varied by the effectiveness of insulation.

There are few metres of pipe either side of the branch on ours, with bends in both, so I suspect that the potential "back flow" problem is mitigated by that. Not sure I'd want two toilets back to back through an arrangement like that with just short pipe connections, though.

I suspect that much of the perceived difference is just from the way we tend to sense body heat loss. One of the first things I noticed with the 3G in the new house was that there is no feeling of it being a bit cooler when standing in front of the glazing on a very cold day. The inner pane of glass is very close to room temperature, and this fools us into thinking that the part of the room in front of the glazing is warmer than would be the case if standing in front of less efficient glazing. This isn't wholly explained by the difference in U value, either, I think much of the perceived difference comes from having two low e coated panes of glass in 3G, versus just one in 2G. The low e coating only reduces the radiated heat loss out through the glass, by reflecting it back inwards, and I think we can sense that reduction in radiated body heat loss.

I fitted one of those at the top of our soil stack. The left and right branches go off to the upstairs bathrooms and the top connection goes to the AAV up in the eaves space. Seems to work fine.

Just to be clear about all this stuff about radiated heat etc. Our UFH operates with an absolute maximum floor surface temperature of 23.9°, when the room temperature is 21°C and the outside air temperature is -10°C. 99% of the time the floor temperature never exceeds about 22°C I challenge anyone to feel any difference at all between the floor at 22°C and the room at 21°C. I can measure it, but certainly can't feel it. I doubt that anyone could feel any noticeable radiant heating effect, either. Not even my thermal imaging camera shows anything up.

Just to be clear, Part G3 does not apply to unvented water heaters that have a volume of less than 15 litres, so the small under-sink units don't need all the complex stuff that a large unvented hot water tank will need. This may include the requirement for a tundish to be waived, although manufacturers instructions (MIs) over-ride the requirements of Part G. For example, our under-unit unvented water boiler (for the boiling water tap) is well under 15 litres, yet has to be installed with a pressure relief valve and a tundish to the discharge pipe, as it's in the MIs.

Got to be better than the small instant heater hand wash units, that's for sure. My experience of those over the years at places I've worked has been like that of @SteamyTea, they are universally rubbish. I suspect that the snag is that a small Sunamp would be far too costly for an application like this, as it would need the same charge control system as a big one, and I suspect that's where a fair bit of the cost is. It may be that they just haven't thought about entering such a low selling price market yet. I suspect that it would be possible to make a very small Sunamp, with maybe a litre or two of PCM, that would very much outperform the existing electric units.

First off, they are not "my numbers" at all, they are just bog standard heat loss calculations that use well-proven laws of physics. The difference between an insulated suspended floor and an insulated ground bearing slab is that the worst case heat loss for the suspended floor will always be higher, irrespective of the form of heating, because the worst case temperature differential will always be higher. A suspended floor will have a temperature on the underside that mirrors the outside air temperature, as it will be ventilated, so cold air will flow underneath it. A ground bearing slab has a near-constant temperature all year round of about 8°C underneath it, so is warmer underneath in very cold weather and cooler underneath in very warm weather. This difference effects the heat loss rate for each floor construction method. It's pretty easy to calculate the heat loss rate for any floor type, build up and temperature differential, and the bottom line is that if this heat loss rate is increased, for any reason, then that will increase the heating cost for the same heating requirement. Of course, if other work done decreases the heating requirement that may outweigh the additional heat loss from fitting UFH, but that doesn't change the basic fact that heating the floor always results in higher losses than heating the room.

I'm saying two things: 1) UFH always, without exception, has a higher heat loss rate than any other heating system that does not directly heat an external surface (unless you can provide evidence that shows otherwise). 2) UFH provides a clutter-free way of heating that can feel more comfortable, but that comfort and convenience comes at the expense of higher running costs, due to the additional heat loss.

