GSHPs: space heating vs DHW

alfa_aficionado

Currently at the planning stages. House will be ~4000sq ft and built to A3 standards, with regards to insulation etc. Like most people the more I research heat pumps the more confused I get! Some questions that I can't seem to get answers for are:

1. Do you run the system all year round or would you switch it off in the summer months? (as we currently do for the OFCH)

2. Is it more economical to heat domestic hot water by an alternative source? And if it is, can you program the GSHP to only provide space heating?

3. One of the suppliers we are considering have a product which can be linked in to solar panels in order to provide DHW - I am thinking that this might be a good option as the solar panels could heat the water in the summer months when space heating requirements are minimal and in the winter both systems could run concurrently - does anyone else have any thoughts on this?

3. I see people mentioning the use of ESB's Nightsaver tariff - how would this work with regards to space heating by a GSHP?

Thanks in advance

quentingargan

In a well insulated house, you should be able to switch the system off for half of the year or more. If you are using under-floor heating, the response time is slow, so you need to watch the forecast in spring and autumn.

All well insulated houses should think seriously about combining their heating system with solar for the very reason you outlined. That would apply whether you are using a GSHP or not. You may need to fit panels anyhow to meet your part L requirement for renewables.

All heating systems can be integrated to solar. There should be no need to buy a particular system for this reason.

I worry a bit about houses relying on a system that needs to use electricity between 5 and 7 in the evening. During this peak time, wholesale electricity prices often reach 25c and smart meters are going to ultimately penalise useage at this time. But a GSHP working off-peak will be knocking off at 8.00am, just as you head to work. In the evening, your house may have cooled and you will have a top-up period. (Or, you may find that it was warm and sunny all day, and the heat generated the night before wasn't actually needed).

It all depends on the nature of your house and its thermal mass and ability to hold heat (which is diminished with timber frame construction).

I like the idea of using a very large buffer tank with GSHP so you can store the heat using off-peak rates, and then distribute it through the house when needed, but I have never seen this done in practice.

beyondpassive

Quentin, can I challenge you on the point that timber frame cannot store heat, heat storage in the fabric isn't necessarily diminished by timber frame. In theory the mass of concrete can hold heat, however the massive cold bridges at the edge of the floor slabs and at eaves and window, door reveals and heat loss through the roof due to workmanship, mean that the only surfaces holding temperatures over about 16 degrees are the floors. Timber frame has concrete floors and you can pour a screed on a suspended floor. Where modern timber frame, say SIP passive spec excels over block built, is in the ability of materials such as softboard and cellulose to keep surface temperatures at comfort level. the density of the natural materials also holds thermal inertia which is released slowly to even out any durnal fluctuation in temperature. Check this out with any A-rated concrete block walls, put your cheek close to the wall and you can feel the block draw heat from your face. most the heat absorbed by concrete is drawn to the outside of the building rather than stored.

An A3 house isn't an energy efficient house, its just a bog standard build with solar and geothermal eco-bling bolted on. BER is an asset rating which bears no relation to built performance. Would it not be far more effective to put the €30-35k into a passive specification and meet the substantially reduced space heating and winter hot water with a small 7kw air source heat pump combined the MHRV. Its much more prudent to spend money on preventing heat loss, than spending on a big heating system to compensate for heat loss through the cavity walls.

I contend that the current spec of build of timber frame stores more heat than the current spec of block cavity wall build.

quentingargan

beyondpassive wrote: »

Quentin, can I challenge you on the point that timber frame cannot store heat

Thanks. That is an assumption that may indeed be wrong. The SEI DEAP software asks for an assessment of thermal mass which would favour concrete & Kingspan over timber frame, but as you say, that may be a false assumption, although it should be discounted by thermal bridging.

alfa_aficionado

beyondpassive wrote: »

An A3 house isn't an energy efficient house, its just a bog standard build with solar and geothermal eco-bling bolted on. BER is an asset rating which bears no relation to built performance. Would it not be far more effective to put the €30-35k into a passive specification and meet the substantially reduced space heating and winter hot water with a small 7kw air source heat pump combined the MHRV. Its much more prudent to spend money on preventing heat loss, than spending on a big heating system to compensate for heat loss through the cavity walls.

As I said earlier I'm no expert on these matters but just want to get it right now before the house is actually built. My understanding is that A3 standards are the highest that can be achieved without needing to install an MVHR system - is this correct?

Can ASHPs be combined with an MVHR system? This would surely improve the performance of the ASHP given that the exhausted air (from the MVHR) would presumably be somewhat warmer than outside air, esp in winter - is this a correct assumption?

Finally would it be worth paying for one of these companies who undertake BER assessments/energy audits etc to give our plans the once over and make suggestions?

Thanks again

sinnerboy

Answers in reverse order .

1. You will by law be required to have a BER cert before you occupy the house . So a pre start of works BER assessment carried out now can be used to inform your choices - a BER assessment now will put all into context . Your plan area , area of exposed walls , roofs and windows , level of air tightness and how you heat and ventilate all combine to affect the energy performance .

2. You can combine any ventilation strategy with any method of heating the house . Provided you achieve good air tightness ( Q50/5 or better ) MVHR is an energy efficient method of ventilation and will therefore reduce demand from whatever method of heating you use .

3. Your opening statement / query would be correct in some cases only . It is not as simple a proposition as you have phrased it . A BER assessment based on your unique design and spec will tell if it applies in your case .

In very simple terms - if you invest in expert design input to reduce energy demand in the first place - how you heat it becomes very simple . Maximise solar gain , insulate well , get your ventilation and air tightness right and you can can practically eliminate a conventional heating system .

alfa_aficionado

sinnerboy wrote: »

1. You will by law be required to have a BER cert before you occupy the house . So a pre start of works BER assessment carried out now can be used to inform your choices - a BER assessment now will put all into context . Your plan area , area of exposed walls , roofs and windows , level of air tightness and how you heat and ventilate all combine to affect the energy performance .

I was discussing this with my architect a few days ago - he tells me that it is not possible to calculate a property's likely BER prior to the construction of working drawings. I'm not sure if this is correct? At present all we have are the 1:500 planning drawings - could a BER assessor work from these?

He did suggest inserting a clause in the contract to specify the BER level required, which does seem like a good idea but I'd like to become a little more involved in the construction process than this, by perhaps directing the building contractor in how the house should be constructed.

I should probably have mentioned at the outset that "our" house is being purchased from a small local developer. No work has started yet. At present we are going through the planning process again in order to make some alterations to the original plans. Do you think the requirement for a provisional BER (for the purchase of a house from plans) would be applicable in this case? When did it become a legal requirement? (the planning permission was originally granted in 2008)

Finally, given the above scenario (and the fact that the architect is principally employed by the developer) would it make sense to employ an independent BER assessor?


P.S. Please feel free to move this to a separate thread as I realise we are getting a little off-topic here!