Air Source Heat Pumps

zeupater

jeepjunkie wrote: »

So if I understand you correctly.

If both a ASHP and GCH are both timed for 8 hours the ASHP trickles out the heat whereas the GCH goes full tilt then modulates down so the net result is pretty much the same?

Cheers

Hi

Pretty simple really .... if you consider that a pretty typical wet heating system would contain ~100litres of water and that the thermal mass of the components would be ~1/2tonne, then raising the components by 25C to the typical operating temperature range for heatpump operations would take ~15kWh, that's somewhere near 2Hrs for an 8kW heatpump, or ~40 minutes for a ~20kW gas boiler ... the boiler however will continue to efficiently raise the temperature for an additional 40 minutes to achieve deltaT50 ... that's ~30kWh in 1.5Hrs, (if switching on at 5pm that's just enough time to sort out meals etc then settle down for a cozy night in), whilst the heatpump after 1.5Hrs has delivered 11.25kWh and will still be running for another 2.5Hrs before it catches up ... just about enough time to become comfortable before retiring for the evening .... Of course, the above would assume that the wet heating system was based on convection radiators, so let's just not consider the thermal mass in underfloor heating .... :D

I agree that the heat provision over time must equate to the heat-loss within the same period, but, as Cardew has raised, in a typical installation scenario, the heat-pump would need to run for a considerably longer period and is less flexible when responding to an immediate heat requirement.

HTH
Z

zeupater

Cardew wrote: »

... Anyway the point being made to the ex-pat in France is that a large thermal store would enable a heat pump to run during the cheaper periods and avoid the expensive periods.

Hi Cardew

Agree to a point, but it really depends on the thermal mass and insulation in the building. You'll find that, if the thermal mass & insulation are sufficient, once the building's core temperature has reached somewhere around the thermostatic set-point, there should be no need to consume energy at peak times - it's a little like using the building itself as a storage heater ... charge it up when the energy costs least, allow the heat to dissipate whilst the cost is high, then replace the heat-loss on the next cycle ... it's effectively what we do with our log-burner. If this can be achieved then there would be no need for an additional large volume thermal store.

HTH
Z

Swainyjo

Phew...logging in took 10 mins and I've got to get back on my bike in sec and point some more walls...!.but -
The main points are
1, gsh not a poss, as I'm sure you were aware but I understand the comparisons
2, It's not rocket surgery to get that all systems operate in a different way and need to be understood so that that particular system can be ' optimised
3, we have a wood burner for ' immediate' heat
4, zeupater, great imput especially interesting the idea of using the building as a storage heater. I don't know what delta t 50 means and although not considering ufh, that's what we will have. Am I right in thinking this would not have to be so hot so therefore would get to temperature quicker or is it the actual room temperature you are talking about? If so isn't the heat obtained a bit different as it is rising through you not radiating out from a wall?
5,thermal heat store ...is it a very modern, very well insulated water tank or does it remove the heat from something ( directly from the pump? The water water? ) and store that?
6 if a systems is on for longer it doesn't NECESSARILY mean it will cost more does? Rhetorical really... I know that's just a question of maths!
7..... No that's it for now. Sorry if any of that sounds particularly dense but this is the first time I've had to install a whole new system. Usually when you move the house conveniently has one! Please assume I know nothing and you won't be for from the mark!
Cheers!
Oh and just as an after thought....we had our gsh running nearly all day in winter in Sheffield. And that was a small terraced house!

Cardew

QUOTE=Swainyjo;63248942]
5,thermal heat store ...is it a very modern, very well insulated water tank or does it remove the heat from something ( directly from the pump? The water water? ) and store that?
6 if a systems is on for longer it doesn't NECESSARILY mean it will cost more does? Rhetorical really... I know that's just a question of maths!

Please assume I know nothing and you won't be for from the mark!
[/QUOTE]

Yes it is basically a very large tank of water holding several hundred litres. The idea being to heat that when electricity is at the cheapest rate, to buffer against the times when electricity is very expensive.

Obviously Zeupater's input on the building as a thermal store is valid, but that IMO is for the mid-winter periods when full heating is required. In fact he alluded to this in post#1489

For example, many with GCH would often use the heating for around 1 to 2 hours in the evening (Spring/Autumn) just to 'take the chill off' very quickly when necessary, whereas operating a heatpump to achieve the same effect would result in additional hours of discomfort before the temperature reached the same levels.

