Heating

I have been using an oil filled electric radiator for several years with good results. After it warms up, it turns off and gives off residual heat. (free) It has settings for 600 watts, 900 watts or both. I find the 600 watt setting is fine for my greenhouse.

 

I do wonder, hypothetically, about this type of solution, but I've never been able to find side-by-side comparative research. I'm not doubting that it works, but to my mind:

Once the "off" Max temperature is reached there is a lot of residual heat, this carries on pumping out into the greenhouse taking the temperature above the desired Max, and that in turn means that the heat losses are greater (because the difference between Outside and Inside is greater, so heat is lost at a greater heat, meaning that more, overall, is lost as a result)

Second when the Min temperature is reached my perception is that this type of heater has to itself "heat up" before it delivers heat, so there is a delay before the heating effect is available - and if the temperature difference to outside is large then the temperature inside will fall quite a bit before it starts to warm again.

Ultimately 600 watts is 600 watts, however the electrically-generated heat is delivered.

And lastly does the heat adequately get into the corners of the greenhouse (i.e. with no fan to move the air, and just relying on convection).

I have read that putting convection heaters (whether paraffin or gas or whatever) under a metal bench means that the bench heats up, acting as bottom-heat for the containers on top, and this may well enable the plants to survive lower air temperatures, plus all the benefits of bottom heat on rooting etc.; can't get that with a fan heater

If anyone sees any comparisons on the web I'd be very interested to hear :_)

 

All I know is that it works for me. I have a remote thermometer and can monitor the greenhouse from indoors and the temperature remains fairly constant overnight. I do have a small clip fan running on slow to move the air. Also, I keep a small fan forced hot air heater, with the thermostat set much lower, under the bench. My thinking is that if the radiator fails, or can't maintain the temperature, the fan forced heater will assist.

 

Yes, I agree, that's an excellent approach. Specifying two heaters of half the power needed, and setting one a bit lower, keeps the target temperature much more closely - as only half the required power is being used, and then if the temperature cannot be maintained the second one does the job. My worry is that we get a power cut on the coldest night of the year ... I have no fallback to electric (well, I do, but its a gas heater that I have to light manually, so I require that the powercut is before I go to bed please!!)

I'd be interested to know what the temperature range that your heater maintains is. If you remember next Winter I can send you a logging device which you could put in the greenhouse for a week or so and that would give us some good data to show what the fluctuations are. (My expensive fan heater and highly accurate thermostat keeps mine to within 0.5C, which is very financially efficient (well ... it still costs a lot! just doesn't waste any necessarily)

 

Actually, I don't heat the greenhouse all winter, as I grow indoors in the winter. I open the greenhouse usually in April, though I will move plants into it for an afternoon in the winter if the sun is strong and the temperature is in the 70s F. As a for instance, this last week on Monday we had frost, and Tuesday we had a hard freeze, so I placed the radiator in the greenhouse. I checked the temperature before retiring and again in the morning; it was about 52 degrees F. The small fan forced heater is set to click on at 45 degrees F, and I leave that in there all year. We have a very reliable power supply, and in worst case, I have a generator in the event that it does fail, as we can expect about six or seven feet of snow during a normal winter. That is the reason for growing indoors. The greenhouse is operational from about April through November. There is thermometer that records high and low temperature and humidity levels on a daily basis in the greenhouse and I just give it a cursory check once in a while. Recently I purchased a wireless web cam on eBay for less than forty dollars and I put that in there so I can see the temperature and check the automatic vent openers. I just took this photo as I finished planting the cucumbers, on the left, today. They will grow on a trellis and the tomatoes are cordon type supported by twine. The large tanks on the shelves in back gravity feed nutrients and everything else is on timers or remotes, so I get to sit back and relax.

 

I have my first greenhouse being erected tomorrow,a wooden one from Swallows. It's 8x6 and I'm going to fit a four foot tubular heater but I don't see them mention on here do they give out enough heat?.
I'm going to bubble rap when it gets cold.many thanks.

