[fred2]

Combining a Heat Pump with traditional oil heating system:
There was an interesting article in the Sunday Times here in the UK a couple of weeks back about the use of heat pumps. Basically it suggests that you can get savings of up to 70% on heating / electricity by using a 'slinky' or vertical borehole to heat water a few degrees and then with a heat pump you can use this to provide effective heating and hot water. And, that such systems are feasible for 'normal' sized houses.

But on looking into this further it seems that heat pumps are best suited to underfloor heating systems. They will only efficiently heat water to appx 50 degrees C, which isnt enough to drive a traditional radiator system (which expect to work with 80-90 degree water). So you need a greater flow of the less-hot water through larger radiant surfaces to get an equivalent amount of heating.

I have a fairly large house with an established 'normal' oil fired central heating with radiators. Does anyone know if there are any realistic ways of using a heat pump in conjunction with such a system without ripping the whole thing out and starting again? E.g is there some way to add a heat pump to an existing system, so as to offset the fossil fuel costs?

[Devil]

Some calculations are in order; only you can do that. The starting point is the fuel consumed on the coldest day with the present system and convert that to joules. You will need at least that as your heat output. Multiply that by ~1.3 and that will be the amount of heat you will need to input your system. Ideally, your cold end should be dimensioned so that, with that heat flow, the cold medium, be it soil or water, will drop in temperature by no more than 2-3°C (less if there risk of freezing, more if you have a geothermal source). This can be calculated. If you do not have access to flowing water or a largish lake, the rule of thumb is that you will need pipes buried at 1 m depth, 50 cm apart, covering 10 times the floor area of your house. This is because the heat transfer from soil to pipe is poor. If you have flowing water, above or below ground, then the heat transfer will be improved (less pipework), depending on the temperature and flow rate.

Generally, a single stage heat pump will have a delta-T of water of about 25°C from the input to output heat exchangers. You could have a two stage one, which should be suitable for pumping water round your radiators, but your electricity consumed will, of course, be doubled. However, you should still be well into positive territory.

You would be well advised to consult a local expert; this is not for the faint-hearted and you may have to have various permissions (electricity authority, hydrological department etc.)

[Starvid]

Many people in Sweden use heat pumps as their only heating system, and we get cold winters (-20 degrees Celsius). I don't see why you should need an oil burner, though this is not at all my area of expertise.

[fred2]

$this->bbcode_second_pass_quote('Starvid', 'M')any people in Sweden use heat pumps as their only heating system, and we get cold winters (-20 degrees Celsius). I don't see why you should need an oil burner, though this is not at all my area of expertise.

[MOCKBA]

$this->bbcode_second_pass_quote('fred2', '
')My point is this: I have a large house with an existing oil fired system installed throughout. I'm investigating whether it is feasible to modify this existing system by adding a heat pump.

[Aedo]

$this->bbcode_second_pass_quote('fred2', 'T')hey will only efficiently heat water to appx 50 degrees C, which isnt enough to drive a traditional radiator system (which expect to work with 80-90 degree water).

[Madpaddy]

fred2,

I investigated this at length and came to the conclusion that in my case putting in a heat pump was not feasible. I have a bungalow with single glazed windows and oil fired central heating. I typically have the oil heating set at 70deg c. Ripping up my floor etc and putting in a borehole was a cost I could not justify. Not only should you have underfloor heating but your walls etc. should be very well insulated and you should have double or triple glazed windows for the underfloor heating system to be very effective. UF heating is very unresponsive meaning that it takes a long time for it to heat up the floor slab which means that you need to have it on at a low setting ALL the time.

In my case my solution is to put in a solid fuel stove which can be left burning at a very low setting for the night. We also have a back boiler so if we light a fire we can put slag coal on the fire before we go to be which leaves the radiators at about 45-50 deg for the night. Our solar panel system for the domestic hot water also has the water in the system at at least 20 deg all the time so when the boiler comes on it has less work to do, it was a benefit from the system we hadn't even factored in. The solar panels feed water back into my heating system through the second coil in the rapid recovery cylinder.

Anyway, it is up to yourself but that is my 2 pence worth.

[Starvid]

Why not use a heat pump which uses the heat in the outside air? You make a hole in the wall and put the unit there, you don't have to rip up the floor.

But maybe they are useless when it gets really cold outside?

[Devil]

$this->bbcode_second_pass_quote('Starvid', 'W')hy not use a heat pump which uses the heat in the outside air? You make a hole in the wall and put the unit there, you don't have to rip up the floor.

But maybe they are useless when it gets really cold outside?

[chenopodium]

We're planning to build a low energy house with solar panels, solar thermal heating system, very thick walls and good insulation etc.

The question remains is which heating system should we install?

One option is a ground heat pump.

The problem is that it is quite expensive, and requires about 1/4 or so electricity of its heating capacity. If there was no outside power, we would have to run it on our solar panels, which means we would have to spend a huge amount of money buying enough solar panels to run the pump in winter.

The other option is a wood stove.

The advantage is price and that you can run it even if there is no power, also you could use it for cooking in a worst case scenario.

But I worry that we are not the only ones turning to wood, and that there may not be enough wood available to buy! I just read an article that in the US somewhere there is already a wood shortage.

