Green Building Forum - ASHP control - how is it done

ASHP control - how is it done

andytk — Tue, 06 Nov 2007 12:50:10 +0000

Hi, Iâ€™m new to the forum. Excellent forum btw.

Does anyone know how the control on heat pumps work. In particular air source heat pumps.

Iâ€™ve been looking into buying a house and many of the properties Iâ€™ve looked at only have access to electric heating. Pretty much without exception these have been storage heaters. I ripped out the storage heaters in my last house and replaced them with normal convection electric heaters. This was much more comfortable and user friendly and didnâ€™t cost too much as the house was unheated during the day and was very small <45m2.

This time round the house will be bigger so Iâ€™d rather go down the ASHP path, but am wondering how it all integrates into the house.

I was planning to run a 8kW ASHP against a thermal store with a heat exchanger for direct potable hot water, and use the water in the thermal store itself for the wet radiator based CH system. I know the radiators will have to be oversized but this isnâ€™t a problem as new rads would be needed anyway.

My question is, how does the ASHP know what level of heat to deliver to the thermal store. In summer it runs on a timer and senses tank temperature and runs as needed within its time window. In winter youâ€™d need it to run at roughly the same heat output as the heating system is withdrawing from the thermal store. Is the ASHP a â€œdumbâ€ system. Can it only fire up based on thermal store temp (as this seems to be its only input to its controller).

If so wouldnâ€™t it run cyclically? If Iâ€™m drawing 4kW off with the CH system, then wonâ€™t it wait until it sees the tank temp dip and then fire up at 8kW (with a lousy COP) until the system is back to temp.

Iâ€™m looking at the Trianco range of heat pumps but havenâ€™t contacted them yet as I donâ€™t yet have a house to design a system for. I was just wondering if anyone knew how it was done. I know that heat pumps off all verities prefer to operate with large water tanks as heat loads. ASHP work with much higher COP when operating at part load (ie. 3kW from a 8kW system)

Also how would the system know what the heat loading from the CH system is? Essentially this is a system of two halves with the thermal store in the centre acting as a boiler/thermal dump.

I was hoping to fit up the system myself as with a vented thermal store self fit is possible.

Any help much appreciated.

Cheers

Andy

ASHP control - how is it done

jamesingram — Tue, 06 Nov 2007 13:05:15 +0000

I presume you have no access to mains gas?

GSHP work using a weather compensation system adjusting the flow temperature,
to counteract the heat loss of the house, which varies with outside temperature.
with the system sized to suit the house and its heat loss
perhap ASHP have a similiar set up?

ASHP control - how is it done

andytk — Tue, 06 Nov 2007 13:16:08 +0000

in most cases access to mains gas would be too expensive to be worth bothering with, or would be technically impossible.

Most of the smallish houses we've been looking at wouldn't have enough ground for GSHP, hence the interest in ASHP.

possibly ASHP work the same way, but then the pump would have to know the heat loading. I know that the cheaper 3kW and 5kW don't seem to have such an input (just the tank temp and timer) but maybe the bigger 8kW units do.

Cheers

Andy

ASHP control - how is it done

tony — Tue, 06 Nov 2007 13:27:03 +0000

Why bother with a thermal store? Why not use air to air heat pumps two or three would do you costing less than 2k for 3 pluss HW how about solar HW?

ASHP control - how is it done

Paul in Montreal — Tue, 06 Nov 2007 14:10:14 +0000

Posted By: jamesingramGSHP work using a weather compensation system adjusting the flow temperature, to counteract the heat loss of the house, which varies with outside temperature.
with the system sized to suit the house and its heat loss
perhap ASHP have a similiar set up?

I must be missing something obvious here since it seems you guys are making a simple problem rather complicated.

I have a GSHP and it doesn't have any "weather compensation" system. The GSHP itself has four modes of operation:

(1) Off
(2) Full-on heating (no auxiliary heat)
(3) Full-on heating (plus 10kw auxiliary heat)
(4) Full-on cooling

How does if "know" what to do? Simple, we have a two-stage heating one-stage cooling thermostat. In heating mode, if the thermostat calls for heat (house is losing heat and therefore getting colder), mode (2) kicks in. The GSHP continues to run until the thermostat is satisfied (it measures the rate of heat rise and overshoots by a pre-set amount) and then the GSHP turns off. If the rate of heat rise is insufficient or the thermostat is not satisfied, then it switches to mode (3) and turns on the 10kw auxiliary heating. In our case, this only starts to occur when it's below -15C outside - but is really determined by the rate of heat loss of the house. After a while, the thermostat switches back to mode (2). In cooling mode, either it is full-on cooling and will run until the thermostat is satisfied, or it is fully off. In cooling mode, the run-time is also affected by the humidity since removing latent heat from the air doesn't lower the temperature.

An ASHP should use a similar setup. If it has more than one power output, then a multi-stage thermostat should be used - it should run in "low" initially and switch to "high" if the thermostat can't be satisfied in low mode.

If an intermediate thermal store is used, the same multi-stage thermostats can be used but these would be based on tank temperature and rate of change. This is directly proportional to the heat loss of your house (or hot water usage) and so should work in the same way. I have a friend with a water-to-water GSHP that uses a thermal store (and then a fan-coil air handler). He just uses a regular thermostat based on the air temperature of the house. In more (3) his auxiliary heat is an oil boiler which turns on if the heatloss requires it.

Hope this helps,

Paul.

ASHP control - how is it done

andytk — Tue, 06 Nov 2007 14:26:59 +0000

If I didnâ€™t have a thermal store then Iâ€™ll still need a hot water tank. Seeing as a thermal store is a hot water tank I canâ€™t get away without one. Plus if I went solar HW later on Iâ€™m going to need a thermal store anyway. Might as well have one from the start. I was also under the impression that any air to water heat pump system needed a thermal load/dump to operate.