What do your calculations of the heat loss through the floor show, then? Are there factors other than the total floor U value, the surface resistances and the temperature differential that we are all unaware of (including those in the business of specifying and installing heating systems)? If you know of specific factors that everyone else is unaware of, then it would be useful to state them here, so we can all correct the mistakes we've been making for decades.

The convective properties of heat don't change anything at all, as the key thing as far as heat loss through the floor is concerned is really just the temperature differential across it. Increase this and the heat loss increases in proportion. Increasing the temperature differential by 50% increases the heat loss through the floor by 50%. The bottom line is that UFH will always increase the heat loss through the floor. Even with our 300mm layer of EPS under our floor the heat loss (in cold weather) from having UFH is about 24% higher than it would be if we had radiators. The bottom line is that if you are prepared to pay for the increased heating requirement that arises from having UFH then that's fine. You are, and we are too. In our case the extra we pay because of the higher heat loss from the UFH is tiny, because our overall heating bill is pretty small, even with the additional losses from having UFH.

No, that's been removed now, so the only plumbing on the UniQ we have is a cold water pipe going in to the unit and a hot water pipe coming out. I added a thermostatic mixer valve to mix the hot water to the house down to about 48°C, but that was just a personal preference, as by default the Sunamp delivers hot water at about 55°C right until it's discharged. Our's stores between 9 kWh and 10 kWh (dependent on how long since it was last charged), so about the same as a UVC of around 200 litres or so.

If you want simple for hot water then it's hard to beat a Sunamp, TBH. JUst a cold water pipe from the supply going in and a hot water pipe coming out. No other plumbing needed, unless you want to add a thermostatic mixer valve to reduce the hot water temperature down from the ~55°C that comes out of the Sunamp.

There's a big difference in heat loss between UFH and radiators though. Radiators don't change the heat loss through the floor at all, whereas UFH does, by a great deal. This is simple physics, in that the rate of heat loss is directly proportional to the temperature differential. For a room at 21°C with radiators, then the floor will be a little cooler than this (because of surface resistance), say 20°C. The differential temperature from the floor to the underlying ground will therefore be about 12°C for typical UK ground conditions (tends to be about 8°C all year around). For the same room, at 21°C, heated by UFH at a rate of 50 W/m², the floor surface temperature will be 25.8°C and so the temperature differential will increase from 12°C to 17.8°C, an increase of nearly 50%.

As mentioned, multifoil type "insulation" just doesn't do anything at all in a location where there is no radiant heat to reflect back, so would be wasted space and money. The very best insulation is either vacuum insulated panels or aerogel, but both are eye-wateringly expensive. The next best is PIR foam (Celotex etc), then EPS/XPS. This spreadsheet will allow you to calculate the floor temperature needed for your house heating requirement, and also give the heat loss from UFH to the ground for any insulation used: Floor heat loss and UFH calculator.xls As above, UFH will always be less efficient than radiators, as there will always be a greater heat loss down to the underlying ground, or cold undercroft space for a suspended floor. UFH may also struggle to heat a house that has a high heating requirement. As a reasonable rule of thumb, UFH can provide around 50 to 60 W/m² of floor area at the most, and works best if delivering a lot less than this. 30 to 40 W/m² is a fairly good target. You can determine the heat output per m² of floor area needed by just dividing the total house heating requirement (in W) by the UFH area (in m²). For example, for our house the UFH area is about 70m² and the heating requirement in extremely cold weather is about 1,600 W, so the worst case for the UFH is about 23 W/m², well within the range where UFH will work reasonably well.

They are often supplied with a kit that includes the pressure reducing valve and the pressure relief valve. There may well be a thermostatic mixer valve included as well, to regulate the hot water output down if needed. The pressure relief valve has to vent to a tundish and the tundish needs to be connected to a discharge pipe with specific constraints. These are given in Part G3, ( https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/504207/BR_PDF_AD_G_2015_with_2016_amendments.pdf ) on page 24.

