Hi Guys,

I'm looking at the possibility of installing an air source heat pump (Viessmann AWO 110) in a new build property with a single phase electricity supply and a heat demand of 12kW. The AWO 110 has a heating output of 9.3kW with a power consumption of 3kW, therefore a COP of 3.1 at an operating point of 2 degree's air temperature to 35 degree's water temperature. The property will be inhabited by two occupants and so DHW heat requirement is 0.5kW (presuming there is no solar yield, therefore worse case) which means I need to achieve an output of 12.5kw.

In order to get to the 12.5kW output I'll need to use a 3kW Immersion heater as a weather compensated 2nd heat source in the buffer vessel, and in addition to the heat pump and the solar heating for the DHW (Vitocell B-300 twin coil cylinder) I would like to install another 3kW immersion here in order to activate periodically to eliminate any risk of legionella.

This means I'll have a maximum power requirement of 9kW and as I'm not an electrician I'm concerned the heating system could be pulling too many amps?

If anyone can explain the In's and out's of how much power I can pull from a single phase supply it would be very much appreciated.

Kind regards

Paul

Hi Paul in Montreal,

Sorry, perhaps in all my waffling I made it unclear I intend to use a 3kW immersion in the buffer vessel as a secondary heat source and another in the DHW cylinder, so with the heat pump a total load of 9kw.

The property is in fact quite energy efficient, but it's also quite big at over 250m2 across a single story. The fabric heat losses only came to 5kW and the air change losses to 7kw, however , I did leave some margin for error on the air change losses but understand a heat recovery ventilation system is in the plans so this may be less.

Kind regards

Paul

Hi Paul,

I based the entire build on an air change rate of 1.5 with the maths coming from this website

http://www.plumbingpages.com/featurepages/Heatloss.cfm

I'm confident the fabric losses are on the button at 5kW as U values for the entire build, however wasn't as confident with the air change as I couldn't find a way of taking into account the heat recovery system.

While on the subject, would it be worthwhile directing the heat recovery exhausted air into the air source heat pumps intake??

Thanks for all your help Paul, I'll take a look at the software now.

Paul @, all the heat loss calcs are too much for me so I'll leave you two Pauls to thrash it out.
You did mention using exhaust air, there is an ASHP on the market that utilises exhaust air and has it's own inbuilt cylinder, immersion and programmed to boost every so often to kill the bugs. Product is the Fighter by NIBE, comes in various sizes. From memory the immersion was 9kW and a standard UK supply could cope.

Dnfh,

you're being a little dim You have to register to download the software.

hot2000: http://www.bcd.rncan.gc.ca/software_and_tools/hot2000_e.asp
hot2xp: http://www.bcd.rncan.gc.ca/software_and_tools/hot2xp_e.asp


Here's a list of what hot2000 supports:

HOT2000TM models:

* electric, natural gas, oil, propane and wood space heating systems and domestic hot water systems (DHW)
* space heating and DHW systems from conventional to high-efficiency condensing systems
* air, ground and water source heat pumps
* central air conditioning systems with conventional or economizer controls
* primary and secondary DHW systems, including solar DHW
* inputs of steady state or seasonal efficiencies for heating and cooling equipment

HOT2000TM calculates

* the seasonal efficiency, based on the characteristics of the specified house, for:
space heating systems, including part load curves and on and off cycling; and hot water tanks, including hot water load, standby losses and location
* heating and cooling loads on the basis of house design
* design heat loss rates for space heating system sizing
* fuel consumption and costs

Air Infiltration and Mechanical Ventilation:

* models central ventilation systems, including heat recovery ventilators (HRVs) and fans without heat recovery
* models secondary supply and exhaust fans
* enter actual blower door test results or select one of HOT2000TM's pre-defined airtightness levels
* accounts for stack and wind effects and their interactions using AIM-2 (the Alberta Air Infiltration Model-2)
* accounts for the interaction of air infiltration and the mechanical ventilation

HOT2000TM calculates:

* effective R-value of envelope components, including thermal bridging of the framing materials
* effective R-values of attic ceiling structures, taking into account insulation compression at the eaves
* effective R-value of windows, including glass type, fill type, spacer and framing
* a typical Energy Rating (ER) for each window type
* utilized solar and internal gains
* heat gain arising from:
insolation; and exterior roof and wall colour choices

