What makes more sense in a highly insulated building (0.15 U value) with low heating needs?

We have approximately 2-3 months where the temperature drops to between 10-15 deg C (we are in the Mediterranean, where summer is over 40, so under 20 feels cold).

Two options using systems that are readily available here. We dont have pellet suppliers at the moment and diesel boilers are just too inefficient.

1. High Efficiency Condensing LPG Gas boiler, low temp underfloor fluid heating loops, evacuated solar tubes to high insulated hot water storage, with heat exchanger giving pre heat to boiler. Installation cost $$$

2. Electric underfloor heating mats in bathrooms, flat convection panel heaters with central zonal control system.
Installation cost $

Looking at installation cost vs running cost, the cost here of LPG per KW is half that of electricity and should be a lot less in that we can use solar power to preheat on a lot of the days. But if your energy requirements are low and the heating season short, would I be correct to assume that the 2nd option makes more sense as you would need many more years to offset the installation cost over saving in fuel?

There are companies here now offering heat pumps and PV's etc but with this level of insulation I feel that the simplest solutions are often the best.

Hydronics allows you to change your mind. Tubing filled with water doesn't care what sort of boiler you use.

In well insulated homes I look to condensing water heaters with plate heat exchanger and sub-system with reset for space heating. With such a low load you are unlikely to wear out any system.

If you are both heating and cooling this building, a heat pump that does both would be most-appropriate, and works very efficiently at 10C, and would cost less to operate than a condensing LPG boiler.

Using a heat pump that heats/cools water rather than blowing air through a coil you'll get better heating comfort (and somewhat higher efficiency) by warming the floor or panels rather than blowing tepid-air. You may still need to run a fan-coil in cooling mode to reduce humidity, but most of the Mediterranean the humidity is low enough to cool they building with a chilled floor or ceiling (or panels) without risk of condensing water on the surface.

Combining any heating/cooling system with evacuated tube solar-thermal could never be described as one of "...the simplest solutions...". At your temperatures an air-source heat pump would be both most-simple, AND most-efficient.

Heat pumps and coils in the floor and ceilings, plus fan coils is very expensive in plant cost compared to a basic electrical set up. The lower running costs which I an sure will come out of the system you describe would take many years to pay off though, with the short heating season we have, and the difference from the lower plant cost of the electric system.

I agree, Mitsubishi mini-split 26 SEER with heating, cooling and dehumidifier all in one.

Posted By jazzdude on 17 Dec 2010 05:05 PM
Heat pumps and coils in the floor and ceilings, plus fan coils is very expensive in plant cost compared to a basic electrical set up. The lower running costs which I an sure will come out of the system you describe would take many years to pay off though, with the short heating season we have, and the difference from the lower plant cost of the electric system.

A heat pump only makes sense if you're using it for cooling as well- one unit for both applications.  At your modest heat loads using a less expensive heat pump with ducted air for both heating and cooling might be the better bet.  Radiant floors/ceilings are a luxury anywhere, and making up the cost in fuel savings is NEVER in the cards. But radiant heating and cooling IS very nice compared to the ducted-air alternatives, and cheaper to operate in the end.

Condensing boilers are also not cheap, nor is solar, so I'm not sure how you would ever rationalize those on simply fuel-cost-savings over a short amount of time either.  If you're looking for it to be only for a 5 or 10 year payback almost nothing makes sense- not even insulation.  In most cases the cost difference between a heat-pump that both heats and cools compared to one used only for cooling is easy to rationalize compared to a separate heating system entirely (at any efficiency.) 

If you aren't installing a cooling system, a heat pump probably won't make sense, but neither will a condensing boiler OR solar for space-heating alone.  An LPG hot-air furnace might, but it won't be as cushy-comfortable as radiant floors.

This is the norm here. We use split units like this for cooling but they can be very dry for heating. In the highly insulated house these would need to come on for about 20 minutes and the room will be very warm.

This is the crux of my post. Electric Mats and wall heaters are inexpensive to buy and fit and although they cost more to run compared to fuel based hydro systems, the total cost of installation plus running in the medium term should be less. Now when the cost of PVs halves it will be a no bread brainer.

Unlike cooling systems, heating systems neither remove nor add moisture to the room air. No heating system is drier than another unless it has features specifically designed for adding or removing moisture. If you're hoping electric radiant mats will improve the humidity situation you will be very disappointed.

Radiant mats will make the floors more comfortable for bare feet, and be marginally more comfortable than a mini-split. The difference in equipment cost between air conditioning with heating capability compared to air conditioning that only cools would be LESS (not more) than the cost of installing radiant mats or resistance heating panels that cost 4x more to operate in my part of the world.