Sounds like a man with a (partly political) mission to me. Thermal stores aren't generally the most efficient way of providing hot water, though, especially vented ones. A UVC is a bit less lossy and more efficient overall, although a thermal store can make sense if you have something like a solid fuel heat source, where there is a benefit in being able to do a short, hot, burn and then store the heat for both hot water and space heating until the next burn. Not exactly environmentally friendly, though, burning stuff that produces a fair bit of air pollution.

Far better to find a way to reduce the heat getting in than to spend a lot of money through the life of the house trying to pump it out, IMHO. We only have around 4m² of glazing facing East and about 10m² facing South and we had massive overheating problems initially. I covered much of the East and South facing glass with solar reflective film, plus we have a big overhang over the large glazed gable anyway, but we still have too much heat getting into the house and have to rely very much on pumping that heat out with cooling systems.

Bear in mind that to work with any reasonable degree of efficiency an underfloor heating system needs at least 100mm of decent insulation under the pipes, ideally more (we have 300mm of EPS and we still lose about 10% of the heat into the underlying ground). UFH is always less efficient, and so more costly to run, than radiators, especially as a retrofit to a building that may not be very well insulated, and so needs the floor to run at a higher surface temperature (the higher the UFH temperature the greater the heat losses down into the underlying ground).

The inlet and exhaust ducts will be at least 150mm in diameter, and need to be spaced apart on the wall by a couple of metres, so that may have a bearing on where you fit them. Best have both of them on the same wall, as I didn't take heed of this and as a consequence our MVHR gets unbalanced if we get a strong wind from the West or East.

Unvented gives mains pressure hot water, vented gives just tank head pressure. Unvented needs a G3 installation (widely, almost universally, ignored, IMHO) if the capacity is over (I think) 15 litres, and in theory an unvented system over 15 litres is supposed to have an annual G3 inspection (again something that's pretty much universally ignored). Unvented is the better system in terms of hot water delivery and energy efficiency, but does need some key safety precautions, including limiting the inlet mains water pressure (usually to around 3 bar) and having a proper pressure relief valve, visible tundish and overflow pipe.

A couple of people here have external solar blinds that seem to work very well. I regret not being able to install them, but we didn't have enough space above the windows to hide them when up, and the planners (well, conservation officer) wasn't happy with anything external, like shutters or a projecting external blind box.

Thanks. I have a vacuum pump that I used to de-aerate mixed resin, home made, from an old freezer compressor, but it pulls a pretty good vacuum and has a gauge. I wonder if I could use that to pump the system out and leak test the pipe connections? Presumably there's a port on the valve block to allow a vacuum pump to be connected. Sounds ideal, especially as it's quiet. Our ASHP is very quiet, we can't ever hear it inside the house, and it's only barely audible outside - I often have to look in the grill to see if the fan is going around, as at low speeds it's just about inaudible. The plan is to mount the outdoor unit on the rear wall of the house, which faces North, and is hidden by our big retaining wall. There's a ~1.8m wide gap between the rear of the house and the retaining wall, so it should be OK there, and if the unit runs quietly that's a bonus, as I'm slightly concerned about noise transmission into the wall of the house. Unfortunately the paved path at the rear of the house is right up against the house wall, so I don't have the option to ground mount the unit. The indoor unit will be in our bedroom, but as I doubt we'll have it on at night (the house stays cool for a long time once it's cooled down) noise shouldn't be a problem. I think we'll probably run it during the day when the weather's hot (and the PV system is generating) and shut it down over night. and rely on the cool air from the MVHR

Funnily enough, the unit I've been looking at is a Mitsubishi SRK25ZSP-S From what I can gather it looks like this unit needs a hole about 65mm in diameter for the pipes and cable to pass through, which seems pretty easy to arrange. I reckon I can probably use a bit of 68mm plastic downpipe as a duct to run through the wall, with a bit of luck, then fill it up with foam to seal it after I've run the pipes through.