Although the houses are small theyâ€™re still 2 bedrooms. So to heat them with ai to air AHSP Iâ€™d need 6 hot air units in the house (2 bed+bathroom+2 in living area+kitchen)

Having just 3 units and then having to suffer drafts as the air is circulated doesnâ€™t appeal.

Also from a resale-ability standpoint, people expect to see radiators not A/C units and I think it would put a lot of people off. You might be able to convince buyers of the benefits of underfloor heating but not random A/C units blowing out hot air in every room.

As an aside can I really buy an air to air heat pump for Â£650??

Andy

ASHP control - how is it done

tony — Tue, 06 Nov 2007 14:37:32 +0000

installed for 650 even!

ASHP control - how is it done

jamesingram — Tue, 06 Nov 2007 14:42:37 +0000

Paul

your system seems simple and effective
as to why some GSHP in the UK(swedish kit) uses weather compensation i'm not sure
its all built in the pump , it has a thermostat for outside temperature and sometimes one inside the house
your right it does seem over complicated, perhap its to do with efficency, i dont know
but Canadas got lots more experience in this field than the UK

andy I thought the thermal store/accumluator tank was to improve the effiency of the ASHP so as its not switching on and of all the time

Jim

ASHP control - how is it done

Chris Wardle — Tue, 06 Nov 2007 15:29:58 +0000

I have just had one of these fitted to a shop:-

http://www.cooleasy.co.uk/product-wall.htm

It is sold as an air conditioning product but is actually an air to air heat pump. I think this might be the kind of thing Tony could be referring to? Can be had from Â£399 delivered each. I can't comment on how effective they are as it has only just gone in. They certainly seem like a bargain and the customer service is good (they delivered a new compressor next day when we reported a fault with the original one).

ASHP control - how is it done

Peter A — Tue, 06 Nov 2007 18:30:42 +0000

Chris, what was the COP of your Â£399 air to air heat pump?

Andytk, don't rely on solar thermal for your hot water, at best they will only do 60% of your needs, that means a big hit on your immersion. If you can run to one check out the NIBE fighters (ASHP with exhaust heat recovery), downside is that they are air to water, but they do have an integral thermal store, failing that UNICO do an air to air, but you still have to produce some hot water. Make sure you are on a good elect. tarif when the 9kW kicks in it might a well be on low tarif.

ASHP control - how is it done

Chris Wardle — Tue, 06 Nov 2007 20:34:25 +0000

COP is 3 out for 1 in, so they say, and it should heat a 25m2 room. They do bigger ones as well.

Doesn't the COP get down to about 2 out for 1 in when you're heating to 60degrees for hot water? So if you can get 60% from the solar and the rest with the immersion and you are only getting 2 to 1from the ASHP, there is nothing much to choose between them overall, unless you can afford the solar and the ASHP. Even then, there might be better uses of the money. how much does one of the NIBE Fighters set you back Peter?

ASHP control - how is it done

Peter A — Tue, 06 Nov 2007 20:56:32 +0000

Chris, they do quite a few models, from memory Â£4K will get you the fighter 410 which is all singing inc heat recovery, but on top of that there is a fair bit of ducting required. On the plus side they are a fit and forget technology, if you're still in the house 25yrs on you may need to think about repacing. They do say that COP is more like 6 to 1 for heating and several hundred have been fitted for an HA in Leicester. Not cheap, but when I last looked at them it was for new build where the cost would be offset against what they would be replacing, at the time the problem was that the local planners didn't consider them a renewable technology, they did GSHP's which was strange as they are both the same technology, just differing efficiency, perhaps I might start another thread on that one!

ASHP control - how is it done

Paul in Montreal — Tue, 06 Nov 2007 21:06:46 +0000

Posted By: Peter AThey do say that COP is more like 6 to 1 for heating

No way. Unless they're quoting unreasonably high outside air temperatures - not much point in heating if the outside air temp is already 21C (where perhaps such a high COP could be had).

Paul.

ASHP control - how is it done

Peter A — Tue, 06 Nov 2007 23:12:08 +0000

I did say they were quoting 6 to 1, with several hundred installed I am waiting to see proof. I was an unbeliever but given the choice I would install one in a new build for myself as part of a package of cost effective measures.

ASHP control - how is it done

Paul in Montreal — Wed, 07 Nov 2007 02:03:54 +0000

After some googling and reading of specs, the NIBE Fighter 410 is actually a MHRV which has a heatpump on the exhaust which is used to heat a hot water cylinder. The compressor is rated at 650W but they don't quote any COP figures. Looking at other NIBE air-to-water heatpumps, the COP is a more reasonable 3.5ish at 7C outside air temperature.

All that said, the Fighter 410 sounds quite interesting in that it does recover the heat from the exhaust air in a way that's usable (i.e. putting it into a tank of hot water) but would probably not do a good job of heating except in passivhaus kind of applications where the load is pretty low anyway. In my cold climate a regular HRV in winter recovers all of the latent heat in the exhaust air because the incoming air is so cold so a heatpump recovery would not be of any benefit - and we're not to passivhaus standards anyway (except for air tightness).

It's quite interesting to view these discussions from this side of the pond where air conditioners and heatpumps are very commonplace - but hydronic heating systems are very rare. When I first moved here it seemed odd that no-one had boilers and rads, but now it seems odd that people would as it's hard to imagine how a place would be kept cool and dry when the temperatures stay above 25C at night with high humidity. My Northern English ancestors would think I've turned soft

Paul in Montreal.