air to water heat pump

kerryplumber

good afternoon people,
not sure if I should be putting this in renewables or here. plumbing a house at the moment which is now first fixed by me. origionally speced for gas system boiler, 300 litre cylinder, steel rads and solar. now the customer has decided that they want to install air to water heat pump.

I have two worries really,
NO 1 : how well would a2w work with alumium rads all thru the house ( first fixed so no chance now of using ufh)

NO 2 : company 1 is suggesting using a2w hp and no back up system.

company 2 is suggesting a2w with back up gas boiler.

option 1: is replacing gas boiler with a2w, removing solar and keeping 300 litre tank

option 2 : remove solar, remove 300 L tank, changing boiler to one wit 54 litre internal storage. therefore gas doing hot water needs and a2w doing heating

option 3 : remove solar, keep 300L tank, keep system boiler and add a2w hp

im new to a2w so would really like to get it right, since it looks like it will be popular,

all opinions extremely welcome.

shane 007

IMO, A2W is only really suitable for UFH. Once you take the flow temperature above 35C with A2W, the efficiency/COP nose dives for prolonged periods of use.

Either way, a back up heat source with A2W is always a good thing due to the times when we get below our system design outdoor temperatures.

kerryplumber

shane 007 wrote: »

IMO, A2W is only really suitable for UFH. Once you take the flow temperature above 35C with A2W, the efficiency/COP nose dives for prolonged periods of use.

Either way, a back up heat source with A2W is always a good thing due to the times when we get below our system design outdoor temperatures.

ya, I agree shane. I am very worried about the customer being talked into going with a2w alone without a back up. and I don't know enough about a2w to properly challenge that company. hence why I brought on board the second company that wants a back up.

air

Is mains gas available to run the boiler?
If so I'd be inclined to ditch the a2w heat pump given the lack of underfloor.

kerryplumber

air wrote: »

Is mains gas available to run the boiler?
If so I'd be inclined to ditch the a2w heat pump given the lack of underfloor.

No, it will be LPG.

kerryplumber

air wrote: »

Is mains gas available to run the boiler?
If so I'd be inclined to ditch the a2w heat pump given the lack of underfloor.

No, it will be LPG.

froshtyv

Have you looked into a high temperature air to water heat pump?

http://www.daikin.ie/for-your-home/needs/heating/renovate-replace-your-oil-and-gas-burner/daikin-altherma-high-temperature/index.jsp

Tommyboy08

froshtyv wrote: »

Have you looked into a high temperature air to water heat pump?

http://www.daikin.ie/for-your-home/needs/heating/renovate-replace-your-oil-and-gas-burner/daikin-altherma-high-temperature/index.jsp

That's not gonna help, I don't have all the figures but at 65C the cop is 2.4, so you can only imagine it's well below 2 at 80c

air

To be honest if you're putting in LPG anyway I can't see the capital costs of the A2W unit ever being recovered.

A2W units have 2 huge disadvantages versus ground source heat pumps:

1. Their input temperature is not stable and is lowest when you need the most heat - this means the average COP is always going to be lower than a ground source unit (apart from perhaps days with marginal heat requirements - but then you don't use much heat those days anyway)
2. They perform best during the day - when air temperatures are highest, which is when electricity is at it's most expensive, and furthermore is generally not when people want heat - unless they work from home or don't work

Further small disadvantages are:
1. The fans are likely to use more energy than the circulation pump of a ground source unit.
2. They are by their very nature out in the elements so don't last very long - hence why manufacturers are keen, they see potential for repeat sales. I'd only consider one under some kind of open shelter.

Personally, if ground source is not an option, I would advise the client to go with LPG and a small solar thermal installation, together with some grid tied PV if they have the inclination to spend a few more euro.

froshtyv

Tommyboy08 wrote: »

That's not gonna help, I don't have all the figures but at 65C the cop is 2.4, so you can only imagine it's well below 2 at 80c

Who said its not going to help?
It allows the OP to keep steel rads and a standard cylinder if required and do all heating and hot water from a heat pump.

The OP has said that the pipe centres are already in so this rules out sizing aluminium radiators correctly for use with a low temperature heat pump.

Im not sure what size unit you are basing that COP of 2.4 on?

From the data sheets i was looking at the 16kW model A7 W65 has a COP of 2.87.

Yes its low but its still better than an oil boiler will ever be.

You would have to do the figures regarding pay back but its still an option.

air

froshtyv wrote: »

From the data sheets i was looking at the 16kW model A7 W65 has a COP of 2.87.

Yes its low but its still better than an oil boiler will ever be.

You would have to do the figures regarding pay back but its still an option.

What input air and output water temperature is the COP 2.87 do you know?

As regards payback, I'd be dubious enough on any kind of return on investment when you look at the expected lifetime of an ASHP (<8 years from anecdotal evidence I've heard) and the labour required to install a new one.

