Air Source Heat Pump

[crusty]

Hello

I am just looking into this now.

We currently have 2 rooms using wet underfloor heating, approx. 35 sq metres each.

We are using a gas combi to heat currently.

I want to add another room approx. 25m2

Does it make sense to heat these rooms with an Air Source Heat Pump? and keep the boiler for hot water and rads in bedrooms

I have seen them listed on ebay for about £1000, so would like some idea on return on investment timescales, and whether this is DIYable or not

[guindilias]

If you heat all three rooms via the ASHP, you are looking at around a 6kW pump minimum for the underfloor. I would go for a 10kW to reduce the defrost time in cold weather, which puts you around £10k for a Worcester one, which we have been using on a couple of recent jobs with great success.
As always, add the extra for installation, as much insulation as you can get packed in, and your underfloor system itself (in the extension).
Is it worth it? Up to you...

[crusty]

Thanks for the response.

I am trying to work out disparity of costs, you say I am looking at around £10K, yet I can get one off ebay, fitted, for about £1K

For example http://www.ebay.co.uk/itm/Air-to-Air-Source-Heat-P...

What am I missing?

[guindilias]

That's an air to air unit - it'll blow out hot or cold air from an indoor unit - same as you see in offices. You want an air to water unit - and that's only rated up to 4kW for the price, you're looking at needing twice that!

[crusty]

Ok, that makes sense

Would something like this work?

http://www.ebay.co.uk/itm/NEW-HOME-AIR-SOURCE-AIR-...

[GetCarter]

JD said:

Why not Ground Source?

Ground Source (which I have) are more efficient, but much more money up front.

[guindilias]

crusty said:

Ok, that makes sense

Would something like this work?

http://www.ebay.co.uk/itm/NEW-HOME-AIR-SOURCE-AIR-...

It might do - I have never heard of the brand "Dream", so it could be useless Chinese trash. Or, it might work fine. I only spec ASHP from well-known manufacturers, as they are always available to give advice and information.
For instance, who fixes it when it breaks down? Is the CoP officially measured and at what external temperature?
Panasonic, Ariston, etc. will guarantee their stuff and send out a qualified engineer to fix them if and when they break down - who fixes these, and are parts available off the shelf?

Think of it as the difference between a nice reliable Mondeo, and a freshly painted up Lada Riva - you could of course buy 10 of them, and chuck them out as they fail, for the same price - but there is a reason that brand names are many times that price.
And, a life expectancy of 20 years? I wouldn't even expect a Mitsubishi to last that long, even wen serviced properly. I mean hell, are they saying even the brushes on the motor will last 20 years?
I have a Miele washing machine and dryer, and the brushes have lasted 20 years - but they don't run for 15h a day...

[guindilias]

GetCarter said:

Ground Source (which I have) are more efficient, but much more money up front.

GSHP I love and adore. But you need enough land to bury your slinkies, or boreholes drilled - I had a client who spent megabucks having 18 boreholes drilled, lined, piped up - and ended up with about 1kW from each one, which was largely lost in transmission. He gave up.
I have another client with a huge field, who buried slinkies over the whole place - he could pretty much heat a palace. He is a farmer, so it was no issue for him to dig and backfill. He heats the house for pennies!

[peterpeter]

this is fine but surely a 6kw ASHP is going to give you some killer leccy bills?

[guindilias]

6kW is it's maximum output - what you need to look at for the bills is the Coefficient of Performance (at a given temperature).
So say for example it has a genuine CoP of 4.0 at 10 degrees, it'll only be using 1.5kW of electricity, but putting out 6kW of heat if it's on full blast. The same applies for ground sourced heat pumps.

Edited by guindilias on Friday 23 October 15:55

[Zoon]

Harry H said:

We used to be quiet good friends.

Deliberate faux pas?

[The Moose]

Of you with ASHPs, who had them designed by an engineer? Was it worth it?

We're looking at having circa 4,000sqft over 2 floors heated (underfloor heating only). Hot water for 6 bath/shower rooms and 2 w/cs and heated towel rails in the bathrooms.

Sounds not insignificant in terms of the type of system we'll need!

[The Moose]

Anyone used an engineer?!

[944fan]

Not sure if it has been mentioned but if your ASHP is reversible (for cooling) then it wont be eligible for RHI.

At least that was the case back when I was working in the industry a couple of years back.

[guindilias]

The Moose said:

Anyone used an engineer?!

I'm an engineer, and also design UFH/ASHP/GSHP/biomass systems. It's easy stuff, allow 50w/m squared for a modern, well insulated building. 150w/m2 for an older building. Overspec what that gives you as the necessary output by about 50% for your heat pump, and have the rads/UFH sized to suit.
That's what I do for a quick estimate, anything further than that you need to get into heat loss calculations and the like - which you need someone to do for you.
But you can never oversize a heat pump, nor the actual heating element, they can always be turned down.
It's all down to how confident you are about how much heat you need.
In the old days, even with oil or gas fired boilers, we'd work out the exact need - then add 50%, then get the next size up boiler from what they had.
We didn't have the computers to calculate exactly what we needed then, so it was a case of "Brand new? - call it 50w/m2, that makes 75w/m2, and the next boiler would be able to give 100w/m/2 - ring the manufacturer and see when he can deliver"
Nowadays it is a good bit more complicated, but I can not stress enough - if you can afford it, oversize your heat pump. Particularly air sourced. They just love to freeze up and spend 20 minutes defrosting, in the middle of winter, when you need the heat the most!

[bob-lad]

As I undersantd it, ASHPs don't heat the water to very high temperatures, which would be fine it wasn’t for a bacteria called Legionella. This bacteria breeds in warm water and if inhaled (in the tiny mist droplets you get in a shower for example), can cause a potentially fatal disease called Legionella disease.

You need to kill off this bug, which is possible by increasing the temperature of the water to over 60C. So you'll need an immersion heater or something to boost the water temperature.

Just a thought.

[bob-lad]

As I understand it, ASHPs don't heat the water to very high temperatures, which would be fine it wasn’t for a bacteria called Legionella. This bacteria breeds in warm water and if inhaled (in the tiny mist droplets you get in a shower for example), can cause a potentially fatal disease called Legionella disease.

You need to kill off this bug, which is possible by increasing the temperature of the water to over 60C. So you'll need an immersion heater or something to boost the water temperature.

Just a thought.

[skeggysteve]

Our system heats the water to 75 (I think) degrees once a week to kill any bugs.

To add to my earlier post our system is coping fine with the cold weather.
We pay our electric bill based on meter reading once a month and every time when we submit the readings we get asked 'are you sure the reading is correct'.
Last month the electric company asked us to send photos of the reading!