We've decided against a heat-pump tumble drier for the same reason that we didn't go for the condensing type. We can use a vented drier with a very short exhaust duct because the drier backs on to an outside wall. The running cost is appreciably higher than that of a condensing one, and a lot higher than that of a heat-pump one. However, we have found vented driers very reliable, as well as far cheaper to buy than the more complex designs, and regular maintenance involves only keeping the air filter clean and, very occasionally, vacuuming out the exhaust hose via the external terminal. Leaving environmental concerns aside (which we cannot expect to keep doing for ever!), we are convinced that the lower running costs of the more sophisticated/environmentally more friendly designs are unlikely to offset the much higher purchase price over a normal tumble drier life of, say, 10 years (our current Bosch drier is 22 years old and still running as well as ever).
We also reflect that the more complex the drier (this is more true of the heat-pump type than the condenser type), the more likely it is to need service visits, and more expensive such visits will be.
CONDENSING TUMBLE DRIER
Unless it has a cold water tank to create a cool environment in the condenser, this type of machine relies on the ambient air being cool enough for its air-to-air condenser to condense enough water out of the exhaust air to make this air only slightly, rather than very, damp. Our tumble drier location is in our utility room, which is a single story extension to the kitchen. Despite massive insulation of the flat roof structure, the room gets too hot in summer for a normal condensing drier to work well. The high ambient temperature in the room is increased by the central heating gas boiler which, even in summer, when the radiator circuits are off, heats our domestic hot water (dhw). We can avoid this by using the high or low level immersion heater in our dhw cylinder, which is, as usual, remote from the boiler. However, although the boiler works inefficiently with only the load of heating the dhw, the cost of water heating is still MUCH greater with electricity, so we still use the boiler except in VERY hot weather.
There is the added concern with a condensing tumble drier that not only is the air that it exhausts into the room where is is located still warm, but it is also still damp, and, however good the air filters in the drier, dusty. Also, again if the air filter is not very effective indeed, moisture in the warm damp air that does condense out in the condenser is dusty air, and some of that dust is deposited, with condensed moisture, onto the condenser heat exchanger. So, compared with a basic vented drier, there are two extra chores: regular emptying of the condensate tank, and occasional (?) cleaning out of the fluff-filled condenser - or so I read.
HEAT PUMP TUMBLE DRIER
This is basically a condenser drier with a heat-pump-operated heat recovery system, which allows the amount of electricity needed to heat the incoming air to dry the clothes to be MUCH lower than with a basic vented or condenser drier.
Because the heat pump uses, like a fridge or freezer, a vapour-compression circuit to chill the exhaust air from the drum (and transfer a high proportion of that heat energy to pre-heat the air drawn into the drum), the rate of efficiency of condensation should be much less sensitive to ambient temperature - though some machine specifications advise what sounds an unsuitably narrow band for ambient temperatures.
For an installation where it would be impossible to use an externally-venting drier, the basic design of the heat-pump machine is obviously a lot better than that of a simple condenser one. However, the following objections still apply, at least in our case:-
1. Exhaust air will still be warm, though less likely to be unacceptably so except in hot weather.
2. Exhaust air will still be dusty. It is not realistic to expect a level of filtration which avoids this completely.
3. The condensation compartment, where the warm air ex the drum passes through the very cold EVAPORATOR of the vapour compression circuit, will gradually get clogged with dust and "lint", and may be awkward to clean.
4. There is still a condensate tank to empty regularly.
5. The machine is so complex that unfailing reliability over many years of service may be difficult to ensure, and servicing will be expensive.