AIR Source heat pump calculator.

[sooty~surfer]

Hi.
I have a biomass heating system that is made of the following;
20kw log boiler
1020 ltr solus 11 tank
6m2 of water panels on the roof

it runs

3 rads (smallish)
2 towel rails
and approx 67 m2 of wet under floor heating pipes (set 100mm apart) in 5 zones

i want to install an air source heat pump and need to know

1; what size in kw i.e 18kw
2; should i connect it directly to the accumulator to charge the tank only ever achieving 55 degrees or
3; connect directly to underfloor heating and/or to rads as well.

Also i have space to run the unit in the plant room if running it in a warmer environment would work or do they have to be outside?

Just to help the house is timber frame with an insulation value of .21 so a constant low sustained heat would be preferable as we can up the air temp with a 7kw wood burner.

hope someone can help as i am really struggling for info/installers in cornwall (business oportunity for someone?)

regards

nick

[Bigfreeze]

Hi.
I have a biomass heating system that is made of the following;
20kw log boiler
1020 ltr solus 11 tank
6m2 of water panels on the roof

it runs

3 rads (smallish)
2 towel rails
and approx 67 m2 of wet under floor heating pipes (set 100mm apart) in 5 zones

i want to install an air source heat pump and need to know

1; what size in kw i.e 18kw
2; should i connect it directly to the accumulator to charge the tank only ever achieving 55 degrees or
3; connect directly to underfloor heating and/or to rads as well.

Also i have space to run the unit in the plant room if running it in a warmer environment would work or do they have to be outside?

Just to help the house is timber frame with an insulation value of .21 so a constant low sustained heat would be preferable as we can up the air temp with a 7kw wood burner.

hope someone can help as i am really struggling for info/installers in cornwall (business oportunity for someone?)

regards

nick

Whats the total area of the house?

[sooty~surfer]

Hi the total floor area of the house is 210m2
1/2 of that i.e downstairs is the wet underfloor at 105m2
all upstairs is thew 3 rads (yet to buy and fit) and 2 towel rails
thanks nick

[MikeHolm]

Oversize the rads by 100% (I'm guessing at this as i don't know the heat loss of the house) so they will run at 40C. If you can do this the heat pump will be a good choice.

[Bigfreeze]

At that level of insulation roughly 12kw should cover your max load. When mixing rad's and underfloor then always use a buffer tank

[mad fridgie]

At that level of insulation roughly 12kw should cover your max load. When mixing rad's and underfloor then always use a buffer tank
How does the client choose a heat pump at this size.
Is this just a nominal rating at some pre-determined condition. If so what figure should he use.
(Not a trick question BF)

[Bigfreeze]

How does the client choose a heat pump at this size.
Is this just a nominal rating at some pre-determined condition. If so what figure should he use.
(Not a trick question BF)

Obviously it's not 100% accurate as I've not seen the full plans and insulation levels but from experience I know an air to water of between
10-12kw is What's required on a house of this size at a u-value of .21. With a location such as Cornwall.
A house with that u-value requires about 35w per sq m with a ground source unit. 210x35=7030. Allow 2kw for hot water production and 20% because its an air to water unit out put will drop and thats how you arrive at the figure.

[mad fridgie]

Hi BF,

Yes each house will have its own specifics. But once that is established, what ambient temperature do you pick for the heat load selection,
For example (not real numbers) we have temp data for most places, this data is broken down into '%'
95% of the time the ambient is above -2C,, then 98% the ambient temp is above -4C (3% of the time is between -2C and -4C) then we have a minimum temp reached -12C.
So in this case I would choose -2C as my base, lets say this calculated to 10Kw (or total loss 240Kwhrs/day)
This what the house requires, regardless of the heat source.
I would then choose how long i want the heating source to run, at these conditions. '240/runhrs= heating size'
Now in the case of an Air Source heat pump, because I have chosen -2C as my design, I will have derate, the heat pump sizing a, because of the lower ambient and the influence of the defrost cycle. (of course the water temp for the application also has an influence, but in my case I use for underfloor, so not dramatic changes)
I am fortunate that I am able to work out, how machines will operate/perform away from standard nominal designs figures given.'A2-W35'
How do approach this?
And is this lack of understanding causing many of the issues we see with ASHPs?

[MikeHolm]

Some older compatriots over here would stand across the road, put one thumb up and if it covers the front of the house....8kw, 2 thumbs....10kw and so on. It's easy but i think you are on to something. Maybe I should tell them to change their ways.:D

[taz24]

.

Sizing a heat pump this way is the wrong thing to do.

You need to size it for the house and to do that you need to know
the heat losses through walls due to insulation.

