Heat pump new build

granty353

guys,

I'm currently looking at designing a 220m3 floor area house, I'm looking at installing a heat pump system heating system and the suppliers offer a 9kW and 12 kW air to water heat pump supplying under floor heating for the ground floor and rads and heated towel racks for the first and second floors. I also want the heat pump to provide hot water for taps and showers/baths. I have sized the full heating for the house at 7kW and trying to figure out what sized hot water tank is required for a 2 hour peak, I don't want to oversize. The house has 4 bedrooms and bathrooms.

Tifosi

Heat pump sounds huge in modern day housing, what kind of specification are your aiming for on the build, i.e insulation, air tightness, windows etc

granty353

Hey,

We are going for an A3 rating so I have used the following figures:

U Values
Wall U Value 0.21 W/m? K
Roof U Value 0.15 W/m? K
GF U Value 0.15 W/m? K
Glazing 1.25 W/m? K
Glazing G Value 0.35 W/m? K
Doors 1.3 W/m? K
Infiltration - 7m?/hr/m?

I'm just not quite sure how big a HW tank would be required for a reasonable amount of hot water for 2 hours.

granty353

I just got revised infiltration of 3 m3/hr/m2 so thats revised my heating load down to 5.3kW

Metric Tensor

I was about to say that 7 m3/m2/hr is a ludicrous value for a new build! Even 3 isn't particularly good.

granty353

I know but that's the figure that I have been given by architect. I just need a bit of assistance with sizing the hot water tank, I'm thinking 300 l but wondering if the 9kw or 12kw heat pump would be required.
the mechanical ventilation is another thing I'm looking for some design help with, if any knows typical ACH(air changes) for use when designing domestically.

Metric Tensor

I'm afraid the sizing of the heat pump or other M+E stuff is beyond my experience - probably shouldn't have chimed in with my smart ass ACH comment when I have nothing useful to contribute - sorry!

granty353

Haha no problem, I'd imagine myself doing the same!

MicktheMan

granty353 wrote: »

guys,

I'm currently looking at designing a 220m3 floor area house, I'm looking at installing a heat pump system heating system and the suppliers offer a 9kW and 12 kW air to water heat pump supplying under floor heating for the ground floor and rads and heated towel racks for the first and second floors. I also want the heat pump to provide hot water for taps and showers/baths. I have sized the full heating for the house at 7kW and trying to figure out what sized hot water tank is required for a 2 hour peak, I don't want to oversize. The house has 4 bedrooms and bathrooms.

What do you mean by "for a 2 hour peak"?

You don't say who is advising regarding your space and dhw heating needs but I see a few red flags which suggests that no one is looking at the overall picture. (Is it a case, I wonder, of satisfying part L of the bregs by bolting together "stuff" to get it over the line!) For instance, you are looking for a low temp engine (hp) to supply high temp water for radiators and dhw. While I'm not an expert on hp's by any means, I understand this setup will generally not be efficient and will shorten the life of the hp.

And I echo MT's comment on your airtightness target.

chillit

hi op

if you are doing solar then for a house that size they tend to recommend a 300L tank. Saying that if you have a heat pump then perhaps your renewables element is catered for already and no solar required.

granty353

@micktheman - 2 hour peak is the period of time during the day that the most hot/cold water is used, usualy 6-8pm as this is worst case requirements and usually will be replenished in 2 hours depending on reheat time and heating method. I am designing myself with input from the suppliers, I am a mechanical engineer but not much experience designing domestically. I have been assured by the supplier that the HP will be able to provide heating upstairs to rads and hot water at 60 degrees flow but there may be an issue during a bad winter which there is a coil that can be used in this rare instance. I didn't know that there was a risk of shortening the life but good to know and I will bring up with him.

@chillit I was originally considering solar but the pay back time isn't great and your right the HP alone for heating removes that requirement. I had originally wanted to go for hybrid gas boiler/HP but that does not meet requirements.

My big design issues at the moment are seeing what kW HP is sufficient, dont want to over or under size and the mechanical vent flowrates, I have read the requlations and know the minimum but just wondering in general what ACPH/design flowrates people have extracting from toilets and supplying living spaces.