We wouldn't have this problem with a heat pump.

So, would you:
- spend the extra money (which is quite a lot) on a heat pump plus more solar panels to cover it, but then we would have to save on other things such as the amount of land we could buy

- or get a wood stove and hope that we will always have wood available for heating....?

Any other ideas?

Thanks

[Quinny]

I've got a similar problem and I've decided that wood is the way for me, but I think it depends where you are and access to local forest. Buying by the cord will become prohibitively high.

[chenopodium]

where we live the law won't allow us to just cut down trees. We are at 42 degrees and have a climate similar to say Boston. There are forests nearby, but if people start to cut down trees (illegally) then there won't be much left...

I guess in the worst case there is always the sleeping bag/warm clothes solution (especially in a very well insulated house)?

I like the idea of a renewable source (wood) plus that we could cook with it in the worst case.... (plus that it is a lot cheaper than the whole PV/heat pump/floor heating installation)

I would guess wood would become as expensive as oil?

[killJOY]

No-brainer.

Woodstove, or wood/coal combination.

Less complex, cheaper, easier to maintain.

Wood ashes are good for the garden (but keep them away from potatoes.)

Coal ash is neutral and can be dumped in the woods without harm. (I'm assuming anthracite as fuel, which is hard, high-quality, clean-burning.)

You can cook on a wood/coal stove.

You can heat your water with it.

You can fire it up quickly, on demand, during a cold snap.

[purdum]

We were in a similar situation several years ago, when deciding how to replace an ancient oil furnace, and geothermal was the clear choice for us. Your mileage may vary of course, especially if your heating needs are substantially less than ours.

Have you done a complete cost analysis and sizing of the geothermal system yet? You should be able to work backwards from the degree day numbers for your location and estimate the heat loss for your house using software like HVAC-Calc. We designed our geothermal system for 81 million BTU/year, which required a 6-ton unit (cutoffs for smaller units are 73 MBTU for 5 ton and 68 MBTU for 4 ton). If your house is superinsulated, you might get by with an even smaller unit. We had 6500 degree days of heating last season and the heat pump used 9000 kwh of electricity. In our location, with an average of 3 hr/day sun, that's a lot of solar panels (I recall doing a back-of-the-envelope estimate for panels, batteries, inverter etc and coming up with $45K).

You can save some money on the geothermal by designing it to cover 90% of your house's heat loss and rely on auxiliary heating or blankets for the other 10%. For us, when it is below 7 degrees F outside, the house loses heat faster than the heat pump can replace it. But the average winter here (central Michigan) only has 130 hours below that temperature. We figured we could tolerate a bit of a chill for 130 hours rather than spend a few thousand dollars more for the bigger heat pump unit.

As for the wood supply, do you have enough land to maintain your own wood lot? And you may have other options for wood - here you can get a permit and harvest fallen logs and dead wood out of any state land for free. Our neighbors also have good tracts of wooded area and we can barter for firewood. We are installing a wood stove as backup for the heat pump.

Another factor which persuaded me to switch to geothermal is that our utility uses coal and nuclear sources for most of its power rather than natural gas or oil, so our electric rates have gone up very little compared to other parts of the country.

PM me if you want a more complete $$ analysis of our heat pump system.

[Blacksmith]

I can't see wood as a problem. There is always scrap wood , broken pallets, scraps from a manufacturer, shorts and cutoffs from a lumber mill, scrap wood from construction sites, and people who want to get rid of branches or trees they have cut down.

As a last resort you can build a Russian stove and burn straw.

I myself get a lot of wood from the burn pile at the local dump, not pretty and have to look out for nails, but it burns.

[dunewalker]

$this->bbcode_second_pass_quote('chenopodium', 'W')e're planning to build a low energy house with solar panels, solar thermal heating system, very thick walls and good insulation etc.

The question remains is...

Any other ideas?

Thanks

[korosten]

(deleted)

[chenopodium]

Thanks for the replies so far!

I guess you're right that there is still scrap wood in some form in the worst case. Could straw also be burned in a wood stove to generate enough heat, or do you need a special (Russian) stove for that? Can you turn a regular stove into a Russian stove?

Our land is pretty small (considered "large" in this area, around 10000 sq feet - it is very expensive here, because we want to live near public transportation), and we could probably plant 3-4 trees or a few more Smile (we still want to have room for as many vegetables/edibles as possible).

Re Georgia: I guess I need to read those threads... I admit I have no real idea why this should be such a problem for us... unless the war spreads and turns into a bigger war... I'd better start reading

[The_Virginian]

GET BOTH !

Being prepared, or having ability to switch to wood as a back up is the best assuming you can afford it.

[chenopodium]

We "could" get both in theory... but that would mean we could get less land and the house would have to be smaller to make it cheaper overall. Currently we are planning for it to be around 1400 sq feet "living area" plus cellar, so comfortable but not huge for a family of 4...
True, wood stoves are not very expensive, so if we do decide on the heat pump, I think we would end up getting both (with the smaller land/smaller house). You may ask, why not wait a few years to save more money? I guess because I think we may not have a few more years until things go downhill and building might not only be more expensive but also maybe impossible