I would speak to Peter Warm for advice about PH calculation costs. I found them very helpful. I don't know what you're getting for the 12k but it seems steep.

maxelaine,

....assuming you are building in UK, look at your (design stage?) SAP calcs for the projected heat loss, and give the SAP calcs to your HP system designer/supplier.

It can save you £'000s in the short term (& long run possibly?).

Good luck

Posted By: maxelaineWe're building a 90sqm floor area house, on 2 storeys, to near-passive-house levels of insulation & airtightness. Our designer, who is PH-approved, says that she could only do heat load calculations if we were going for PassivHaus certification, which would cost £12k (and therefore well past our budget), and that the heating engineers will do it.

We've had one proposal (Earth Save Products) who are recommending a 4 to 12 Kw ASHP, and another, from Ice Energy, recommending a 5Kw (max) ASHP.

If you're building to PH levels of insulation/air-tightness, there's NO WAY you have such a high heatload, unless the plan is to use the ASHP to also provide hot water.

I live in a 185m^2 (plus 90m^2 heated basement) ancient house with hardly any insulation, and my heatload at an outside temperature of -23C (!) is only 15kW.

Paul in Montreal.

The designer who specced a lot of my house insulation and AT details, upon receiving my complaint that rdSAP wasn't accurate enough, said that I'd really need to go for full PHPP if I wanted to know with any measure of accuracy. He then bandied a figure of around 4K, which was for 2 houses (semis) of 250 and 280sqm, a complex design as a retrofit of an existing building.. so the other suggestions of 1.2 - 2K seems more like the ballpark, rather than 12.

Both the rdSAP and the BSEN12831 figures for my place (done by 2 independent people) have been high, maybe 30-40% over say each of the professionals involved. Alas, it's what they have to use when sizing a heat pump if it is to be properly registered for RHI (because the legislation requires that a pump cover 100% of the heat demand on the coldest day ever in the darkest winter than never really happens, lest Mrs Miggins be misled by some unscrupulous installer about just how much of the heating comes from the heat pump versus the immersion heater/hybrid fossil device as it sits there covering some lacking percentage when it's baltic outside and sending the electric bill through the roof)..

Upshot is in your well insulated house you may well end up installing something that is oversized by following these guidances - if youre going for the RHI. Oversizing can have consequences for the length of time a pump will last before the compressor fails :/

Thanks for comments. I now have the design SAP, which shows the highest space heating requirement as 606.52 KwH in the month of December.

You should probably do something more sophisticated, but if you divide that by the number of days you get roughly 20KwH per day, which is a bit less than 1Kw if the load was constant.

Anyone suggest a reasonable maximum heat load? Seems to me 5Kw would be more than enough, but I'm really only guessing.

Anyone know if ASHP's are available which will run at less than 1Kw constant output, because it seems that for much of the time we'll need less than that.

maxelaine,

Posted By: maxelaineWe're building a 90sqm floor area house, on 2 storeys, to near-passive-house levels of insulation & airtightness. Our designer, who is PH-approved, says that she could only do heat load calculations if we were going for PassivHaus certification, which would cost £12k (and therefore well past our budget), and that the heating engineers will do it.

How do I begin to make sense of this?

I sympathise, as we have a not dissimilar situation. We recently bought a low energy house of 148sq.m, built in 2008 with high levels of insulation and air-tightness of 1.45 air changes/hour @50Pa.
We planned initially to apply for EnerPHit certification, and retained a PHI certified consultant, who produced a full PHPP assessment. EnerPHit was achievable, but with some debatably thermal bridge issues, we were put off by the PHI certification fees and thermal bridge checking calculations quoted by a second firm of around £4000 inc VAT.
We therefore decided to settle for AECB Silver Performance certification, which was obtained in October.
Our total fees for full PHPP analysis, creation of full CAD files from combination of various original suppliers CAD files (chiefly timber frame supplier plus GBS for triple glazed windows) & certification process was nearly £5,000 inc VAT. We are satisfied with that, but appreciate that there were few options on a complex second hand building.
We have just put out a performance spec to several ASHP installation specialists, assuming they will install an inverter HP to satisfy a peak demand of 2.6kW, calculated in PHPP, plus provision for DHW. That will probably result in HP of 5/6kW.
The main problem will lie with the MCS calculation to BS EN12831, which are necessary to receive RHI payments. The mandatory software assumes a heating load which is roughly twice our needs, due to assumed ventilation losses. It will not permit use of our tested level of airtightness.
We are anticipating a HP of around 12kW to be able to make the RHI application.
Maybe this will all become academic if The Chancellor alters RHI within the spending review – in which case the choice is made for us – and we will probably specify the correctly sized Panasonic Aquarea G/High Performance HP, or 5th Generation Samsung EHS. Both have the right sized pumps with good COP performance and low noise levels.
I hope our experience and some of the figures I have mentioned will help you.