Condenser

froshtyv wrote: »

Who said its not going to help?
It allows the OP to keep steel rads and a standard cylinder if required and do all heating and hot water from a heat pump.

The OP has said that the pipe centres are already in so this rules out sizing aluminium radiators correctly for use with a low temperature heat pump.

Im not sure what size unit you are basing that COP of 2.4 on?

From the data sheets i was looking at the 16kW model A7 W65 has a COP of 2.87.

Yes its low but its still better than an oil boiler will ever be.

You would have to do the figures regarding pay back but its still an option.

An oil boiler would give you an output equivalent to a COP of about 2.1 so its hardly a world beater. Fingers crossed the temp doesn't drop below 7 then!

Condenser

kerryplumber wrote: »

good afternoon people,
not sure if I should be putting this in renewables or here. plumbing a house at the moment which is now first fixed by me. origionally speced for gas system boiler, 300 litre cylinder, steel rads and solar. now the customer has decided that they want to install air to water heat pump.

I have two worries really,
NO 1 : how well would a2w work with alumium rads all thru the house ( first fixed so no chance now of using ufh)

NO 2 : company 1 is suggesting using a2w hp and no back up system.

company 2 is suggesting a2w with back up gas boiler.

option 1: is replacing gas boiler with a2w, removing solar and keeping 300 litre tank

option 2 : remove solar, remove 300 L tank, changing boiler to one wit 54 litre internal storage. therefore gas doing hot water needs and a2w doing heating

option 3 : remove solar, keep 300L tank, keep system boiler and add a2w hp

im new to a2w so would really like to get it right, since it looks like it will be popular,

all opinions extremely welcome.

Option 4: use oil or gas because the application is all wrong for a heat pump especially an air to water. Square pegs in round holes and all that.

air

I should have clarified above that I would only install ground source with underfloor, which isn't really an option here.
Condenser, can you advise what type of usage pattern or installation scenario you see ASHPs being suitable for domestic installations?

Condenser

air wrote: »

I should have clarified above that I would only install ground source with underfloor, which isn't really an option here.
Condenser, can you advise what type of usage pattern or installation scenario you see ASHPs being suitable for domestic installations?

The cheaper air to water units should really be only installed in small low energy properties. The bills can really spiral in large properties if you get a harsh winter. Top end air source really should play second fiddle to ground source everywhere bar where its not possible to fit either a horizontal or vertical collector which is rare.
I don't see ASHP's taking off in housing developments however as the noise from multiple units running at once in a small area would be quite intrusive.

Also, one thing thats often overlooked with ASHP's is that its not always when it coldest that they're at their least efficient, its often in the 4-8C range as the gas in the evaporator will be below zero in order to gather energy but the air will often be heavy with moisture which causes rapid icing and very regular defrosts. And as you mentioned the cheaper units have a very limited lifetime.

air

Thanks for that Condenser, sounds like they're pants basically!

Interesting info on the efficiency in the 4-8C range also, especially as that's a very common winter temperature range here, not to mention we have massive humidity.

Does relative humidity tends to be lower here sub 4C?
Absolute max humidity is only approx 30% lower at 0C than it is at 4C for example so I would have expected similar issues with evaporator icing.

Condenser

air wrote: »

Thanks for that Condenser, sounds like they're pants basically!

Interesting info on the efficiency in the 4-8C range also, especially as that's a very common winter temperature range here, not to mention we have massive humidity.

Does relative humidity tends to be lower here sub 4C?
Absolute max humidity is only approx 30% lower at 0C than it is at 4C for example so I would have expected similar issues with evaporator icing.

The colder the air gets the less moisture it can hold hence the icing issue gets less but yes its still an issue down to about -5C.

You'll end up buying a cheaper ASHP twice if not three times (especially if near the sea) inside the life you would get from a properly installed ground source. Top end units should get about 3/4 the lifetime of a GSHP but cost more in the first place.

You also have more parts such as fan motors, invertors (generally), reversing valves for defrost so theres more to go wrong.

Condenser

Its no coincidence the boom in ASHP has coincided with the recession. They've been around as long as GSHP's.

Condenser

Double post

air

Condenser wrote: »

The colder the air gets the less moisture it can hold hence the icing issue gets less but yes its still an issue down to about -5C.

Useful table here to help illustrate this point:
http://www.tis-gdv.de/tis_e/misc/klima.htm

Thanks again, I think you've done a good job of explaining why they're pretty much a waste of time and money.

sangrace2004

hi there quick question seen as you seem to be very up on this. building house at mo, got BER done. option 1 oil, log burning stove with backboiler and solar panels OR log burning stove with back boiler and air to water. We would like to get the air to water but dont know any1 with it that doesnt have underfloor....so confused. The house will be airtight and insulated as per the BER i.e. every inch of it!! Thinking vents in the walls...help?

sangrace2004

sorry its a dormer and 2200 sq feet