HOT2000TM models:

* solar gain through windows in eight cardinal directions
* tilted windows, including skylights
* overhangs, taking into account the hourly position of the sun with respect to each window and overhang on a typical day each month
* various levels of thermal mass
* slab-on-grade, crawl space (open, ventilated or closed), basement and walkout foundations, taking into account:
dimensions; resistance and placement of insulation; soil conductivity; water table depth; and weather/climate
* a house as three zones - attic, main floor and foundation, - taking into account the heat transfer between zones

HOT2000TM Reports

* technical report, including monthly tables
* comparison report for comparing results on up to four houses simultaneously
* weather, fuel cost and economic reports

HOT2000TM added features

* English and French versions
* climate data files on 75 Canadian and 200 U.S. centres plus the option to create user-defined weather files
* house templates-predefined files with default values that can be used to fill in commonly used data automatically when creating new house files
* multiple views for editing data
* global editing feature
* fuel costs editing feature for established utility rates
* economics editor for analysing and comparing conservation investment options between two houses
* a graphical user interface for Windows 95 or Windows NT
* a visual tree directory for navigation
* online help files
* context-sensitive help
* defaults
* Metric, Imperial and U.S. units of measure

Paul,
Perfect, thank you, and I'll try not to be so dim another time! Looks very useful.

Dantenz,
Some ASHP people say that because they use inverter-driven HP's they can be run efficiently at part load. But they don't really explain it. Is it like a Triac dimmer, where for half light it only uses half power? If so, apart from the capital expense it would seem you can happily oversize for safety. But I am suspicious of the claims!

inverter driven:

An inverter converts DC to AC. Here is how it works...

AC from supply (single phase) is converted to DC (this is called rectifying).
DC is converted to AC, typically with a variable frequency and voltage. The variable frequency is the most important part.
The variable frequency AC is also typically 3-phase for efficiency.
The variable frequency AC then typically drives a 3 phase motor. The speed of the motor is directly controlled by the frequency of the AC.
The energy loss in the inverter is very little, but the efficiency saving in the motor is great.

I have a variable speed well pump with an inverter does just this.

As for sizing you should size for a little below the "average mean temperature", not the record low, and not the peak low.

Most heat pumps that I am aware of, use weather compensation control. This control method means that the heating flow temperature varies according to the outdoor temperature i.e. warmer outside lower flow temp colder outside increase in flow temp. The flow temp setpoint for a given outdoor temperature is determined by the Heat Curve setting which is usually adjustable. A frequency controlled compressor has no effect on the flow temperature but merely "slows down" and reduces the output when approaching the setpoint temperature. What the compressor is effectivel doing is modulating down so that the output matches a reduced load from the system. This makes for less stop/starts and gives better efficiency.

Posted By: fostertomWhat if the MHRV was of superior quality, designed to condense moisture out of the outgoing air in order to recover its latent as well as sensible heat (not generally available in UK)?

If it has already condensed the moisture, the latent heat has been recovered into the incoming airstream and so the exhaust air is at pretty much ambient (outdoor) temperature and thus there's not much heat to recover anyway compared to just using the outdoor air. The airflow of a MHRV will be a lot less than needed by the ASHP and so it would make very little difference to the overall COP IMHO.

Perhaps they type of ASHP that's used in the MHRV of a passivhaus is worth it - but for a typical "standard" house the heatload is much higher than can be supplied by that kind of system.

Paul in Montreal.

Paul @,

going back to one of your originial questions about your electrical supply being able to cope - there are several factors to consider:

1. You will have an incoming fuse (on the supply side of the meter) which will likely be 80 or 100 Amps

2. Your consumer unit/fuse box may have a master switch and/or RCD which will have a max rated Amperage

3. How you split the required power across different circuits will define what size of cables (and therefore fuses/MCBs) you use

4. Diversity (different circuits being in use at different/the same time) should be taken into account

5. Fuses don't blow as you might think, i.e. a 100A fuse doesn't blow at 101A, and different types of fuse have different characteristics in this regard

6. Your premises will have a specific earthing system and the characteristics of this also needs to be taken into account

All of these factors need to be considered to design the correct electrical system for you needs.

Any good electrician should be prepared to talk you through this and give you guidance on exactly what work will need to be done to get the system working safely.