The cost of PV is falling but the cost of power storage is on a different track, which means you would have to rely on the grid. The days/hours when you have the greatest power output coincide with the highest passive-solar gain (==lower heating requirements.) This makes PV (and solar in general) a LOUSY approach to primary space heating. Even as solar technologies go, the efficiency would be well under half what you would get with much cheaper and smaller thermal air-panel. If you go with a hydronic solar solution the installed price (with thermal storage) is also pretty expensive, but done right will be far less (heating system radiation included) than a PV system of equal energy output. Committing to that much capital expense for a small load makes little financial sense in much of Europe or N. America, but for energy-import islands with expensive utility pricing it can sometimes be financially rational. What are your electrical rates (denominated in Euros or USD per kilowatt-hour), and what do you pay for LPG per liter?

It depends.

Unless your house is very tight. Central forced air will increase infiltration when compared to radiant heating systems which also lower the heating load given the same structure.

True, but at the extremely low loads we're talking here that "extra" infiltration is going to be in the statistical noise.

In a fairly leaky house if the ducts are designed & built to Manual-D the effect is measurable but still small even at moderate heat loads. In the Mediterranean region in a leaky house you're pretty much at the same absolute humidity as the outdoor air no matter what heating system you have, but in a moderately but not extremely tight house and an unbalanced duct system you can probably measure the difference with careful metering. But here we're talking 10C (50F) outdoors at heating design-conditions, which isn't going to present very high duty-cycle for whole-house air exchanges through the heating furnace with 40-55C output air for a U 0.15 house.

In the much colder drier prairies of MN it'll be a more significant factor, and easier to measure, since the duty-cycle will be 10x higher and the infiltrating outdoor air 5-10x drier. In Gibraltar or Malta, Alexandria, or Santorini, if it's too dry inside in winter it's an indication that the house is just too leaky, the heating system type has no bearing. NO mini-split is going to present a drying factor unless it's designed to always introduce a fraction of ventilation air when operating in heating mode. (I don't know of any models that do, but maybe they exist...)

Posted By Dana1 on 20 Dec 2010 01:44 PM

  What are your electrical rates (denominated in Euros or USD per kilowatt-hour), and what do you pay for LPG per liter?

We are charged €0.75 ($0.98) per litre for LPG and electricity between €0.10-0.12 ($0.13-0.16) per KW/H.

Running a mini-split for heating & cooling will be substantially cheaper than a condensing LPG burner. There's about 7.1kwh/L of source-fuel energy in LPG, so burned at ~95% efficiency you deliver ~6.7kwh of heat to the building for €0.75, which works out to €0.11/kwh. With an average coefficient of performance of 3.0 on the mini-split (you'll probably do somewhat better than that at your temperatures) even at an electricity rate €0.12/kwh you deliver heat to the house at €0.04/kwh.

In your case the operating cost of burning LPG in a condensing burner is roughly equivalent to the mat-radiant and electric panel radiators, which are much cheaper to install. But a mini-split or ducted heat pump that both heats & cools is a much better long-term bet, even with maintenance & replacement every 15 years factored in.

Just to be clear, when you talk about a mini split you mean a split air conditioning unit, that has both heating and cooling with the compressor outside and a wall mounted fan coil unit inside. You see, I always assumed that they were more expensive to run than an electric heater or under floor radiant mats. But presumably you are referring to the greater than 3.0 types or A efficiency. It thus seems that a combination of split unit a/c units in the main rooms and under floor electric mats in bathrooms, hall ways and kitchens is the way to go. Can split units be set up to work on a central control device, like you would get with central heating?

While mini-splits are set up with one compressor per room, multi-splits use a single outdoor compressor and multiple room units for distributing the heat & air-conditioning, essentially a central system, but with plumbing replacing bulky ducts.

Almost any heat-pump using more standard refrigerants will get at least a 3x (or even 4x) multiplier of power in to heat-delivered at outdoor temps above 10C. See:

http://www.maro.net/robert/images/lennox.jpg (Divide output kw by input kw for the coefficient.)

When outdoor temps drop below ~5C the efficiency begins to fall, and at 0C it's usually something like 2.2x, give or take. At -5C outdoor temps most are beginning to lose advantage over resistance-heaters and with the lower overall output they're designed with resistance heating strips to make up the difference. those that use CO2 (R744) as a refrigerant can go quite a bit lower, and can even make hot water for the home with decent efficiency, but if your coldest temps are +10C you'll get higher average heating & cooling efficiency with standard refrigrerants. A seasonal average COP of 3.0 would be on the low side for you, but would be typical in areas with the coldest outdoor temps in the -5C to 0C range, and occasional use of the internal resistance heating to meet the peak heating loads. You should expect something closer to 4, and your would never need a unit with internal resistance heaters.