Posted By: maxelaineWe've had one proposal (Earth Save Products) who are recommending a 4 to 12 Kw ASHP, and another, from Ice Energy, recommending a 5Kw (max) ASHP.

Incidentally I completed the lengthy Ice Energy pro-forma with our details, plus notes, following phone calls to them, and never heard back. Caveat emptor!

I would like to offer the following observations based purely on my own experience.
Our house is an end of terrace home built about 1975. After a small extension it now has a floor area of 98 sq metres and an internal volume of 252 cu metres. 
We have insulated as much as we can with cavity wall insulation, 3 layers of insulation in the loft, some internal wall insulation in the bathroom and above the stairwell, ( using plaster board backed by 42 mm of foam ), good double glazing throughout, and a well insulated front door. 
The heating before the extension was an 11.7 kw gas fired back boiler with radiators. Just before building work started we were informed that our gas fire was obsolete and could not be repaired if it broke down. To cut a long story short we ended up in an extended house without any central heating system. To survive the first winter we bought a 2kw electric fire which we had in the lounge and being open plan it also kept the bedrooms warm. We now have an air to air heat pump ( no hot water ) with the inverter the lounge. It is rated at only 1kw. That has proved quite large enough to keep the house warm even in the coldest winter. We are now fully electric but even so we have reduced our consumption from 18500 kwhrs per year to just over 5000 kwhrs . We rarely use more than 25 kWh's per day and at christmas with a house full of people we can sometimes switch the heating off. I hope that helps.

In reply to Ed Davies.

When we did our small extension we extended the kitchen and turned the garage into a useful room. Half of the kitchen now has an insulated floor and the garage has a completely insulated floor. We didn't attempt to dig up the lounge and insulate that, so that it remains the original uninsulated concrete slab.


When it comes to ventilation , I did intend to install a whole house ventilation system with heat recovery. Sadly it was not to be. It soon became obvious that there were so many problems with the installation that I decided not to go ahead.


I have experimented with my own pressure test but it didn't yield any measurable results. However simply trying to do it led me me discover leakage routes that I previously hadn't given much thought to. So not a complete waste of time. 

With only 3 of us living in the house ventilation doesn't seem to be much of a problem. I can't find any obvious draughts and it never seems to get stuffy so I can only assume that the normal traffic through the doors covers our needs.
We dry all our washing indoors too, on drying rack in the stair well but even that doesn't cause problems either, although we have got a dehumidifier on standby, but again thats hardly ever used.


Last year the heating season was from Oct 6th to 9 April, This year I waited until Oct 10.


The small amount of stored hot water that we need comes from an immersion heater. We have a bath thats hardly ever used, instead we use an electric shower. Likewise the dishwasher and washing machine also heat their own water. 

Before we modified the garage, the end wall was a single brick wall which separated it from our lounge. We were in effect separated from the outside world by a single brick uninsulated wall. Now of course it is an internal wall.
I have 'nt got any figures for it but I think that that has produced some useful gains on its own.
It would be nice to have a pressure test done but so far the cost has put me off. 



We live about 6 miles from Buckingham.

Greenkeith, that does get things into perspective, thanks for sharing your experience.

According to the SAP, our space heating demand for the year is roughly 2,400 kWh, and DHW roughly 1,900.

We're thinking of an Ecocent for DHW, and whilst some of that demand will be pumped up via the ASHP, some will come from excess MVHR exhaust heat. Still sounds as though a 1 to 5 kW ASHP is more appropriate than a 4 to 12 kW one!

One issue is that most people expect heating to work QUICKLY and think of the thermerstat as a on/off switch.

So a heat pump sized for study state, does not work well if the tenants/owners are "normal people", hence some of the very bad real world results in social housing with heat pumps.

Should a system be design for how we wish people to use it, or how they WILL use it......

Posted By: maxelaineWe're thinking of an Ecocent for DHW, and whilst some of that demand will be pumped up via the ASHP, some will come from excess MVHR exhaust heat. Still sounds as though a 1 to 5 kW ASHP is more appropriate than a 4 to 12 kW one!

The Ecocent DHW system will take heat from your house which will need to be replaced by the ASHP. In theory, you can connect an Ecocent to the MVHR exhaust & thereby make use of waste heat. However, there are 2 issues with this:

1. The Ecocent requires an air flow rate of 350m3/h (200 litre unit). This is unlikely to be a match for the MVHR exhaust air flow rate;

2. For a reasonably efficient MVHR, the exhaust air temperature will be close to outside air temperature. So the Ecocent power output and COP will be significantly reduced. Last time I looked at this the Ecocent power output & COP fell off very quickly below room temperature. So I concluded that its only good for recycling waste or excess heat.

David