You need to read up on ASHP principles, but essentially heat pumps are at their most efficient when the water temperature delivered to radiators is low - which can be as low as 30C.

This is why Under-floor heating(UFH) is ideally suited to heat pumps. If not you need very large radiators or fan assisted radiators.

The heat output of a heat pump is low compared to a Gas CH boiler - typically 8kW to 30kW. The output decreases as the outside ambient temperature falls. This is why they need to run for long periods.

The Energy Saving Trust carried out an extensive 12 month trial of heat pumps(ground and air). The results were poor, so they extended the trial and have produced a further report with better results.

Whatever you think of the results, the conclusions are required reading. It must be properly sized and installed in a suitable house and the user must know how to operate the system effectively.

[Deleted User]

Cardew wrote: »

You missed your vocation jeepjunkie, you should have been a politician(or heat pump salesman);)



The point, as I am sure you are aware, is that you can put on the gas CH, say, 20 minutes before you get up or return from work and the house is warm. That isn't the way you can operate a heat pump.QUOTE]

Nah, salesmen can rarely afford to buy what they sell and I hate politics

20mins warm time up for GCH... I wish. In five of my houses with GCH one of which was a brand new large detached house in Bristol and when I moved in in December it took almost a whole day of having the heating on to get the house comfortable then at least 8-12 hours a day of background heating to maintain a comfortable temp. Yes it was a cold winter but...

Exactly the same as an even bigger house I had in the Bristol countryside but on oil. A comfortable house meant long burn times therefore not cheap to run but if you want a warm house instead of academic run times of an hour or so simply wastes fuel as there is no beneficial heat.

What has this got to do with ASHPs... Dunno other than I have personally not found a great deal of difference in run times between any of the technologies.

This morrning would be a good example... Freezing outside this morning heating timed to come on at 5:30AM. I get up at 5:30AM. House lovely and warm Fancoils are almost silently blowing warm air for immediate heat. System will time off at 9AM for everyone else to get up and leave.

Cheers

[Deleted User]

Cardew wrote: »

You need to read up on ASHP principles, but essentially heat pumps are at their most efficient when the water temperature delivered to radiators is low - which can be as low as 30C.

As pointed out this is excellent advice, I have even tried 25c for the shoulder periods on the heat curve with good results but plumped for 30c. Fancoils were fine but the UFH was not great at 25c :-(

This is why Under-floor heating(UFH) is ideally suited to heat pumps. If not you need very large radiators or fan assisted radiators.

Again excellent advice

The heat output of a heat pump is low compared to a Gas CH boiler - typically 8kW to 30kW. The output decreases as the outside ambient temperature falls. This is why they need to run for long periods.

Again correct but as long as ASHP/Rads/UFH are sized correctly is as relevant as saying there is no air on the moon

The Energy Saving Trust carried out an extensive 12 month trial of heat pumps(ground and air). The results were poor, so they extended the trial and have produced a further report with better results.

Yup I was pesonally pestered by them about this. I think, on paper initially, they tried to cherry pick systems...

Whatever you think of the results, the conclusions are required reading. It must be properly sized and installed in a suitable house and the user must know how to operate the system effectively.

Absolutely critical !

Hope you don't mind me commenting

davecooper71

We have used these guys (A UK Company based up in Lancs) for our air source heat pumps in the past and they were very professional:- http://www.globalenergysystems.co.uk/air-source-heat-pumps/how-it-works

Air_Source_Heat_Pump_Man

The sizing on heat pumps is not actually restricted. Any property has a given heat demand and any heat source has an output, if the two meet then the property will be warm. With a heat pump the question is at what outside air temp are you going to base the calculation on.

The limitation on heat pumps is cost, few single units can offer more that 10kW at sub zero temps and so you need several to heat a large or poorly insulated house, which leads to poor paybacks.

Match the outputs though and you should get a heat and cost effective system

waltsalt

I heard, allegedly, apparently, a source told me, that Goodman are absolutely crap.

TiredGeek

I always doubt a recommendation of a company given over an open forum, smacks of advertising to me.....