Also normal temps have downstairs at 21 deg C and sleeping areas at 19 deg C
so that needs to be factered in also. The radiators for upstairs need to be high
efficiency and almost certainly double paneled to alow for the lower delivery temps.

Also if you are using biomass fuel and solar panals you would be advised to upgrade
your hot water storage cylinder to cater for the 3 seperate heating coils.

As has been advised earlier you should also think about installing a buffer tank in the
circuit.

Will you be doing the work yourself or will you be employing someone to do it for you?

If you employ someone they should do a full survey of the house, sample the wall insulation
and callculate exact heat loads on the property.

If you do not do it correctly now, don't be suprised if you have problems later on or
if you do not get the efficiency returns you are expecting.

A few hundred quid spent now on doing a correct survey will save you a whole load of grief.

Also if you want the thing to conform to MCS accreditation then it will have to be sized and
installed by authorised installers.

All the best

taz

.

[MikeHolm]

All joking aside, I would find someone like BigFreeze who could integrate a coil from the Solus tank (which is non pressurised, IIRC) into the evap of the HP. For this I don't think he could use a Japanese (Mitz, Daikin, Fujitsu etc) but that would be up to him.

[knighty]

do you take your hot water direct from the thermal store via an upper coil ?

and is your heating via a coil or are you pumping the store water ?


when/why do you want to use the ASHP ?

is the wood burner not putting out enough heat ?
if it for when you run out of wood ?
because it'll be more convenient than the wood burner ?
you want to use it in light conditions when it's not worth running the wood burner ?


what type are the solar panels ? with the right number of the right panels (evacuated tubes) setup correctly they should provide you with all your heat/hot water for 8 months of the year :-)

[Bigfreeze]

Hi BF,

Yes each house will have its own specifics. But once that is established, what ambient temperature do you pick for the heat load selection,
For example (not real numbers) we have temp data for most places, this data is broken down into '%'
95% of the time the ambient is above -2C,, then 98% the ambient temp is above -4C (3% of the time is between -2C and -4C) then we have a minimum temp reached -12C.
So in this case I would choose -2C as my base, lets say this calculated to 10Kw (or total loss 240Kwhrs/day)
This what the house requires, regardless of the heat source.
I would then choose how long i want the heating source to run, at these conditions. '240/runhrs= heating size'
Now in the case of an Air Source heat pump, because I have chosen -2C as my design, I will have derate, the heat pump sizing a, because of the lower ambient and the influence of the defrost cycle. (of course the water temp for the application also has an influence, but in my case I use for underfloor, so not dramatic changes)
I am fortunate that I am able to work out, how machines will operate/perform away from standard nominal designs figures given.'A2-W35'
How do approach this?
And is this lack of understanding causing many of the issues we see with ASHPs?

Hi MF,

As you said, it depends on the the weather conditions at any given location. A house in Cornwall would have different design specs than a house in Inverness. I tend to look at the mean winter temps and allow about 5-7C below this temp to build in some tolerence. The HP should be able to cover this heatloss for the day in a 12hr run period. This ensures that its not too oversized and will have a decent run period in even mild weather. It also allows enough tolerence to be able to cover the heatlosses in extreme weather due to the fact it can run an additional 12hrs in order to do so.
Air Source units are so variable its impossible to size 100% correctly as one day they're correct and next week they're vastly over sized or undersized. Invertors can overcome this but not the way they're being employed by the big name A/C manufacturers.

I would say most people installing heat pumps have done nothing more than a generic calculation as to what the house may need. I know of one plumber not far from me who installs a 15kw heat pump regardless of house size or insulation because "it doesn't matter that much if you put in a 20kw boiler or a 25kw and shur isn't it the same for heat pumps"

[sooty~surfer]

The tank has 9 ports. 7 being used . top hot water. middle underfloor and rads. lower and upper is inlet/outlet for gasification boiler via a laddermat pump. 7kw(5kw nominal) woodburner in open plan house is just enough in milder weather but finding colder weather the underfloor (no rads installed yet) and towel rails hammer the solus 2 tank. so the plan is to put the underfloor and rads on the ASHP so i can leave the stats looking after the house when im out. The reason for wanting ASHP is that although trying to do the green thing i install woodstoves for a living and the excitement of heating my house with biomass is wearing off. so ASHP will keep it toasty and i only have to use the Wood boiler to heat water for showers etc. 1 burn raises 1020 litres from 35 degrees to 65-80 degrees (i actually burn pallets) and lasts 3-4 days before recharge unless summer then 3-4 week, as solars keep it topped up. the house has been built in the last 2 years. we were meant to achieve at least .27 value but upgraded to .21 to stay ahead of the game so to speak. Warm roof but very open plan so the rads will keep the higher level warm keeping the ground floor at a better ambient temp. After reading the expensive running costs and poor performance of undersized ASHP i want to get it right. We put a plant room on the back of the garage so we have space to put a further buffer tank. Also wondering if it would be advisable to run the ASHP inside the building to get better winter efficiency? Sorry for the dragged out explanation but hoping this clarifies the requirements. nick

[MikeHolm]

What is the water temp currently going to the floors? Is it weather compensated with a mixing valve? I like this idea because I am designing the heat pump for my house and it will have a 900L open storage tank with coils for DHW, floors and rads. Inputs are from a wood fireplace with back boiler, solar (10m2) and the heat pump (with possible back up by my current gas boiler).