 

I'm not a fan of them. They take a while to get up to full heat, the heat has to circulate by convection, by the time that happens the tubes will be fully hot and the air around them so the thermostat will cut out and they will keep on pumping out heat for quite a while. When the thermostat senses its getting cold again it will take a while for the tubes to heat up and convection to get that heat into the corners and crannies of the greenhouse.

All that means that they are more expensive to run that something more efficient.

An electric [greenhouse] fan heater is best IMHO. What you need is a really good thermostat (and they aren't cheap, probably £50). The fan will blow the hot air all around the greenhouse, so no cold spots, and a good thermostat will keep the temperature within 1C - perhaps even within 0.5C, whereas the el-cheapo thermostats that come fitted with cheap fan heaters will probably manage 5C at best, and maybe as much as 10C swing at worst. Overheating a greenhouse by 10C costs a fortune ... under heating it by 10C will mean all your plants are dead :(

I would expect that a good thermostat will save you at least £50 in the first year - by reduced exlectiricty consumption.

Insulation is the cheapest form of heating so good idea to do the bubble wrap. Make sure you tape all the joints so that the bubblewrap is a continuous "sheet" inside the greenhouse (bar windows and doors, but taping the bubblewrap to the glass at those junctions (so that hot air doesn't just flow between the bubblewrap and the outer glass) will also save money.

 

Wow thanks for your lengthy reply.I may consider bringing the plants into my conservatory at night or the delicate ones.

 

Conservatory, attached to the house, is likely to be far more air-tight than a greenhouse, so much easier (i.e. cheaper) to heat. Also likely to benefit from being attached to the house (although modern levels of insulation may mean that the house-wall the conservatory is attached to doesn't leak much heat into the conservatory!).

My conservatory was attached to the utility room wall, and the utility room was unheated. The conservatory remained frost-free, without heat, even when the temperature outside was -10C - although I did install a fan heater to make sure it stayed frost free, and to heat it to 10C Minimum in Spring.

 

I've just ordered a Peresene high top propagator which I'm going to leave in the conservatory,I have French doors from my main living room to the conservatory which are open all the time and it never gets freezing because of the living room wood burner,I really want to start chillies early as I've never had great success.

 

If you want to start them early then as well as heat, to get them to germinate, you may need to consider supplemental lighting to get them growing too. Sufficient warmth in late Winter / early Spring won't be enough to get plants going without also giving them the light that they would expect at those temperatures :(

 

My new first greenhouse which should be arriving in the next two hours is going to erected on my decking,so I've been to the builders merchants today and bout some treated 4x2 to ask them to sit it on raising it 2 inches,the reason I'm doing this is because I'm going to put a 2 inch insulation on the deck followed by treated ply,its a wooden green house and I'm going to do the same up the 18 inch walls.
Getting excited at my age.

 

Good idea. Extra cropping height too

Worth thinking about the thermal mass as well, if you have an opportunity. Paving slabs for the floor is probably the only obvious thing? (but I suppose you could use the most dense ones you can lay your hands on?)

The Delta-T (temperature difference) across the walls of a greenhouse is much less than a centrally heated house, but the principles are the same for insulation: the most you can sensibly install, and as air-tight as possible (I would definttely tape all the joints - can't really do that with the glass, although if yours is Wood it may be that the glazing is putty'd in? which would be better than butt-joints I guess - bubble wrap will help with that, but if you can reduce air-changes through leageake that's still important.

A wall / hedge to keep the prevailing wind (well, any cold wind in Winter - so may not actually be the prevailing wind??) off the structure will also help. Not so tall / close that it casts a shadow though

 

I have a sort of micro climate as my property is in a plot of a old Georgian quarry and three sides have 50 ft rock face some with caves,so wind is not a problem.
I'm sitting watching the guys erect it now,oner the moon.

 

It would be nice to see some photos when it's finished Roy.