[Bigfreeze]

If I was you I'd use the ASHP just for the UFH. This would keep temp in the house while you're out during the day and maximise the performance of the ASHP. Then use the woodpellet to raise the buffer temp in evening for hot water, rads and ufh. That way will get the best performance possible from all facets of the system. Running for just hot water on the pellet burner would be too short a burn to get it burning efficiently. I'd imagine you'd be mucking out the ashes every day.

It would also mean a smaller heat pump.

[sooty~surfer]

You have more m2 on solar we use approx 6m2 on flat panels (not tubes) i have adjustment on UFH from 25-65 or max degrees. The Hot water has a blender valve from tank. I like the idea of direct UFH from ASHP and topping up with wood boiler . (20kw SHT (from euroheat uk) pallets and timber not pellets) wood burner back boiler is good as you are heating airspace as well as water.

[sooty~surfer]

Just want a ASHP that will fill the gap for underfloor while out. just want air heat and not worried about hot water as the tank is fine for week to week usage. thinking 18kw is the one.

[MikeHolm]

Yes, I use flat panels. Tubes don't last as long as flat panels and need more maintenance unless the system is really undersized for the load. Unless you NEED to heat water to 90C there is no need for tubes anyway.

BF is right, of course, that the ASHP be used for UFH alone. And I would get a weather compensation control for the floor heat, it is worth the money.

[Bigfreeze]

Just want a ASHP that will fill the gap for underfloor while out. just want air heat and not worried about hot water as the tank is fine for week to week usage. thinking 18kw is the one.

You've already been told that 18kw is far too big.

[knighty]

ufh is perfect for heat pumps, especially when you won't want any hot water from it at all - the lower running temps will make it much more efficient

but it seams a but strange to install a second buffer tank when you already have your nice big thermal store...

it would also be a bit tricky to plum the heat pump into the ufh without making things overly complicated - simple is always best in a system like this

if I were you I'd plum the heat pump in just before the ufh manifolds - that way the water for the ufh will be pulled through the lower coil in your tank then through the heat pump heat exchanger to boost the temperature, sent around the ufh and then back through your return manifold, through the lower tank coil and back to the heat pump

the big advantage there is you don't need the extra buffer tank or pumps, and the plumbing is very simple, and you can set the heat pump to run via thermostat so it runs completely automatically coming on whenever your ufh water temp falls below x'C

you're hardly going to loose any heat to the thermal store at all (if any) as the ufh coil will never really be cold - it will just be a couple of 'C colder than you need - the heat pump will only really be keeping up with heat loss

and the stratification in your tank will mean you're only ever heating the very bottom of the tank

this also gives you a lot more flexibility when you're buying the heat pump.... the massive thermal store makes an excellent buffer tank to stop the heat pump short cycling

how long does the wood burner burn for to bring the tank up to temp ? (with the ufh running) ?


from what I've read, you really want to aim for 2kw minimum with a heat pump, if that's too big you make the buffer tank bigger

and 2kw should be more than enough to meet your heating demand if a 5kw wood burner can heat your tank in a few hours

the navitron forum...
http://www.navitron.org.uk/forum/index.php?action=unread

is a great place if you want to talk to other people with thermal stores/wood burners/heat pumps/solar all combined into one :-)

[MikeHolm]

The ASHP should take its heat from the tank directly and there is no coil in the tank for the HP. The only coils are for solar in (at the bottom) and DHW out at the top.

If you read some other posts there are a lot of systems, typically inverter driven that feed directly to the floor without a buffer many of which have defrosting and flow issues. The system must be designed to meet the HPs flow rate needs if you do not use a buffer or large low loss header.

I have 10m2 only because i have un-obstructed south facing room on a wall for the panels and at 90deg, I should not get major overheating issues. My entire flat roof is covered with PV panels.

[sooty~surfer]

Knighty thanks for the great advice. Got the plumber round this week and will show the posts to him
Just to clarify i have a free standing 7kw dry wood burner in the main house. The 20 kw log boiler is next to the tank in the garage. But loving the idea of incorporating existing tank as it was a lot of money.
The small ashp sounds great and alot cheaper than i 1st thought as i was looking at the big 10kw+